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Adding Freon to Car AC – Gauge Readings Explained

7:29 am Air Conditioning, DIY, How To Auto Repair

Get your A/C checked at a local shop or check it yourself? Freon will need to be added by someone.

Summary © Adding freon or topping off a cars A/C system is the most common task performed to restore performance and get cold air blowing again. However, adding refrigerant isn’t always the solution for car air conditioning problems – troubleshooting may be required before just charging a car AC system.

There can be many other things wrong besides a system being low on refrigerant. To diagnose/troubleshoot problems, an A/C manifold gauge set is needed to read high and low side pressure readings. Avoid adding refrigerant with a simple charging kit like the ones sold at parts stores. Don’t add any stop leak, this can cause problems in the compressor, expansion valve or condenser.

The image above was taken at Moog and US Hwy 19 in Tarpon Springs Florida, it made me laugh. The sign says “Your wife is hot! She wants her A/C fixed, let us check it for free”. For some help taking care of your AC problems yourself continue reading.

This is where it starts to get serious. Keep in mind that using an A/C gauge set and seeing BOTH high and low side readings can help in diagnosing the problem when you know what to look for. First, on a 134A system the high and low side service ports are different sizes. AC gauge sets have color coded hoses, the blue color coded hose has a connection that fits on the low side service port and the red hose has a connection that will only fit onto the high side. The yellow hose won’t hook up to anything if just checking the readings; it can be used to connect to a vacuum pump or attached to a refrigerant can or tank.

*Make sure the condenser fan comes on when the readings are being checked. Below are normal car AC pressure readings with 134A.

* Normal readings on high and low side with AC OFF (static pressure) – Depends on outside temperature, but normally is between 80-105 PSI * Normal low side reading with AC on high speed and MAX & engine at 800-1000 RPM’s – Ranges from 25-35 PSI – Note that on many Chrysler products a normal reading on the low side may be 15-25 PSI

* Normal high side reading ranges from 200-350 PSI Don’t assume that if adding little Freon is good that adding a lot is better! Overcharging just a little can decrease the performance of the system and possibly damage the compressor. Additional Car A/C Info – Troubleshooting Gauge Readings

With the AC on the coldest setting, use a thermometer in a middle vent. Normal vent temperature readings will vary depending on the (ambient) outside temp. The vent temperature should range from around 42-55 degrees in my experience. If normal gauge readings are obtained and the vent air is cold – STOP don’t overcharge the system. The only proper way to remove refrigerant is with a AC recovery machine so if this is being done at home I can’t emphasize enough NOT to over-charge the system. And actually the best way to insure the proper charge is in a system, is to use an AC machine to recover the freon and then evacuate and recharge the system with the correct amount. Most cars have the factory specified amount on a decal under the hood. See the next page for gauge readings and images. Continued on Page 2

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163 Responses
  1. Ronnie Kephart :

    Date: May 21, 2009 @ 10:48 pm

    Is my high side gauge bad if I can get the correct low side readings and the vent temperature is great, but the high side is only 50 psi?(Also, it reads the same on multiple vehicles) Will the gauges read the system pressure on both sides with the gauge valves closed. In other words, under what circumstances should the high side valve be opened(how about during vacuuming system down)?

    Thanks for the help and the website!!

  2. dennisb - Auto Tool Sales :

    Date: May 22, 2009 @ 6:30 am

    Ronnie, Sounds like the high side gauge is bad. The AC manifold gauge should NOT be open to read the car’s readings.

    Without an AC machine, charging is always done through the LOW side, so the only time the HIGH side ac gauge valve should be opened is during vacuuming the system down.
    Click the image below to see our AC gauge sets for sale on our main website.
    Ac Gauge set for sale

  3. greg :

    Date: June 27, 2009 @ 5:25 pm

    Using a/c gauges I’m getting 25 psi and low 150 on the high side and the fan keeps going off, any common causes?

  4. dennisb - Auto Tool Sales :

    Date: June 27, 2009 @ 6:52 pm

    Greg, assuming the blower speed is on high, AC is set on maximum and engine is around 2,000 RPM’s, those readings seem low. Seems like the system is undercharged. The condenser fan is most likely going off because of the high pressure not being high enough to require the fan.

  5. Todd :

    Date: June 29, 2009 @ 9:13 pm

    My silverado is blowing hot air. i am getting a reading off the low of 100psi…any ideas?

  6. dennisb - Auto Tool Sales :

    Date: June 30, 2009 @ 6:58 am

    Todd, as stated in the article BOTH readings should be checked to have a better idea of what is going on with an AC system but based on the limited information… the compressor is not engaged or is not pumping. Below are some things I would check;
    1. Check the high and low gauge readings with the AC on & set on MAX/Recirculate, middle vents with the engine at 2,000 RPM’s, blower speed on high.
    2. Check the vent temperatures with a thermometer from the middle vents, far right & far left vents?
    3. What is the outside temperature at the time the readings were taken? 4. Check to see if the condenser fan blowing strong?
    5. Notice if compressor clutch is cylcling excessively.

  7. Nathan :

    Date: July 5, 2009 @ 5:29 pm

    My a/c is blowing out semi cold air. When I check the low-side pressure with the a/c on max, high blower setting, and idle speed, the reading is 55psi (95deg F ambient). But when I hold the engine at 2000 rpms, the reading drops to about 40-45psi and then the clutch disengages. When i let the engine come back to idle, the clutch kicks back on. It does this constantly. Am I low on freon because of the reading at 2000rpm?

  8. dennisb - Auto Tool Sales :

    Date: July 9, 2009 @ 10:29 am

    Could be overcharged, not enough air going across condenser fins, compressor not pumping sufficiently… BOTH gauge readings are really needed to get a better idea.

  9. adam :

    Date: July 10, 2009 @ 12:58 am

    I have 1993 acura integra. I installed new compressor, orfice tube and drier. Its still not cold.The reading for the low side 40 psi to 55, the high side is 200 to 350 psi. The condenser fan works perfectly at a very high speed. When I drive at highway speeds, the temp inside the vent reaches 40f when I stop it reaches to 60 to 70f, whould you please help me out?

  10. dennisb - Auto Tool Sales :

    Date: July 10, 2009 @ 6:19 am

    Adam, I would have thought your car would have had an expansion valve NOT an orifice tube.

    But from what you are telling me, the AC works great at highway speeds and not at stops. My first thought would be the condenser fan, if there are two fans I would see if they both working. Also if the car’s engine temperature is running higher than normal at idle, there could be some heat transfer from the radiator to the condenser causing the AC not to work as efficiently.

  11. A/C Re-Charge - Chevy and GMC Duramax Diesel Forum :

    Date: July 15, 2009 @ 4:42 pm

    [...] low side may be 15-25 PSI •Normal high side reading ranges from 200-350 PSI Here is a link Adding Freon to Car AC – Gauge Readings Explained | __________________ 05 Duramax LLY Bullydog Tripledog(Crazy Larry) w/ outlook monitor Last [...]

  12. Mike :

    Date: July 24, 2009 @ 8:24 pm

    AC blowing warm. Took to service tech and he claimed air in the lines. They connected to ac recovery machine, reclaimed old 134a, pulled vaccuum, and charged. The process added .7 lbs to the sys which requires 1.7. So, I take it that my sys still had 1 lb before recycling. Air blows very cold now. Obviously I am concerned about how long this will last (after $149 to add 134a) Car is 1996 and ac issues began 2 yrs ago. Is it safe to say there is a very slow leak if there is still 1 lb of 134a after 2 yrs of warm air? If warm air reocurrs, can I just add 134a or will it need to be in a vacuum before adding?

  13. dennisb - Auto Tool Sales :

    Date: July 25, 2009 @ 7:12 am

    Mike, sounds like there has to be a small leak. On any older car like that, it wouldn’t be unusual to have small leaks. Some of the common causes are the front compressor shaft seal, compressor housing leak, service ports, hoses & evaporator. If UV dye is in the system, it may help later to determine the source of the freon loss. Also using an electronic refrigerant leak detector should be used if the problem isn’t obvious. I wouldn’t be too concerned at this point unless it got worse. The service technician was not aggressive and has got your air blowing cold for the lowest amount compared to most other repairs he could have suggested. By doing the evacuate and recharge (& most likely adding dye in case you come back) he’s given your car’s AC a new lease on life. Hopefully this will last at least a year before it needs some sort of service, but don’t be upset with the technician if something fails. To answer your question about adding freon. Most likely you can add if needed, this would be the least expensive option… if that doesn’t work then further diagnosis will be needed at that time.

  14. dane :

    Date: July 25, 2009 @ 2:34 pm

    03 toyota avalon, gauges read normal readings for temp, but the compressor is constantly cycling, and the normal readings turn into up and down movements, and high side reads really high (after cycle) then repeats… any ideas? ac is cold then hot then cold then hot….

