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 allot 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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392 Responses
  1. Nate :

    Date: June 13, 2015 @ 3:43 pm

    About 2 months ago, my AC stopped working if the gas was not pressed. It would get hot/warm at a red light, then cool as soon as I started moving again. Two weeks ago it started getting warmer even when driving. A week ago it was warm all the time. I thought I had a freon leak. I bought the freon charging can. When I connected to the low side port (car on, AC set at max) the gauge peaked out at 200+ PSI. It is 90 degrees today. I think the AC compressor is running but am not sure where to look. 2005 Saturn Ion. Any DYI suggestions, or am going to a shop?

  2. dennisb - Auto Tool Sales :

    Date: June 14, 2015 @ 7:56 am

    Nate, I would check the gauge readings and go from there. Saturn’s do have a common problem with compressors though. Good Luck.

  3. 98Expedition :

    Date: June 15, 2015 @ 2:53 am

    Hi, I am hoping you can help me figure out what is going on. Last summer, the A/C on my Expedition started getting weak. I had it checked, and there was a leak found on lines going to the Rear A/C. The cost of the repair was too high, so I decided to chance it with A/C Pro. I added a full 20oz can. The A/C worked much better after adding the refrigerant. That was near end of summer season last year. The A/C was adequate until the last couple of weeks when the temps here in Phoenix went above 100F. I thought that the leak was not fixed, and that I needed more R134a. I hooked up manifold to High side and another can of A/C Pro to Low side. Pressures at idle with approx. 105F ambient was about 60psi on low side and 260-270psi on high side. The static pressure was about 110psi. I also noticed that the compressor engaged when I turned on the A/C, and then never disengaged. Isn’t it strange for compressor to just stay on? The temp of output air was about 68-70F on both front and rear vents. Based on what I have read here, now I am wondering if I actually over-charged the system last year, and causing poor efficiency. Any advice would be greatly appreciated. Thanks

  4. dennisb - Auto Tool Sales :

    Date: June 15, 2015 @ 7:51 am

    Could be overcharged, could also have issues from the stop leak in the “AC Pro” you’ve added. Stop leak can wreak havoc on AC systems and recovery/recycle equipment. Not sure why products with stop leak are allowed to be sold. Many shops without added filtering for their AC recovery machine will turn customers away if they know there’s stop leak gunk in the vehicle’s AC system. I would take to a repair shop and have the Freon recovered and the system pulled down into a vacuum and the proper charge (with the correct refrigerant) put in. And then recheck. It may also be possible to cap off the rear AC, if that’s where your only leak is and you can’t afford to fix it. Good Luck.

  5. Jeff :

    Date: June 22, 2015 @ 2:42 pm

    My AC vent temp is 70 degrees at 1500rpm, ambient air is 90 degrees.
    Static pressure is 100 w/ everything off.
    Low side pressure 53, high side 250.
    System is blowing cool but not cold air. Compressor is new, condenser is new. I flushed all lines and evaporator w/ Dura II. Vacuumed for 1 hr to 30mmHg. Held vac for 15 mins afterwards. Then charged with 4 oz pag oil and 1.76 lbs 134a as dealership told me.
    My low side seems hi? but everything else looks fine….why are my vent temps so lo poor?
    2003 Silverado 2500HD.
    THANK YOU!!!

  6. dennisb - Auto Tool Sales :

    Date: June 22, 2015 @ 2:50 pm

    Jeff you can’t flush the expansion valve (or compressor) so, unless it was not hooked up when you flushed, I’d bet that there’s solvent in the system. Even with the expansion valve and compressor NOT hooked up in the system, it’s really hard to get all of the solvent out. That’s why I always preferred using an extra add on filter on systems that are really dirty instead of trying to flush.

  7. Jeff :

    Date: June 22, 2015 @ 2:53 pm

    my pressures are at idle.
    Also, my compressor seems slightly noisy…….being new I thought it would be silent like the OEM was…….new one is a Delphi.
    New condenser got 1oz PAG 125
    3 ozs went into new accumulator.
    I was told 2 ozs for evap, 1 oz for condenser and 1 oz for lines……totaling the 4oz I put in.
    It seems like I need more 134a but the pressure already seems hi to me and I know I have the correct charge amount in the system anyway.(1.76lbs)
    also, evap inlet cold, evap outlet not so cold…cool at best.

  8. Jeff :

    Date: June 22, 2015 @ 2:54 pm

    I did not flush compressor……just added the amount of oil that was in the old one………..I removed all lines to flush them.
    So you believe my evap is full of flush solvent???

