Adding Freon to Car AC – Gauge Readings Explained
April 17, 2009 7:29 am Air Conditioning, DIY, How To Auto Repair
New cars don’t stay new for long – sooner or later the AC will need Freon added
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. There can be many other things wrong besides a system being low on refrigerant. To diagnose 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.
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 specified amount on a decal under the hood.
Both sides are lower than normal
Both low and high side readings are lower than normal, this indicates a cars AC system is low on refrigerant and is under-charged.
Both readings are higher than normal
If both low and high side readings are too high, this indicates an overcharged system – too much refrigerant. This also can indicate that the condenser fan is not working, is too slow or the car is overheating and heat is transferring from the radiator to the condenser.
Low side goes into a vacuum and high side is too high
When the low side goes so low that it’s reading shows it is in a vacuum, the most likely cause is a bad expansion valve or blocked orifice tube. Another possibility is a restricted condenser. Blocked condensers are not as common as they used to be but if a compressor fails and comes apart inside the remnants can end up in the condenser causing it to restrict the flow of refrigerant.
Low side is too high and high side is too low
When the compressor clutch is definitely engaged and the low side is high and the high side is low, the most likely cause is that the compressor is failing – it is not pumping sufficiently. Rarely an AC clutch could be slipping but usually this will be accompanied with a squeal or chirp. Also be sure not to overlook the obvious, like a loose belt!
Question: I have to add a can of freon every two weeks, where could it be going?
Answer: Obviously, the system has a leak. Some cars come with UV dye in their system already. Usually you can see some of the dye in the service port (a little might show after disconnecting the gauge set hoses). Look for dye or clear refrigerant oil on the AC components that would indicate a leak. In addition to a visual inspection, an electronic leak detector can be used to pinpoint the source of a leak. See our article with more information about Finding A/C leaks.
Question: I see the small line going into the evaporator on my Ford Explorer, it’s really sweating and very cold at one point. Is there a restriction in the line somehow?
Answer: Most Ford Explorers have an orifice tube (expansion tube) in the inlet of the evaporator. It’s normal for it to be cold – if it is frozen and the vent air is not cold then it may be plugged up. If upon removal there is metal debris caked onto the orifice tube, the compressor will need to be replaced.
Question: Why is there such a big temperature change in the AC lines? Some are very hot and some are cold is this normal?
Answer: The line going from the compressor to the condenser is the discharge line – it is normal for it to be very hot to the touch. The other line going from the drier or accumulator to the compressor is normally colder. The liquid line can be hot up to the point an orifice tube is in place. Just remember that LOW Pressure = COLD and HIGH Pressure = HOT.
Adding 134A to Car’s A/C - Topping Off a System - articleurge.com :
Date: April 23, 2009 @ 8:28 pm
[...] amount of freon to the car’s A/C system without causing harm to the system. Learn about car A/C gauge readings to know what normal readings are and to help diagnose AC problems. With the correct tools and a [...]
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!!
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.
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?
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.
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?
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.
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?
dennisb :
Date: July 6, 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.
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?
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.
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 [...]
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?
Thanks!
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.
dane :
Date: July 25, 2009 @ 2:34 pm
dennis,
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….
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.
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! =(
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.
Alex :
Date: July 30, 2009 @ 1:48 pm
Hi
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!
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.
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!
Alex
VW golf 2000
dennisb - Auto Tool Sales :
Date: July 30, 2009 @ 7:33 pm
Alex,
The low side does seem a little too low. The expansion valve may not be working properly.
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?
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.
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?
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.
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?
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.
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.
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 http://www.a1limobus.com
thanks again for the info i will pass it along to our other office
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
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
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 our 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.
dennisb - Auto Tool Sales :
Date: August 19, 2009 @ 7:50 pm
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