  15. dennisb - Auto Tool Sales :

    Date: July 25, 2009 @ 3:32 pm

    Dane, since you say “the high side reads really high (after cycle)” I would suspect that the high pressure cut-off switch is shutting the compressor down to protect it from being damaged. Usually the cut-off switch will disable the AC compressor at around 500 psi to keep the compressor from being harmed. The most likely causes, could be;

    1. Too much freon in the system.
    2. Condenser not working, or not spinning fast enough.
    3. A restriction in the system, possibly a clogged condenser.
    4. Heat transfer from the radiator if the car is running hotter than normal.

    You may want to try spraying water on the condenser with a garden hose to see if the extra cooling, will allow the system to operate more normally.

  16. john doe :

    Date: July 29, 2009 @ 5:28 pm

    Hi, I have an acura integra 1993. My a/c doesn’t cool very well any more. The gauge set I have only allows for testing of the low side. My low side w/ engine off is at ~74psi, w/ engine on is at 35psi. The evaporator outlet line feels cool but the condenser outlet to the evap. is warm. Advice would be great! =(

  17. dennisb - Auto Tool Sales :

    Date: July 29, 2009 @ 8:00 pm

    Could be a number of things, if both gauge readings are not read it’s really hard to say. The static gauge reading indicates that the system may be a little low (indicating under-charged). The static reading can vary depending on the outside temperature.

    The low side reading (35PSI) if measured on MAX air, high speed blower and fans working could indicate that the compressor isn’t pumping sufficiently. It’s probably best however to have a pro check it locally that will have the benefit of having both gauge readings and physically being there to diagnose it properly.

  18. Alex :

    Date: July 30, 2009 @ 1:48 pm

    vw golf 2000. New drier, exp valve, condenser and rebuilt comp. Evaporator and hoses are old but flushed. Idle, low side 42, hi side 250. 3000rpm, low side 30 and less, hi 300-330 up and down constantly. If running the car for 30min will freeze and blower inside the cabin will blow low even if max blow selected. Turned the car off for few min, started again and blower was working again. Get another compressor? Appreciate the advice!

  19. dennisb - Auto Tool Sales :

    Date: July 30, 2009 @ 2:35 pm

    Alex, the compressor seems to be working sufficiently. The problem could be that the system has moisture in it (try a long vacuum) or that the expansion valve is not working properly. Either of these two things could cause the evaporator to freeze up. If the evaporator is iced up, the airflow will be reduced.

  20. Alex :

    Date: July 30, 2009 @ 5:21 pm

    Thanks for quick answer! Forgot to say that I did vacuum the syst. 40 min with robinair 2 stage pump. Problem is that on high rpm the low goes as low as 15psi, while the high is at 250-300.
    Any additional thoughts?
    Many thanks!
    VW golf 2000

  21. dennisb - Auto Tool Sales :

    Date: July 30, 2009 @ 7:33 pm


    The low side does seem a little too low. The expansion valve may not be working properly.

  22. Oleg :

    Date: August 6, 2009 @ 5:40 pm

    Hi. I got 02 Ford Explorer. Recently I noticed that my A/C is not working good at idle. So I hooked up my gauges and the low side is up in 60s, high about 230 at idle, but with a little rpms (about 2000) low side goes down to 35 and A/C works alot better. Does that means that I need to look for a new compressor?

  23. dennisb - Auto Tool Sales :

    Date: August 6, 2009 @ 7:32 pm

    Oleg, Sounds like the AC compressor could be starting to pump a little less than it should. If the compressor isn’t noisy, I would try an evacuate & recharge to insure there’s no air in the system and the correct amount of refrigerant.

    Since you have an Explorer… If you haven’t already seen our blog on a common problem with Ford Explorer’s AC, check it out in case the recirculate door might be part of the problem.

  24. Trace :

    Date: August 7, 2009 @ 11:25 pm

    I have an after market system from Hot Rod Air in my car. The ambient was 90 deg. High side was 250 and the low was 20… The vent temp was 38-40 deg F. The gauge is working properly as I checked it on my daily driver and both high and low were in the norms. I’m not complaining about the cold air but something must be a little off. Any ideas?

  25. dennisb - Auto Tool Sales :

    Date: August 8, 2009 @ 7:17 am

    Trace, I see what you mean, the low side is lower than you would expect. But as long as the system isn’t getting so cold as to freeze up, I would enjoy the ice cold air! It’s possible though, that with a little cooler outside temperature, the evaporator could get iced up. If the system uses a temperature probe, I would try to make sure it is correctly located to accurately read the temp of the evaporator. If it doesn’t have a temperature probe in this aftermarket system, the expansion valve could be contaminated or malfunctioning. You might also check with the manufacturer to see if they have any suggestions.

  26. Chuck :

    Date: August 12, 2009 @ 5:56 pm

    I have a 2001 Ford Expedition w/ rear AC. The high side reads 180 psi and the low side 60 psi at 1500 rpms. Static test shows even presure on both sides. Could I be over charged?

  27. dennisb - Auto Tool Sales :

    Date: August 12, 2009 @ 7:30 pm

    Chuck, If it were over charged, both readings would be too high. The high side should be higher than 180 psi and the low side should be lower than 60 psi. In your case the compressor may not be pumping sufficiently.

  28. dennisb - Auto Tool Sales :

    Date: August 12, 2009 @ 7:31 pm

    More About Dual A/C Systems

    With dual AC systems the readings may not tell the entire story sometimes. For instance, if the rear expansion valve isn’t working properly, the readings may look normal because there is no way to isolate the readings specifically for the front OR the rear. Usually if the front is cooling well, but the rear is not, the rear expansion valve will be the cause.

  29. wayne :

    Date: August 15, 2009 @ 3:15 pm

    thanks for the info.. we have issues all the time and im trying to get better educated in ac arena. this explanation is really helpful. thank you. we have a huge fleet of Hummer limos and if anyone is looking to work on our vehicles you can check us out at

    thanks again for the info i will pass it along to our other office

  30. Jose :

    Date: August 18, 2009 @ 11:07 pm

    I have a jeep an i just used one can of freon and it shows 500 psi the ac works good but the pressure is to high the low is 50Psi

  31. Jose :

    Date: August 18, 2009 @ 11:10 pm

    Why is the pressure of my jeep so high and how do i lower it?
    I just used one can and and it shows 500 psi

  32. dennisb - Auto Tool Sales :

    Date: August 19, 2009 @ 6:23 am

    Jose, The system in your Jeep’s AC is most likely overcharged, assuming the cooling fan/fans are working and the engine isn’t overheating.

    In the old days to remove freon (before it was illegal) we would vent the refrigerant. We would slightly open a the valves and have the yellow hose in a cup so we could measure how much refrigerant oil came out during the process. It is now illegal to remove freon this way due to damage that fluorocarbons can do to the ozone. The only legal method for removing refrigerant now is to recover it with an AC machine. I recommend taking your vehicle to a shop and have this taken care of. They will 1. Recover the refrigerant properly. 2. Vacuum the system down. 3. Add oil if needed. 4. Put the correct amount of 134A in the system. 5. Evaluate the AC system and other systems (like cooling system and HVAC air delivery) to make sure it is working as it should.

  33. dennisb - Auto Tool Sales :

    Date: August 19, 2009 @ 7:50 pm

    Please visit our car questions page listed at the top of the page for help with repair questions you may have. Thanks, Dennisb

  34. Chris :

    Date: October 4, 2010 @ 1:34 am

    HELP! PLEASE!? I have a 92 Ford Escort. The a/c was low on r-12 freon, so I bought the retrofit kit. It says under the hood that a proper charge is 2 lbs exactly. I pulled a vaccuum on it for over an hour. When I did though, I had to keep turning the vaccuum pump on and of and on and off until it stayed 15-20 below 0. Basically it wouldn’t hold the vaccuum untill I got it below 10 and it would rise right back up and then I would turn it on. Anyway, I finally got it to stay vaccuumed and left it there for an hour. I used the included cans which were 14 ounces total each, which led me up to almost the proper charge. Anyway, it did get colder, but the coldest it gets is 62-65 degrees. I can live with that, but as soon as I get in and drive it at all, it stops being cold at all. Its only slightly cold at idle! The pressures are both 85-90 with engine off. With engine on: low is 30-55, and high has gone from 150 to below 100 now. The high side doesn’t move much at all, but the low side moves a lot! what do u think????? PLEASE HELP! btw, this is my first a/c work attempt.

  35. dennisb :

    Date: October 4, 2010 @ 10:11 am

    Chris, the fact that vacuum could not be brought down to 28-30 is a sign of a leak. First thing first diagnose the and repair the leak. Use a freon leak detector like the one below. Click on the image to see the product listing.
    Refrigerant leak detector

  36. Ben Orens :

    Date: October 5, 2010 @ 10:05 pm

    Looking for high pressure A/C line (adaptor.)
    93 Buick Road Master (R12) system.
    Male fitting at rear of compressor has check ball inside that is down inside about 3/8″.
    Need to connect charging line,high pressure and gages.
    Adaptor needs to trip the check ball so high pressure can be read on gages.