  9. dennisb - Auto Tool Sales :

    Date: June 22, 2015 @ 3:40 pm

    It’s possible. The evaporator and condenser are very hard to flush to remove ALL solvent. Lots of tiny tubes, especially the condenser. Anything other than refrigerant in the system, like solvent, excessive oil, air etc, can cut down on the efficiency making it not cool as well.

  10. Akram :

    Date: June 28, 2015 @ 11:55 pm

    Hi, I have a 2010 Ford Expedition not cooling well at idle speed. The compressor and expansion valve are new. The low side pressure is 30 to 40 psi. It is a single ac and is coldest when around 2000 RPM’s.

  11. dennisb - Auto Tool Sales :

    Date: June 29, 2015 @ 7:04 am

    I would check both high and low gauge readings. Also check vent temperature with a thermometer with AC on MAX so the cabin air recirculates. The readings will help determine what’s going on with the system. Good Luck.

  12. spanky :

    Date: July 1, 2015 @ 11:28 am

    Hey I have a Snapon electronic leak detector from the 80’s. Will it work on 134a it was made when we still used R12?

  13. dennisb - Auto Tool Sales :

    Date: July 1, 2015 @ 12:24 pm

    More than likely. You could always test it at a service port to verify.

  14. BurgieDanny :

    Date: July 3, 2015 @ 7:20 pm

    I have A 1999 Dodge Stratus, the A/C only seems to work well when it’s cool outside. Today it was 92 degrees and the air would only go down to 68 degrees. This is what I did to the A/C today. I changed the Accumulator/Drier and the Expansion Valve. I flushed the Lines, Evaporator and Condenser. I removed the Compressor emptied the oil and refilled with 5 ounces pag 46 oil. I pulled vacuum for 1 hour and filled with 19 ounces of Freon after venting lines to evacuate air in the lines. The Gauges are showing 46 on the low side and 200 on the high side, its 92 degrees today so I believe the readings are low, but Ive already filled to manufactures specs. The condenser fan seems to be running on high like it should and Ive cleaned the Condenser with engine degrease and the garden hose. What am I missing?

  15. dennisb - Auto Tool Sales :

    Date: July 4, 2015 @ 9:37 am

    Normally, there’s no reason to flush a system unless the compressor has failed and you are installing a new one. If the oil is black that indicates a worn compressor and it should be replaced. You don’t normally change oil in an AC system like you do in a car’s engine.
    Hopefully you didn’t vent the Freon into the atmosphere. This does harm to the ozone. A shop with an AC recovery machine should be used to recover refrigerant before starting to work on a system. Never heard of using degreaser on condenser. Usually there’s just pollen and dirt, not grease or oil. If there was an oily residue, that indicates a leak. The low side on Chrysler vehicles is usually 15-20 PSI. So, 46 is high. Causes of the low side being too high on your model is typically a bad expansion valve. I know you replaced it, but it could be defective. Could also be the compressor not pumping sufficiently. I would expect the low side lower and the high side higher. So, I would replace the compressor and expansion valve. You could try the compressor without replacing the expansion valve first if you wanted, but I would get it warrantied just in case. You’ve already replaced the drier. Good Luck.

  16. BurgieDanny :

    Date: July 4, 2015 @ 10:57 am

    I didn’t vent the freon, I have an rg5000 recovery machine. I’ve been trying to get this ac working right for a few weeks now so I went ahead and tried every thing except replacing the compressor! When I vacuumed the system I could only get it down to -22 and I’ve read it should get down to -29. could I still have moisture in the system? I used the rg5000 to vacuum, maybe its not strong enough? it sat at -22 for 30 min, I don’t think its leaking. I’m new to this a/c work and my wife’s gonna buy a new car if I can’t get the a/c working right! I really appreciate this!

  17. dennisb - Auto Tool Sales :

    Date: July 4, 2015 @ 11:42 am

    There’s either a leak in the car’s AC, the gauge/hose assembly or the gauge is just wrong. Moisture will not cause an incorrect vacuum reading.

  18. BurgieDanny :

    Date: July 4, 2015 @ 12:36 pm

    I didn’t think the vacuum reading was incorrect, I thought because I didn’t get it to -29 I left moisture in the system and that’s why I was getting the pressure readings and a/c was not cold enough. according to charts, low side should be 45-55 and high side 275-300. but my high side reading was low and I didn’t wanna just keep adding freon when I was already at 19 ounces. I guess next is a new compressor, thanks for the help.

  19. Jeep zj :

    Date: July 5, 2015 @ 5:42 pm

    I have a 1994 Jeep Grand Cherokee Laredo 6 cylinder . I just replaced all the ac components and evacuated the system . I put in less than 1 pound aim getting 48 psi on low side and 90 on high and blowing hot air

  20. Steve :

    Date: July 6, 2015 @ 10:26 am

    I have a 1994 Ford F150 XLT, 5.0 liter. when it was purchased a couple years ago, the air conditioning was weak. The sellers filled it with Freon, it was blowing warm within a couple days.

    Two years later we are starting to work on it. lowside pressure was 8 pounds, high side 0. The compressor would not kick on until I jumped it, then it came on. It started taking the first can of freon (134). Lowside dropped to a vacuum of about 15 and I periodically checked the high side. It was building up gradually, then seemed to stop at 125 psi. I thought the first can was empty. It really was not. I hooked up the second can, and the system would take no more. The low side had gradually crept up to zero, and now was staying there. The high side would not go past 125 psi, and after a few minutes actually dropped to 120.

    I am not good at air conditioning systems, but I am very mechanically inclined. This is the first time I have added freon in my life, and my first set of guages. Any help you can give would be appreciated. Do you think the compressor is tired?



  21. dennisb - Auto Tool Sales :

    Date: July 6, 2015 @ 10:28 am

    Wow, not sure why you would replace ALL components. At a full charge you should have around 30 on the low side and around 300 on the high side at around 1500 RPM’s. At a partial charge, the fan may not be on which would cause a higher than normal low side. Lower RPM’s would cause a higher than normal low side also. Refrigerant not drawn into the system will cause a higher than normal low side, although it shouldn’t take much pumping to bring the low side reading down. Good Luck.

  22. dennisb - Auto Tool Sales :

    Date: July 6, 2015 @ 10:37 am

    Jumping the clutch to engage the compressor bypasses the safety which protects the compressor. It’s not needed to charge a system, so I don’t recommend it. It’s not good for a system to be left empty. This allows seals and orings to dry out which can cause leaks. I would stop and check for leaks since you have enough in the system to check. After leaks are repaired I’d vacuum the system down and charge and recheck. To answer your question about the compressor being worn, yes that’s a possibility. Without a full charge it’s hard to tell though. With the low side in a vacuum the Freon should flow into the system if the valve is opened properly if the compressor is good or not. Good Luck.

  23. Justin :

    Date: July 29, 2015 @ 11:04 am

    Hey, We hooked up my 2005 BMW 525i A/C up and we got a low reading on both sides. We decided to vacuum the system. We got it to -10 and it basically didn’t drop over our wait. We recharged the system to 50 (90degrees outside) on the low side and the A/C was blowing good about 65 degrees. Better than 90 or 95. We turned the car off waited a while and then started it again to test pressures. The low pressure was high about 70 and the high pressure was low about 200 and the needle was jumping up and down about 5 both directions. The A/C was blowing about 80. Any idea of what could be happening?

  24. dennisb - Auto Tool Sales :

    Date: July 29, 2015 @ 11:10 am

    Justin a vacuuming down of the system should only be done on an empty system. It should go down to 28-29 not 10. A leak could be the issue with the system or the gauge set. Since the low side is high and the high side is low, this indicates a weak AC compressor. Good Luck.

  25. Tim :

    Date: July 30, 2015 @ 8:35 am

    I have a 1995 Chevy silverado. When the A/C is off and the recycle is off, the controls for the air direction work fine. I turn it to vent and it blows out the vents, floor and vent and etc. etc. I turn the A/C on and the air flow goes directly to the floor, and the control to change the air direction will not work. It did this for about 3 months, and the air was cold. Now the A/C is not cooling at all. The controls are still doing the same thing. Any ideas will be helpful.

  26. jed :

    Date: July 30, 2015 @ 9:44 am

    I have a simple question, but have looked for the answer for days. 2004 Honda Accord EXL 4 cyl, A/C only cooling to 65 deg in right center vent and passenger vent, drivers side vents basically not cooling. I have has the Compressor and serviced (last year). Things I have noticed that may be a problem 1. High pressure line and low pressure line near the servicing ports are close to the same temp, 2. Bottom 1/3 of the condenser is almost closed off due to fins bent due to road degree.

  27. dennisb - Auto Tool Sales :

    Date: July 30, 2015 @ 10:45 am

    Jed, not sure what the question is, but I would check the readings to see what’s going on with the system. The blockage could definitely affect the efficiency of the system. The high reading would be high if too much air is blocked off from condenser. Good Luck.