    Thanks, Please let me know

  37. dennisb - Auto Tool Sales :

    Date: October 6, 2010 @ 8:44 am

    Ben, make sure you aren’t looking at the high pressure releif valve which is usually on the rear of the AC compressor. Usually you will find the high side service port on the AC discharge line (hose going from comp. to condenser) or in the liquid line near the evaporator. The service manual says the high side adapter required is (3/8-24 threads). I would probably go to the local parts store and get retrofit adapters. Flush system, replace drier and evacuate and recharge with 134A and compatable oil.

  38. Miguel :

    Date: October 11, 2010 @ 6:48 pm

    I have read most of your comments and i know to get accurate readings i have to hook up low and high side but until i do that i will tell you waht is happening on the low side. With ac full blast i get no cooling off the ac. Low side reading fluctuates between 25-100 psi. Initially it starts at 25 PSI, the clutch on compressor kicks in pressure goes up to 100 psi then compressor clutch kicks off presuure then drops to initial 25 psi reading. This cycle happens every 10 seconds.

  39. dennisb - Auto Tool Sales :

    Date: October 11, 2010 @ 7:43 pm

    Miguel, sounds like low freon level, I also think you have reversed what’s happening. The pressure on the low side will read the highest with the compressor clutch dis-engaged and the lowest when the compressor is engaged.

  40. Rolando :

    Date: November 21, 2010 @ 11:32 pm

    I am glad I discovered this post.

    Well, I think in answering Miguel’s question, you might have answered mine, since I am having a similar problem. Right now I have only checked the low side port, and it is cycling up from 22 PSI to 43 PSI within a 7 second time period, and then back down from 43 PSI to 22 PSI within a 19 second period, for a grand total of approximately 26 seconds? What’s going on? This was done today, in Florida’s weather, evening time, I guess about 80 degrees, with the A/C on, in the setting right before max, which is number 3 on my 2003 Ford Explorer.

    I find myself adding freon about every six months, but it is needed a lot sooner than this because after I add freon, I believe it’s about a month after I hear the compressor cycling pretty often like it was before I added freon.

    I bought one of those products that’s supposed to inject a red dye into the system, but I don’t believe I saw any coming out. What I do see coming out after a few minutes of adding freon is a lot of dripping of water. I am not sure if that’s due to maybe the can being empty, but me not being sure, so I continue squeezing the trigger that’s pumping the freon into the system. I use one of those kits that bring the hose with the trigger and the ability to read the pressure.

    When I do add freon, the cycling happens at much longer intervals. Don’t remember the exact details, because I have been struggling with this problem for about 2 years now but haven’t had anything done other than buying cans of freon due to shortage of cash.

    So, what’s going on?

  41. dennisb - Auto Tool Sales :

    Date: November 22, 2010 @ 7:17 am

    Water dripping is normal condensation draining off the evaporator. Refrigerant leaks can sometimes be hard to find. See our car repair article about finding refrigerant leaks for more help.

    Checking for A/C Refrigerant Leaks – Car Repair Tips

    You may also want to invest in a refrigerant leak detector.

  42. barney stauffer :

    Date: January 8, 2011 @ 3:45 am

    without bothering you, i think you answered my question. i should put my hand in my pocket & pay a profesional. thanks

  43. plugged orface and blown compressor :

    Date: February 14, 2011 @ 9:58 pm

    I have a 1990 F250 that was converted from R12 to 134 and the cap tube became plugged and a blow out plug on the compressor released the 134. I need some kind of tool to get to the cap tube and open it up. Also is there any way to repair the blown plug on the compressor without replace?

  44. dennisb - Auto Tool Sales :

    Date: February 15, 2011 @ 6:24 am

    There may be an orifice tube repair kit available, which would require cutting the old one out. Flushing the AC system will definitely be a good idea. As far as the high pressure relief valve on the back of the compressor; usually once the pressure goes down the valve closes – if it DOES NOT close it will have to be replaced. Usually the cause of the relief valve to release pressure is from TOO MUCH pressure from 1. Not enough airflow across condensor. 2. The vehicle overheating causing head pressure to climb too high. 3. A restriction in the system, like a blocked orifice tube or clogged condensor from dessicant material escaping the drier. 4. OVER-CHARGING – putting too much Freon in will cause this everytime. Good Luck

  45. Richard :

    Date: February 24, 2011 @ 12:02 pm


    I am putting in a Vintage Air system in my 56 chevy, I am wanting to use Russell Proflex braided lines, my question is, does this braided hose carry enough PSI rating, I believe it is 1000 PSI. Please let me know and thanks Dennis.


  46. dennisb - Auto Tool Sales :

    Date: February 24, 2011 @ 12:31 pm

    Richard, The most I’ve seen car’s AC systems get up to is 500 PSI. Usually there’s a cut-off switch that will shut the compressor down around 450-500 psi to save the compressor from damage. So if your hoses are good for 1,000 PSI you should have no problems with them.

  47. justin :

    Date: March 2, 2011 @ 1:20 am

    thanks for all the great info. here is what i got. 2001 for expedition 5.4 rear aux a/c. replaced the compressor as well as the accumulator (sorry cant spell tonight lol)recharged the system. i was told 36 oz of freon and 11 oz of oil is needed for the system. all was going well, and then the clutch on the compressor started to dissegauge every 10 seconds then engauge for 7 sec. back is blowing ice cold but the front isnt, do you think i have a blockage in the orifice tube, low side is all i gould gauge but it is static at 60 psi when the clutch is engauged it drops to 24 or so and when it is off it goes up to 36 or so, any ideas

  48. dennisb - Auto Tool Sales :

    Date: March 3, 2011 @ 7:29 am

    Justin, sounds like a restricted orifice or expansion valve in the front. I would be careful not to add too much oil. 11 ounces seems like a lot unless you have thoroughly flushed and removed ALL the oil that was in the system previously. It’s important to know that many compressors come with oil already in them too. It’s best to drain and measure (and refill) them to know how much is going in – unless the compressor paperwork tells you. You have to manually turn the compressor clutch plate while draining to get all the oil out.

    If a system has too much oil, the system’s cooling can be affected and sometimes the cycling switch can be full of oil and not cycle the compressor properly. Since the rear is cooling, the compressor is probably running enough, so I would look into the front expansion/orifice. Good Luck

  49. mike :

    Date: March 30, 2011 @ 11:43 am

    Great info on this site, thanks. My mom has a 97 chevy malibu who’s a/c stopped working. I hooked up my a/c manifold gages to see if I could fix it for her and got really low static readings, 35 low side, about 45 high side. With the car turned on I got the same readings. When I turn the ac on I do not hear a click or anything, so I do not believe the compressor is engaging. Might the compressor not be engaging because the pressures are so low? I don’t want to add any more freon since the low side pressure is already at 35. Is there anything more I can check before I tell my mom to take it to a shop? Thanks for any help!

  50. dennisb :

    Date: March 31, 2011 @ 8:48 am

    Mike, low static pressure (AC clutch NOT engaged) indicates refrigerant charge is too low. When the system is undercharged to a certain point, the compressor will not be allowed to be turned on for it’s own protection.

    A common leak on the Chevrolet Malibu (and most Chevy’s) is the AC compressor, where the housing comes together. Look for residue from a leak on the underside of the compressor. Good Luck

  51. Terry :

    Date: April 4, 2011 @ 8:17 pm

    My dad replaced the a/c high pressure switch cut out on my 95 Olds 88(K)and the 134 blew out (he was told it wouldn’t). Do we need to add oil to the compressor?

  52. dennisb - Auto Tool Sales :

    Date: April 4, 2011 @ 9:03 pm

    Terry, it all depends on how much oil was lost. I would think that adding an ounce or two should be ok. The only way to know for sure is to remove the compressor, drain and measure the amount. Also the drier should be changed. Also it’s important to know that the high pressure relief valve seldom just leak – they normally will release refrigerant if the system is over-charged or if the fans are NOT working. Good Luck

  53. Terry :

    Date: April 5, 2011 @ 2:40 pm

    Thank you for your real quick answer. Actually the high pressure relief didn’t leak. My compressor would not always turn on, and so my dad was told that the high pressure switch should be replaced,and when he replaced the switch the 134 blew out. The low pressure valve does leak though (I have a new one to put on). Now hopefully I can get my dad to remove the compressor, he was hoping that he could just add some oil while adding the 134.

  54. dennisb - Auto Tool Sales :

    Date: April 5, 2011 @ 2:48 pm

    OK, so it was actually the high pressure switch and NOT the high pressure relief valve. I would most likely estimate how much was oil lost and just add UV Dye / oil to the system. And then evacuate and recharge the system. Good Luck.

  55. Terry :

    Date: April 5, 2011 @ 4:12 pm

    @dennisb, yeah just the switch. Thanks for your help!

  56. Terry :

    Date: April 5, 2011 @ 5:04 pm

    @dennisb, my dad doesn’t really want to replace the dryer, since the system was only open for about 5 minutes. Do you think it is necessary to replace it since it wasn’t open for much time? I could possibly talk him into it, if I gave him the reason :)

  57. dennisb - Auto Tool Sales :

    Date: April 5, 2011 @ 5:27 pm

    The dessicant life-span is around 5 years in a sealed system. Anytime the system is opened it is recommended to be replaced. When comes down to it, it’s your decision. Good Luck

  58. Terry :

    Date: April 5, 2011 @ 5:31 pm

    @dennisb, good reason to change it then! THanks

  59. Fritz :

    Date: April 6, 2011 @ 12:14 am

    just replaced compressor on a 95 gt Mustang, flushed system replaced drier.
    Pulled vacuum to 30hg and checked for leaks. All was good. Started adding Freon 1.4 lbs aprx. Compressor cycled, sucking in Freon. The Vent temp dropped from 80 to 65 and than suddenly low pressure went from 35 to 90. High side the same. Compressor quit cycling. Replaced pressure switch on high and low side from another Mustang that we know are good. Wwhat gives? Thanks for any ideas what it could be. Compressor possibly broken? A/C switch not working? Checked clutch fuse as well and is good!

  60. Terry :

    Date: April 6, 2011 @ 12:17 am

    @dennisb, I like your site it is designed very nicely. Thank you again for your help, I got the dryer and my father is going to put it on for me.

  61. dennisb - Auto Tool Sales :

    Date: April 6, 2011 @ 6:37 am

    Fritz, I would check to make sure there’s power and ground to the clutch coil. It needs power and ground to engage the clutch. The coil acts as an electro-magnet and magnetizes the clutch when energized. If it’s getting power and ground, then the clutch coil could be faulty or there’s too large of an air gap between the clutch disc and hub. If there’s no power, you may have to get a wiring diagram and bactrack through the circuit to pinpoint the problem. Common causes of loss of power to the clutch coil could be a poor connection, bad low pressure cycling switch and sometimes a high pressure switch. You are also on the right track to check fuses too. I like to check ALL fuses just to eliminate something simple. Many cars have fuse boxes in two or three locations! The Mustang, I believe has one fuse panel in the passenger compartment and one under the hood. There could be more than one fuse related. Sometimes it’s easier for me to check ALL the fuses, because by the time I look them up in the repair manual, I could have checked them all with my power probe. Good Luck.

  62. Fritz :

    Date: April 6, 2011 @ 9:12 am

    Thanks I will check that out!
    I also was thinking the a/c switch was faulty but kind of strange it would fail during charging.

  63. Fritz :

    Date: April 11, 2011 @ 12:30 am

    Okay so I got a few days to check out
    the Mustang Gt AC. It holds Freon so no leaks! I noticed when car is cold and I turn on AC switch, compressor cycles, however as soon as car warms up, it stops cycling. So I jumped compressor and it cycles on, low and behold Air is cold so I added freon and I got it down to 38 degrees. So I know now that compressor works. It is not a fuse.
    The AC relay is good because I switched it out. Now I’m thinking the control inside the dash is defective. What do you think?

  64. alfactors :

    Date: April 16, 2011 @ 11:57 am

    1994 dodge shadow comp. ingaged whin ac on but low side is 0 hight side 110 y is it running with 0 on low side try to put 134 in wont vac it in.?

  65. wrong :

    Date: April 17, 2011 @ 8:08 pm

    I have attempted to put freon in a 1994 gmc sonoma. Only a small amount of freon will go into the unit. The ac is blowing hot air and when a small amount of freon finally went into the unit, the air conditioner got a little cool, then went back to warm air. Does this mean there is a leak? If so, what is the most common place to check for leaks?

  66. adam :

    Date: April 18, 2011 @ 11:37 pm

    I have 1999 Astro with dual a/c I replace new ac compressor and orifice tube and charged the the system. I have reading for low side 40 psi and the high side 150-180. The rear ac very cold, the front not cold, the dryer is not cold its warm, the line go from dryer to compressor is warm, the evap is warm. The compressor engages at 150 psi high and the low side stay at 40psi not lower. When disengage low side 50-60psi and high, any idea? Thanks

  67. dennisb - Auto Tool Sales :

    Date: April 19, 2011 @ 6:37 am

    Adam, I would have vacuumed the system down at least 30 minutes before charging the dual AC system. Air trapped in the system can make it much less efficient. If you did have the vacuum pump on the system for a good evacuation before charging, seems like the compressor may NOT be pumping quite enough. I would check the readings with the RPM’s around 1500-2,000. If you still have 40 Low and 180 High, the compressor is not pumping sufficiently. If you need more help and suggestions, visit our car question page to ask a Chevy Tech. Good Luck.

  68. Fritz :

    Date: April 23, 2011 @ 11:58 am

    Okay so I was having intermittent clutch cycling problems. I replaced hose with built in orifice,(Ford).Thinking it was clogged.
    I believe the problem is fixed and it all leads to the wiring on the High side pressure cycling connector.
    Two wires are slightly exposed of the four. As I jiggled it,the compressor cycled on. I will now order new connector.
    These Cars are getting older and the connectors,wiring, all are degrading to the high heat temp under the hood in these vehicles. A/C works very well and cools with 24 oz freon,versus 32oz that Ford shows on the sticker.The vent temp at 85 outside air with 50% hummidity is in the mid 30′s on high setting and at normalsetting in the low 40′s.

  69. dennisb - Auto Tool Sales :

    Date: April 23, 2011 @ 6:53 pm

    Glad to hear your AC will be no longer on the Fritz!

  70. PT :

    Date: April 23, 2011 @ 8:33 pm

    I have a Vintage Air system on a 454. My safety switch failed after I installed electric fans. Replaced it and then evacuated the system for one hour. I let it sit for an hour and it maintained 30″ of mercury–no leaks. Next, I added 12 oz’s and then another 12oz of 134a. I still have 4.5ozs to go but it is going way sloooow and my pressures seem out of whack. I’ve quit adding 134a. My low side seems very high and my high side seems real low. Cabin temp at the vent is 60F with an OAT of 76F. I have a 90PSIg reading on the low side and a 100PSIg reading on the high side. Suggestions? Thanks

  71. dennisb - Auto Tool Sales :

    Date: April 24, 2011 @ 8:59 am

    PT, higher than normal low side readings and lower than normal high side readings indicate that the compressor is NOT pumping sufficiently. If both readings were LOW, it would indicate low on refrigerant. Good Luck.

  72. Ron :

    Date: April 25, 2011 @ 6:08 pm

    What happens if you add refrigerant to your auto ac (on the low side) without the compressor on?

  73. dennisb - Auto Tool Sales :

    Date: April 25, 2011 @ 6:12 pm

    Ron, not much will go in. The compressor usually helps to draw in the refrigerant, Shops that use AC machines have the choice of having the car draw in the refrigerant or they can use the machine to put the refrigerant in without the car and AC compressor running.

  74. Jim :

    Date: April 28, 2011 @ 8:29 am

    First, thanks for the great article.

    The article states that on some Chrysler products a normal reading on the low side may be 15-25 PSI.

    Any idea if this is true on my ’03 Neon. I must replace the high pressure hose (small leak) and I am anxious to use my new gauge set and to hopefully learn just a bit.

    Thanks in advance!


  75. dennisb - Auto Tool Sales :

    Date: April 28, 2011 @ 10:45 am

    Jim, It most likely will be between 15-25psi. Keep this in mind when charging. Also more is NOT better and finally, cold air coming out of the vents is the true test!

  76. Jim :

    Date: April 28, 2011 @ 10:48 am


    Thank you for taking the time to answer questions here!


  77. richard :

    Date: May 4, 2011 @ 11:23 am

    Dennis I have 1997 gmc savanna low side is 25psi with a/c on max and compress. engaged. high side is 80psi

  78. dennisb - Auto Tool Sales :

    Date: May 4, 2011 @ 12:22 pm

    Richard, based on gauge readings the system is low.

  79. joel :

    Date: May 8, 2011 @ 11:11 pm

    what will be the proper gauge of low and high freon on my honda civic 1.5 engine 99 model

  80. dennisb - Auto Tool Sales :

    Date: May 9, 2011 @ 6:41 am

    Joel, the readings in the article can be used as a general guide line for just about every car including yours. If you want more specifics you may want to ask a Honda Tech by using the Car question box on our car questions page.

  81. wally :

    Date: May 14, 2011 @ 5:45 pm

    I replaced a high pressure line on my saab 9-5. system was opened, but plugs inserted. recharged w/2 cans freon, low pressure read 40 and climbed to 45 when compressor kicked in. Worked fine for 5 days. Not cooling today, low side at 75, have checked w/yellow dye and nothing obvious. thanks

  82. john :

    Date: May 16, 2011 @ 9:26 pm

    Have a 2002 dodge caravan dual ac. I replaced compressor, expansion valve, and drier, let it sit for a hour on the 2 stage pump and then charged with 2.8 lbs of freon. My gauge reading were low side 40 and high was about 250 the problem is, is the vent reading was only about 60 deg, with it being about 80 deg out side. For a while it sat at low side 45 and high about 160 then it just switched to the readings above. Low 40 high 250

  83. Ralph :

    Date: May 27, 2011 @ 8:52 pm

    1995 Corvette-standard not electronic dash control. A/C blows nice and cold anytime-out.temp 75-100 degrees. However after about 10-15 minutes it freezes up and I have to turn if off for 5-10 min for the evaporator to thaw out. Any Thoughts?, Ralph

  84. dennisb - Auto Tool Sales :

    Date: May 28, 2011 @ 7:15 am

    Ralph, usually evaporator freeze up can be caused by several things. 1. Moisture within the system. A new dryer and using a vacuum pump to draw the the system down can remove the moisture from within. 2. Too low of pressure in the evaporator. This can be because of a partially restricted orifice tube or expansion valve. 3. Water not draining off the outside of the evaporator and out the drain tube quickly. This may be because of a restricted (or clogged) drain tube in the bottom of the drain hose.