  28. dennisb - Auto Tool Sales :

    Date: July 30, 2015 @ 10:47 am

    Most likely a control head issue. Will take some testing with use and knowledge of wiring diagram and how the system works. Good Luck.

  29. Hope :

    Date: August 7, 2015 @ 3:00 pm

    I have a 2002 Dodge Grand Carvan Sport just put new air compressor on it . It has dual air in front and back much freon would I put in it?

  30. dennisb - Auto Tool Sales :

    Date: August 7, 2015 @ 4:46 pm

    Hope, sorry that information may be found under the hood or in a manual. Good Luck.

  31. Jay :

    Date: August 29, 2015 @ 5:01 pm

    My daughter’s 2000 Beetle has been sitting for a couple of years without running while she was off at school. I got it started but the a/c wasn’t working so I hooked up the gauges and got a reading of 90 psi on the low side and 97 psi on the high side. Would this condition occur if it had a low charge?

  32. dennisb - Auto Tool Sales :

    Date: August 29, 2015 @ 6:43 pm

    Based on the static pressures, seems like it has a charge, just maybe the compressor clutch isn’t engaging the compressor. If the compressor is engaging it may not be pumping. Good Luck.

  33. Jay :

    Date: August 29, 2015 @ 6:46 pm

    I checked and the compressor clutch is engaging. If it’s not pumping could this be the RCV?

  34. dennisb - Auto Tool Sales :

    Date: August 29, 2015 @ 7:59 pm

    The RCV (Refigerant Control Valve) is used in some VW’s that have a varable speed compressor. However the high side reading should be higher even if there was a problem with that valve. You could try raising the RPM’s to see if the readings change (to seee if the compressor is pumping any), but more than likely the compressor has failed. Not using the AC for an extended period of time is not good for an air conditioning system, Orings and seals need the lubricating refrigerant oil to circulate throughout the system to keep them pliable so they can seal properly.

  35. Bruno :

    Date: October 28, 2015 @ 9:51 pm

    I have an Excursion with front and rear A/C. If rear A/C is off the front will blow warm. As soon as the rear A/C is turned on, the front will start to blow cold air. Turn the rear A/C off and the front will go back to warm air. Any ideas? Overcharged system?

  36. dennisb - Auto Tool Sales :

    Date: October 29, 2015 @ 7:02 am

    Bruno, I would check the gauges and see the change when the rear AC is turned on or turned off. You could be right maybe overcgharged. The readings should help diagnose. Good Luck.

  37. Mike :

    Date: March 20, 2016 @ 2:42 pm

    2005 Lincoln LS 3.9L. 1750 RPM, L/S=45 psi needle oscillates +/- 5 psi, H/S = 100 psi. Leaning towards replacing expansion valve. Compressor activates. DCCV replaced. Your thoughts? Thank you.

  38. dennisb - Auto Tool Sales :

    Date: March 20, 2016 @ 5:37 pm

    If it’s fully charged, I’d double check the high side reading. If it’s accurate it is not pumping. Good Luck.

  39. Daniel Guin :

    Date: April 24, 2016 @ 4:55 pm

    I have a 96 Nissan truck when I first start the truck the a.c. pressure is between 30 and 45 and after a couple of minutes it goes up to over 100 and shuts off and continually does that throughout the day. Before I replace everything which seems like what happens to me all the time what would usually cause this problem besides low freon.

  40. dennisb - Auto Tool Sales :

    Date: April 25, 2016 @ 7:44 am

    I’d check both high and low AC pressure readings and go from there. Otherwise anything would just be a guess. Good Luck.

  41. John K :

    Date: May 1, 2016 @ 6:49 pm

    I have 2006 jeep liberty. There is a button with a snowflake on it then when pressed a light comes on if its working normally, indicating the ac is on. It won’t come on now and so I was checking to see if maybe the freon was low. Is there a saftey mechanism that keeps the compressor from starting if there is low or no freon in the system? I bought a charging bottle/kit with a gauge on it and the gauge stays in the red (when attached to the low side) with the car running (with ac button turned on and air in general cranked to high) or if the car is not running. When this guage is connected to the low side port, it never gets out of the red. As I understand it, the gauge should be down in the green or blue if the system is working normally.

  42. dennisb - Auto Tool Sales :

    Date: May 2, 2016 @ 7:12 am

    I would check the readings on high and low sides with a gauge set not a parts store charge kit. But sounds like the compressor is not engaging. Yes, there’s a low pressure switch to protect the compressor if it’s low on Freon. Usually below 50 psi and the clutch will not engage the compressor. However, this is probably not your problem. You’ll know more when you find out what the pressures are. Good Luck.

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