  85. Joel White :

    Date: May 29, 2011 @ 10:33 pm

    Have a question, 2000 Grand AM merely blows cold air. I am getting a static reading of 32 on the low and 35 on the high. At operating pressure I am getting 26 low and 35 high. The systems was properly charged with a Robinair 34788 machine. Any ideas? Thank You and have a blessed day.

  86. paul :

    Date: May 30, 2011 @ 1:21 am

    I had a compressor seize up on a 2006 mazda3. I replaced it with a new one, and replaced the expansion valve and drier. I didn’t flush, because there was no debris in the compressor oil when I drained it. I added about half the freon the system takes, to about 60psi static (outside temp is 75). Started car, ac on, and low side pressure climbs to 35psi at which point belt starts squeeling. High side pressure was 100 psi. Any thoughts? seems like compressor is trying to stall, but if I had a blockage wouldn’t low side be really low and high side high.

  87. Mnk :

    Date: June 2, 2011 @ 1:08 pm

    Over the winter, my 1990 Ford A/C (converted to R134a) lost pressure, I guess, in part, because I did not run it to lube the seals. I added a can of sealer, then recharged the system using an all-in-one refrigerant/gauge kit. At about 35psi, the air was 70F at the center vent (ambient temp 85F). Adding another 12oz did not lower the temp.

    I have since purchased a good gauge set and have determined that the leaking has stopped, i.e., over one month, the psi has remained constant.

    But . . . static pressures are LOW-145, High-150; running pressures are LOW-60, HIGH-280.

    Before I purge/exhaust the system, am I correct in assuming that the most likely cause is air in the system? Any other possibilities?


  88. dennisb - Auto Tool Sales :

    Date: June 2, 2011 @ 1:25 pm

    Mnk, judging by the static pressures and pressures when the system is running, the system is overcharged – see the images in the article above. The sealer can cause a restriction in the condenser and orifice, so I would never recommend adding stop leak. It may not be causing a problem at this time for you, but I wanted to mention this for the benefit of other readers. In fact many shops will refuse to even hook up their expensive A/C machines if they know there’s “stop leak” in a system.
    After the system has the correct amount of refrigerant, I would then diagnose it – if problems are still present. Good Luck.

  89. Mnk a.k.a. Monk :

    Date: June 2, 2011 @ 5:41 pm

    You know you’re getting old (69) when you misspell you name.

    First off, thanks for the speedy response. I also know what you are saying about stop leak and gauge sets-A/C equipment. Over the years, maintaining my A/C in a what is now a fleet of rusting old cars, I’ve always blown out my gauge sets/hoses with compressed air. Don’t know if it helps; it’s a feel-good thing, at least.

    So . . . I bled off my refrigerant (into an empty 20lb propane tank which a local shop will purge for me). My current readings in 98-degree ambient air are:

    Static (engine cold): L/H – 90psi;
    Running (engine hot): L – 35psi; H – 230psi;
    Center console temp dropped to 60F.
    These readings were taken with the truck idling. My experience is that the console temp might drop another 5F on the road given better radiator/condenser cooling as well prolonged cooling of the dash and ducts.

    FYI, two years ago after I switched over from R-12, the console temp at idle was about 45F (40ish on the road).

    Thanks again,


  90. dennisb - Auto Tool Sales :

    Date: June 2, 2011 @ 6:43 pm


    69 is the new 39 right? Those readings seem a lot closer to what they should be. If you are not on MAX or (Recirculate) the system may get a little colder after changing that setting. Also the outside temp does play a large role in how cold the inside air will get. 98 outside is pretty warm especially for a car that is sitting at an idle. Condenser airflow is always something to check. Make sure the condenser is clean, has straight fins and the cooling fans are working properly. Good luck

  91. Monk :

    Date: June 2, 2011 @ 8:12 pm

    39? That was Jack Benny. What keeps me young is I still ride my ’96 Harley (FXDL) and trying to keep either the ’90 truck or a ’94 voyager running (when one breaks down the other serves to get parts).
    Speaking of old, I said ’90 Ford: it’s actually a ’90 Chevy 1500 pickup. Yep, OLD!

    I was surprised by the drop in temp after I bled off some refrigerant because when I first recharged the system after the sealer, I could not get the temp below 70???

    So, if it continues to put out 55-60F, that’s good enough for me. Just for future reference, am I correct in thinking that if there is air in the system, that air will cause the system to prematurely reach normal operating pressure but result in decreased cooling since air + refrigerant = less cooling than pure refrigerant?

    I really appreciate your gift of time and expertise. Thanks.


  92. dennisb - Auto Tool Sales :

    Date: June 2, 2011 @ 9:20 pm

    Sure, if there’s air in the system the efficiency will be decreased.

  93. Greg S. :

    Date: June 3, 2011 @ 11:24 am

    I have a 2000 Saturn that a bracket rubbed a pin hole in the condenser coil. Will JB weld fix this?

  94. dennisb - Auto Tool Sales :

    Date: June 3, 2011 @ 11:29 am

    Greg, JB is good stuff, but I don’t think it will work for that. High side pressure could reach 300psi. I would either replace the condenser or take it to a radiator shop to see if they can repair it.

  95. derek m :

    Date: June 3, 2011 @ 4:26 pm

    i have a 1998 chevy mailbu with 113000 miles ac blows cold kind of but not like it should and on the low pressure side it is running at 7psi

  96. dennisb - Auto Tool Sales :

    Date: June 5, 2011 @ 6:31 am

    Derek M, 7 psi is very low. You already know that from reading the article above. High side reading?

  97. clinton :

    Date: June 6, 2011 @ 9:56 pm

    2003 dodge 1500 4.7 engine …low side reads 14 psi and high side is 270 ,,if I add anymore freon the compressor starts to lock up.

  98. dennisb - Auto Tool Sales :

    Date: June 6, 2011 @ 10:38 pm

    Clinton, I would suspect a bad expansion valve. First I would have a shop remove the refrigerant, vacuum and recharge with the proper charge and recheck. The compressor seems to be pumping enough but the expansion valve may not be allowing enough freon through. Fig. 3 shows gauge readings when the expansion valve is stuck closed all the way (similar to your readings). Good Luck.

  99. Chris :

    Date: June 6, 2011 @ 11:28 pm

    Hello I see all your posting and I have a quick question? I have a 2001 Mustang GT just recharged the ac on the low side 25 – 30psi the car is running and blowing cool air…But on the high side 105psi that with the car off. is the ok..I don’t want to overcharge my ac unit. Any advice would be appreciated. Thanks

  100. dennisb - Auto Tool Sales :

    Date: June 7, 2011 @ 6:45 am

    Chris, the ambient temp. will effect the static reading (pressure with system turned off). The higher the outside or ambient temp. the higher the static reading. If you have the proper charge, the system will perform at it’s best. Anything more or less and the A/C system will not cool as well. High side readings? If the vent temp (while controls on MAX) is from 40-55 degrees it’s a pretty safe bet the system is NOT overcharged. Good Luck.

  101. Matt :

    Date: June 7, 2011 @ 9:45 pm

    I have a 2001 Honda Civic and the AC compressor clutch seems like it won’t engage on its own. If I smack it with a tool the clutch grabs and spins but if I rev the engine the clutch disengages again. I only have a low pres gauge and it reads 25 at idle but if I rev to 2000 rpm, the pressure drops to 10 and then kicks off the compressor. When the compressor is engaged, the vents blows cold air. Should I assume its low on 134a?

  102. dennisb - Auto Tool Sales :

    Date: June 8, 2011 @ 6:39 am

    Matt, sounds like a bad coil. I recommend taking it to a shop that can check it out more thoroughly. Good Luck.

  103. Tim :

    Date: June 8, 2011 @ 8:30 pm

    I just replaced the conpressor (reman), evap, condenser, expansion valve, drier on my 95 Maxima due to compressor failure. After pulling vacuum for an hour and recharging, I’m getting 55 on the low side and 150 on the high side with only cool air coming out while running at 1500 rpm and 80F. I’m thinking that the compressor is bad and can’t pull enough low pressure. What are your thoughts? Thanks.

  104. dennisb - Auto Tool Sales :

    Date: June 9, 2011 @ 6:23 am

    Tim, I think you are right. The compressor may not be pumping enough. Good Luck.

  105. Matt :

    Date: June 10, 2011 @ 11:14 am

    Hi Dennis, Before I rip out the compressor and replace the coil let me ask you this. Around the same time the AC clutch started acting up, the car had a tough time starting. When I checked the battery the cables were badly corroded mainly the Red + cable had acid all within the cable. I soaked it in baking soda to neutralize the acid and the cleaned it up as good as possible. The car starts fine now and I have the same battery which is still good. Could a bad battery cable cause the AC clutch to malfunction with a weak current? I don’t know the wiring diagram enough to know if the current is supplied from the alternator or battery but it seems coincidental that both would happen at the same time.
    Thanks again

  106. dennisb - Auto Tool Sales :

    Date: June 10, 2011 @ 11:26 am

    Matt, low pressure can make the coil cut off. When you rev the engine the low side pressure could go down low enough to make the compressor go off. If the coil is getting power but not engaging, it could be a weak coil or too large of an air gap. Which would make sense that when you wack it, it engages. A good repair shop will do some testing to see what the problem is before taking anything apart, which is why I suggested taking it to a repair shop. Good Luck

  107. Dustin :

    Date: June 13, 2011 @ 1:37 pm

    93 Ranger has R12 and gauge reading fo 25-50 on low side and 120-140 on high side. Is it low on Refrigerent or maybe a bigger problem

  108. dennisb - Auto Tool Sales :

    Date: June 13, 2011 @ 2:04 pm

    Dustin, Sounds like it may be low on refrigerant or the compressor not pumping enough, could be a bigger problem depending on the leak… Service ports may leak a little over time on any vehicle. If there’s a fast leak – say more than 1/2 a pound a year it should be fixed. Use dye or an electronic leak detector to pinpoint the source of the problem. It may be a good time to update to 134A. Good Luck.

  109. Todd :

    Date: June 13, 2011 @ 5:24 pm

    I have a 2004 Volvo S80. I am wondering if having too low and overcharged freon will cause the air to blow cold and hot air?
    It will blow cold for an hour or so, and then start blowing hot for 5-15 minutes. any ideas?

  110. dennisb - Auto Tool Sales :

    Date: June 13, 2011 @ 5:43 pm

    Todd, low freon is undercharged, not overcharged. Sounds like the amount of freon may be alright since it’s cooling ok until a certain point when something happens to affect it. The things I would look at when diagnosing the problem when it is acting up, is the possibility of a bad clutch coil or a frozen evaporator.

    1. A bad coil often causes the compressor NOT to come on when it should. The outer part of the clutch, called the clutch disc is engaged by the coil. The coil is like an electro magnet which allows the compressor to be turned on and off. The system will cycle the clutch on and off many times during normal operation depending on several factors, one being the low side pressure.

    2. If the system has moisture inside the system it can freeze preventing operation. Another cause of the evaporator freezing up can be the lack of cycling of the compressor. If the compressor stays on all the time or at least too long, the pressure in the evaporator will become too low and freeze. When the water on the outside of the evaporator reaches 32 degrees, it forms a sheet of ice blocking the flow of air. Restricted airflow means the heat and cold transfer cannot take place properly.

    Either of these things could cause an intermittant problem with cooling. Good Luck.

  111. Sean :

    Date: June 13, 2011 @ 6:35 pm

    Hello and thanks in advanced for any help you may bring.

    I own a 2002 Subaru legacy GT. I regretfully tried to save money and I bought a home refill kit with gauge. I followed all instructions and emptied the 18 oz can I to the low side. After I refilled I let the car and AC run and the compressor belt started screeching very badly and kept stopping and starting again draining power from the engine. The high and low hoses both get EXTREMLY hot to touch. The compressor seems to work fine until the car has warmed up completely. Did I overfill my system and ruin it?

  112. dennisb - Auto Tool Sales :

    Date: June 13, 2011 @ 6:43 pm

    Sean, the compressor could be siezed or “locked up” or the system may just be over-charged. More isn’t always better. If I were checking the system, I would recover the system (use an AC machine to take the freon out) then pull a vacuum and recharge with the proper amount of refrigerant. After that recheck the operation, to see what we have. Good Luck.

  113. Tom K :

    Date: June 13, 2011 @ 10:49 pm

    Thanks for helping folks out. I need some help myself. 1996 Jaguar XJS. A/C got weak last summer and a friend evacuated and charged it and it started working great. What I am assuming is a relief valve let a bunch of refrigerant go a few days later. Put my recently purchased gauges on it a few days ago and it read maybe 5 psi on the and 200 on the high side on a 95F day. Tried charging and the low side came up to about 20, but the high side quickly started going up to 450. Got scared and took it to a chain offering $30.00 A/C diagnostic. They had me pay to evac and charge it, and then said bad compressor. That didn’t make sense to me, since the high side seems to indicate it can compress plenty. Added refrigerant and high side went way up again. Drove for a while before it let a bunch go again. I promise not to donate any more refrigerant to global warming. My best guess is expansion valve is clogged. Any ideas?

  114. dennisb - Auto Tool Sales :

    Date: June 13, 2011 @ 11:52 pm

    Tom, Sounds like an expansion valve to me also. It’s common for the low side to be very low and the high side to go up too high when an expansion valve sticks closed. Adding refrigerant just makes the high side go higher due to the restricted flow. I recommend having a shop recover the refrigerant with an AC machine, perhaps you could get the shop that you took it to previously to take it out for you since you just paid them for a check. Then you can change the expansion valve safely, without harming any Unicorns. Good Luck.

  115. Ben :

    Date: June 14, 2011 @ 4:10 pm

    1995 Ford F350, compressor replaced about 8 years ago, now just blowing warm air, and the compressor engages about every 10 secs or so.

    Static Test:

    Low – 14
    High – 14

    Running Test:

    Low – Approaches down to 10 psi, then when the compressor kicks in, it drops to -5, then when compressor kicks out, it once again travels back to around 10 psi before the compressor kicks in and drops it back to -5 psi again.

    High – 32 psi, then when compressor kicks in, it rises slightly to about 39 psi, until the compressor kicks out again, it drops back down to around 32 again.

    Based on your this article, I could assume an indication of low freon, and since the low side hits a -5 vacuum, maybe a bad exp valve?

    And to top off this, the yellow hose on my gauge manifold has no way to pierce a can of freon. There should be an adaptor or something if using one of those little cans of freon, wouldn’t ya think? haha

    Thanks for any help! Take care…


  116. dennisb - Auto Tool Sales :

    Date: June 14, 2011 @ 4:44 pm

    Ben, sounds like low on refrigerant. I would think that the expansion (orifice) tube is ok. The low side is doing what’s expected with the system so low. Adapters are available on our main website ( There’s a side can tap below, click on the image to see the product listing.
    Can Tap

  117. Tim :

    Date: June 14, 2011 @ 9:09 pm

    Hi Dennis,
    I just replaced the conpressor (reman)again (from AdvanceAuto Parts – a Gilmore Products #67453 unit) on my 95 Maxima due to 1st reman. compressor not operating correctly (see above). After pulling vacuum for an hour and recharging, I’m getting 65 on the low side and 120 on the high side with only warm air coming out while running at 1500 rpm and 65F. I’m thinking that this compressor is also bad and can’t pull enough low/high pressure. What are your thoughts? Thanks.

  118. dennisb - Auto Tool Sales :

    Date: June 15, 2011 @ 6:44 am

    Tim, sounds like this one is pumping less than the last one (could be a bad expansion valve, but most likely the compressor). Remanufactured parts can sometimes be very frustrating to deal with. The cost factor makes it a necessary evil though. I once installed 6 defective starters in a row on a Mustang… finally on the 7th one I got a good one. You start doubting yourself when you get a series of bad parts like that. Here’s hoping that 3′s a charm. Good Luck.

  119. Tim :

    Date: June 15, 2011 @ 12:14 pm

    Just checked it again and it reads 70/110 at 80 degrees. Pressure quickly equalizes when I shut off the A/C. Wll try another one and see what happens – hope I don’t have to argue too much with Advance Auto on this 2nd return. Thanks.

  120. Pat :

    Date: June 15, 2011 @ 4:55 pm

    I have a 2003 Envoy SLT, we live in the Phoenix area, yesterday it was 108, got in my car after coming out of a store, started my truck, the air came on for a few seconds, and started blowing hot air, looked at the outside temp and it read -29, I laughed, and adjusted my temp knobs to 60, still hot air, shut off car and back on, still reads -29. and hot air,, any clue what is wrong?
    Thanks a lot

  121. dennisb - Auto Tool Sales :

    Date: June 15, 2011 @ 5:11 pm

    Pat, I would normally suspect that the ambient (outside) temperature sensor may be be bad because of the negative reading. But, since the AC is not working, all at the same time, it could be a controller (HVAC Control Head) or BUS communication problem. A scan tool and repair manual information will be needed to check this one out. Good Luck.

  122. herb :

    Date: June 15, 2011 @ 9:55 pm

    Great site! Best info i have found. My 2000 town and country cools great on the road but in town not so good. I find if i run the trans in gear 3(more rpm) it cools better. we have 100 temp–texas. Why does it take so long to cool and not cool at low rpm.

  123. dennisb - Auto Tool Sales :

    Date: June 15, 2011 @ 10:46 pm

    Herb, if the AC only cools well while the vehicle is moving – the AC condenser fan (or radiator cooling fans) may be inoperable. If the fan is working properly, based on what you have described, the compressor may not be pumping at a high enough rate. When you increase the RPM’s of the engine, the compressor is turning faster also. So if a compressor is weak, you may notice the air cooling sufficiently only when the compressor is made to pump at it’s fastest. A tech with AC gauges will be able to troubleshoot this further. Good Luck.

  124. Marty :

    Date: June 16, 2011 @ 2:56 am

    Thanks for the gage readings for 134A. Here in sunny AZ and with my ’03 Nissan Maxima, I was having to wait sometimes 3-4 minutes after starting for the A/C to start cooling and the condenser fans to start. Tonight with my new gage set and adapters and a can of R-134 I added a little at a time until I got a ‘steady’ 30 low and about 145-148 high at about 89 degrees ambient. Took me about 1 hour. Now even with the windows open, the discharge is very cold (forgot to do the digital temp gage) then turned it off to allow both sides to equalize. About 45 minutes. Started the car, turned the AC on and seconds later the compressor started and then almost immediately, the high speed condenser fans cycled on. Don’t have a leak detector but Nissan uses a UV dye so now I need to find the leak. Eveything looks very dry at 76000 miles. I just didn’t know the pressure/temp numbers like I used to with 12 and 22 systems. My new gages have a R-134 scale. I think I am darn close to an optimum charge. What do you think? Should I add more until I get close to 35 low and 160-180 high at about the same ambient?? Your advice is appreciated and an answer to my email would be terrific. Thanks

  125. dennisb - Auto Tool Sales :

    Date: June 16, 2011 @ 6:41 am

    Marty, thanks for purchasing the gauge set from, we appreciate your business. The only way to know 100% about the correct charge amount even for a professional is to put the factory specified amount of refrigerant in, after completely recovering and evacuating the system. With that being said, using the gauge readings and dash vent temp reading can help in making an educated guess (without doing the forementioned). The dash vent temperature with the system on MAX/RECIRCULATE, should be between 40-55 degrees (windows closed). The high side of 145-148 you mention seems low (RPM’s at 2-3,000). The high side should be around 200 or a little more. If adding more refrigerant I would monitor the low side to make sure it stays around the 30psi mark. Good Luck.

  126. Tim :

    Date: June 16, 2011 @ 5:14 pm

    Well, Advance Auto replaced the compressor again without a problem and the third one works good – 25 low/230 High at 1500RPM and 82 degrees outside. Hope it lasts at least three years to get my son through college! Thanks.

  127. dennisb - Auto Tool Sales :

    Date: June 16, 2011 @ 5:57 pm

    Tim, thanks for letting me know how this saga turned out.

  128. albert :

    Date: June 18, 2011 @ 10:16 am

    compressor clutch failed on 99 suburban with dual evaporators. replaced orifice tube, accumulator, and compressor. charged and front a/c working great. rear expansion valve was out so i evacuated, vacuumed, replaced accumulator, orifice tube and expansion valve. added oil and r-134. no leaks. low side 40psi and hi at 200psi with compressor running. ambient temp of 84. venting at 65 to 70 f on both front and rear. any suggestions? thanks.

  129. dennisb - Auto Tool Sales :

    Date: June 18, 2011 @ 10:29 am

    Failed clutches are not that common on GM’s. Usually it is a locked up compressor – the clutch will overheat, but is not the root cause typically. I mention this because if the compressor has locked up, the system could be contaminated and should be flushed before installing a new comnpressor and related parts.

    If the readings that you’ve given are with the AC on MAX (Recirculate) at 1500-2000 RPM’s I would suspect a weak compressor. I would like to see the high side reading a little higher (around 250) and the low side should be around 30psi.

  130. John :

    Date: June 18, 2011 @ 11:38 am

    Dennis, I have a 2000 Chrysler Town & Country with rear air. The system takes 46 oz of R-134A. The A/C was blowing warm air, I suspected the front expansion valve, so replaced it. After vacuuming the system, I have added 3 12oz cans so far. It is about 95 degrees out. The low side is reading about 70 psi, and the high side about 200 psi, yet still no cold air. Normally, I would think it the low side should be around 35 psi, and the system should start cooling, right? Bad compressor?

  131. dennisb - Auto Tool Sales :

    Date: June 18, 2011 @ 12:32 pm

    John, too high of a “low pressure” so that would make you think compressor, however the high side seems to indicate it is pumping as it should. If the system was fully charged, you would have higher than normal readings for both low and high sides. This makes me think that the cooling fans are not working properly or the vehicle’s engine temp is too hot – or both. Good Luck.

  132. Brian :

    Date: June 18, 2011 @ 1:21 pm

    Hi Dennis

    Thanks for the website will all the useful information!

    Like everyone else I have a question.

    I have a ’99 F350 7.3 4×4 Crew cab.

    The AC was just “OK” before I started fooling with it and took 20 minutes to really cool off the truck here in sunny Phoenix, it blew cold but I felt it should be colder.

    Upon the advice of a video series I made some changes (search youtube for “powerstrokehelp ac” and you’ll find the videos.

    I changed the orifice tube from a 38635 to a 38639, the 38360 is about .015″ smaller and actually recommended for climates that regularly exceed 105 degrees. While I was in there I replaced the accumulator. I pulled a -30Hg vac for 45 minutes, and recharged with 2.8lbs of R134a (I weighed an empty can and valve along with a full can and valve in order to get exactly 8.5oz out of the last can).

    At idle everything is fine, 35-40 on the low side and about 250 on the high side with 102 ambient outside temp, middle vent hovers around 50 degrees. At 1800 rpm (cruising RPM) the low side drops to 25 and the high side stays around 250-275 and the middle vent shows about 45 degrees.

    Out on the highway the compressor cycles on and off every 20 seconds or so while still blowing COLD. I ran a wire from the compressor switch to a test light in the cab so I could monitor compressor operation as I couldn’t hear it or feel it in the truck at highway speeds. My guess is that the air moving across the condenser is somehow causing the low side to drop past the 22psi threshold of the low side switch and causing it to cycle, but if that’s the case I have no idea why.

    I’m starting to think the orifice tube swap is the problem.


    Thanks for your time.

  133. John :

    Date: June 18, 2011 @ 1:59 pm

    Dennis, I added almost all of the last can of R-134A in the 2000 Chrysler T&C, so now I have approx 46 oz in the system. The pressures are now about 72 psi on the low side, and approx 260 psi on the high. I am getting a very small amount of cooling to the touch on the low pressure line, but not much. The cooling fans are working and the engine temp is in the normal range. It almost seems like there is a blockage of some sort in the system. I did change the compressor and dryer about a year ago due to compressor failure. Could there be residue in the system yet? The only thing that has not been changed are the condensor and evaporator (there are two evaporators – front & rear.) Any suggestions? Thanks for your help.

  134. dennisb - Auto Tool Sales :

    Date: June 18, 2011 @ 2:20 pm

    Brian, the smaller orifice tube is allowing less flow which equals lower pressure. If this is causing a problem, I would install the original one. Good Luck.

  135. dennisb - Auto Tool Sales :

    Date: June 18, 2011 @ 2:25 pm

    John, a blockage would result in too high of a high side and too low on low side since there would be reduced, or no flow. If I were checking it in person, I would check readings at different RPM’s, front and rear vent temps, line temps, condenser temp, etc. Good Luck.

  136. dennisb - Auto Tool Sales :

    Date: August 15, 2013 @ 4:50 pm

    Just opened comments back up on this article. Be sure to check out the 2nd page for gauge reading explanations. If asking questions (and cannot find the answer to your question in our AC articles) please provide as much info as possible. Thanks.

  137. Wilson :

    Date: August 19, 2013 @ 4:15 pm

    2002 Ford Crown Victoria – replaced accumulator and orifice when new compressor installed.
    Ambient temp. = 95℉
    Static: Low: 105, High: 105
    Idle: Low: 35, High: 275-Vent Temp:58℉
    1500: Low: 30, High: 310-Vent Temp:52℉

    Vent temp should be cooler…too much 134a? Bad compressor? Air/Moisture in system? (Pulled vacuum for 6+ hours and held at 30Hg for 2 hours after).

  138. dennisb - Auto Tool Sales :

    Date: August 19, 2013 @ 4:52 pm

    Looks about right actually with 95 degrees outside. The higher the outside (ambient) temp is the higher static and operating readings will be. You should know how much refrigerant was put in since the system was completely empty and on the vacuum pump for SIX hours? (15-30 minutes is the usual amount of time to keep on the vacuum pump.)

    I would expect maybe a little cooler vent temp with:

    1. Thermometer in the vent closest to the evaporator.
    2. AC set on max / recirculate and blowe speed on low.
    3. Running engine at 15-1800 RPM’s
    4. Max cooling achieved – Meaning the car was running long enough for the temperature not to be decreasing anymore.

  139. art barnes :

    Date: September 9, 2013 @ 9:47 am

    Hey, I have a 2000 3500 454. Yesterday I found a reading of 35 low and only 60 high but pretty good cold air blowing. The outside was 67 degrees. I just wanted to test my new cheap manifold gauges. Seems high side should be higher, about twice the temp, or 130 or so. Is this normal or perhaps my high side gauge is not reading correctly. Any thoughts, thanks so much!

  140. dennisb - Auto Tool Sales :

    Date: September 9, 2013 @ 10:43 am

    Art if you are really hooked to the high side, the reading should be 250 plus psi. So I would take the cheap set back and buy one of our gauge sets. :)

  141. Joe :

    Date: September 11, 2013 @ 1:02 am

    1986 Toyota Pickup, converted to R134a, very little cooling, just replaced expansion valve, still getting readings of 80 on the low side and 200 on the high side, I’m thinking compressor at this point. Your thoughts?

  142. dennisb - Auto Tool Sales :

    Date: September 11, 2013 @ 7:07 am

    Joe, could be the compressor but 200 psi is pumping pretty good. If the compressor is failing it usually will not be able to pump high enough pressure (200-300psi). The other thing could be that the expansion valve is not working properly. The expansion valve should be slowing the flow into the evaporator to a trickle. This creates low pressure (around 30psi on the low side is ideal at service port) and therefore cold temperature.

    Low Pressure = Cold.
    High Pressure = Hot.

    The other issue is that the expansion valve (on a 1986) was designed for use with R12 not 134A. So I would expect approximately 10-15% higher readings with the 134A, but 80psi on the low side is too high to achieve anywhere close to acceptable cooling. Good Luck.

  143. kelly :

    Date: September 18, 2013 @ 10:35 am

    hello, what causes horn of my honda accord 2000 model to work intermittently

  144. dennisb - Auto Tool Sales :

    Date: September 18, 2013 @ 11:07 am

    Kelly, the horn has nothing to do with the AC gauge readings that this article is about.

    But, normally when a horn “blows” by itself, the horn pad (aka horn switch or button) is faulty. The contacts touch completing the circuit just as if you pressed the horn yourself. Sometimes the switch (or horn contacts) can be changed without changing the entire airbag assembly in the steering wheel and sometimes it cannot. Good Luck

  145. kelly :

    Date: September 21, 2013 @ 2:12 pm

    Hello Dennis, its me again. I have this AC issue. My cars AC chills nicely in the mornings and evenings but only cools in the afternoons. Could low refrigerant be the culprit? By the way, I recently replaced the compressor and noticed this almost immediately.

  146. dennisb - Auto Tool Sales :

    Date: September 21, 2013 @ 3:47 pm

    Kelly, what are the low and high readings? Most likely low on refrigerant but I would see what the readings are to help diagnose. Good Luck.

  147. mike :

    Date: September 26, 2013 @ 5:08 pm

    2004 hyundai santa fe first time problem started blowing hot air noticed clutch on compressor not kicking in, added freon from local parts store cold air but noticed a bubbling around low line port schrader valve next day blew hot again, changed schrader valve topped off freon to 45 on low side blew fine but by the next afternoon was hot again got home to find a lime yellow/green oily spot on floor in garage. what do you think

  148. dennisb - Auto Tool Sales :

    Date: September 26, 2013 @ 5:16 pm

    Mike, it’s common to have a few bubbles from the schrader when you first disconnect. 45 is too high on low side, may of overcharged. The dye on the floor could have been from the actual leak or from the system relieving itself of the excess pressure if it was overcharged. I would clean everything, make sure at least 50 psi is in the system with it off (static pressure) and check for leaks using electronic leak detector and/or UV light. Good Luck.

  149. mike :

    Date: September 26, 2013 @ 5:47 pm

    thanks will try may get back to you soon

  150. mike :

    Date: September 26, 2013 @ 8:31 pm

    i checked around, under, and atop engine compartment where a/c lines and compressor lies…using black light found dye in the crease on the two piece housing of the compressor and nowhere else does this mean i may have overpressure the unit and now need to replace it

  151. dennisb - Auto Tool Sales :

    Date: September 27, 2013 @ 7:25 am

    You know there was a leak already, that’s why you added Freon. The dye on the seam of the AC compressor housing indicates a leak regardless of if you caused it or not, it should be replaced and the rest of the system checked (in case there’s more leaks). With an electronic leak detector you can check components that are hidden like the evaporator (and expansion valve) in the dash. You most likely didn’t cause a leak because there’s a pressure relief valve that will vent pressure if needed. Sometimes the pressure relief valve is located on the back of the compressor and also most systems will turn the compressor off when the pressures reach around 450 to 500psi on the high side. Good Luck.

  152. mike :

    Date: September 28, 2013 @ 8:32 am

    if no other leaks are found can i replace the compressor or do i need to replace expansion valve or other components….finances are tight. thanks for your help.

  153. dennisb - Auto Tool Sales :

    Date: September 28, 2013 @ 8:42 am

    Mike, some compressor warranties will require the expansion valve and dryer to be replaced. Some will only require just the dryer to be replaced. If I had my choice and the compressor just leaked like yours and didn’t “lock up” which could put trash in the system, I would replace the compressor and dryer and skip the expansion valve. Check with the parts store though to see what is required for the warranty. Good Luck.

  154. mike :

    Date: September 28, 2013 @ 8:58 am

    do you know if the drier is easily accessable on the santa fe

  155. dennisb - Auto Tool Sales :

    Date: September 28, 2013 @ 7:45 pm

    Mike, you may want to refer to a repair manual. Or just look at the physical location of the dryer.
    To others; just a reminder that DenLors Tools does sell quality Auto A/C Tools. Let us know if we can be of assistance.


  156. Freola :

    Date: February 28, 2014 @ 10:17 am

    Low side is 25psi and high side 180psi with AC on. When I turn the engine off I don’t hear the pressure equalizing in the oriffice area and low side reads 65psi and high reads 120psi. It should level its self to same pressure. Thanks.

  157. dennisb - Auto Tool Sales :

    Date: February 28, 2014 @ 11:01 am

    Yes the pressure will equalize but may take a little time. I would not be concerned as long as the AC is cooling well.

  158. hawthorne :

    Date: February 28, 2014 @ 5:31 pm

    Hi Dennis, I have a 2005 E55 MBenz with 65K mileage. I am having issues with the ac. Withing two months I have refilled freon once. I am getting an alarm with the freon sniffer in the vent located in passenger side. When I attached the ac gauges the static pressure reads low side 35 high side 26. There is a manual switch to turn on/off the ac compressor. However the compressor is off all the time even if I switch to on, it will just blink. So I belive that the compressor does not turn on. With a static pressure of 35 on the low side, this should be able to turn on the compressor……but this is not happening. I also have to look for the leak. Thank you for your advise in advance. Hawthorne

  159. dennisb - Auto Tool Sales :

    Date: February 28, 2014 @ 5:52 pm

    Electronic AC leak detectors can have many false alarms. If there’s an indication it’s usually best to double check the possible leak by looking for oil or dye if possible. 35 psi is not normally enough static pressure to allow the compressor to come on. I would say at least 50 psi. You can tell if the compressor comes on by looking at the front clutch disc – it should be engaged and spinning if the electro-magnet (coil) comes on. Blinking could mean there’s other problems but based on the static pressure it could just be insufficient refrigerant. The system will not let the compressor run if there’s not enough because it gets lubrication form the oil that circulates with it. Good Luck.

  160. rasheed :

    Date: February 28, 2014 @ 8:54 pm

    Hello Dennis,
    My a/c temp at Max cool is 60 degrees at 80 degree ambient temp and at 1500 rpm my low side pressure is at 25 psi and high side is 250 psi. The problem that I am having is when my rpm drops around 1000 rpm, I get a vibration in my high pressure line from the compressor to condenser. what will cause a vibration like this?

  161. dennisb - Auto Tool Sales :

    Date: March 1, 2014 @ 8:22 am

    That’s not much of a temperature drop. The max air temp coming from the vents should be in the 50′s or possibly 40′s. Maybe the vibration is from air in the system. I know compressors can be a bit noisy if there’s air in the system which I suppose could cause vibration. I would have a shop recover, vacuum and recharge the system with the correct amount of refrigerant (adding oil if needed) and see how it reacts. Good Luck.

  162. seanw :

    Date: March 22, 2014 @ 8:53 pm

    How’s it going. I have purchased a 1990 chev 1500 with warm air. I have checked pressures and found low side around 40 psi and high side reaching 175. I am only getting about a 15 degree change in temp. All parts on the system appear to be new. Any help would be great thanks

  163. dennisb - Auto Tool Sales :

    Date: March 23, 2014 @ 9:19 am

    This time of year, the high in many parts of the country is around 70-75 degrees (South Carolina included). If you are checking the readings with that range of outside temp; at 15-2000 rpm’s, on max setting, blower speed on high. The compressor may not be pumping sufficiently. I would expect a low side of around 25 psi with such a low outside temperature. Good Luck.

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