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Correct A/C temperature

We had a relatively warm day so I tested my car’s a/c. The ambient temp is approx 72F.

The compressor is cycling on and off just fine but the air is only cooling down to 45-50 degrees.

Isn’t it supposed to be closer to 39-41? Could the pressure in the system be low ?

I know how to DIY-Recharge but I don’t want to unless the thing just isn’t cooling at all. Advice ? Would it be prudent to have my mechanic do a pressure-test on the system and then advise ? Thanks.


45-50 degrees sounds about right to me, especially if you tested it during the day. That 39-41 degrees you expect might be realistic when the sun isn’t in the sky.

Recharging won’t help unless you have a leak, and if you have a leak, you really should get it fixed.

Yes, I think that’s correct for R-134.

should be a MIN of 25F drop in output temp from ambient, and at only 72F should be even more. Have it leak tested and the accumulator possibly replaced. Charge the thing with the proper oil/refrigerant mix and amount. I would expect that a good dry system with the proper mix should do better than this.

You need to look at a chart that takes into account the realitive humidity. 80 degrees ambient and 90% humidity gets you 55 to 60F outlet. 70 ambient and 20% humidity gets you 35 to 40F.

How are you determining your compressor cycling “is just fine”?

What I want to know is, under what conditions was duct temperatures measured at?


The temps you have are too high but the temps can vary based on if the car is stationary or moving, in shade or sunlight, etc.

A 25 degree drop is not very good at all but it’s impossible to say what the problem is without knowing the system pressures. Odds are it’s low on the refrigerant charge.

At elevated RPMs (and with a fan moving air over the condenser or the car moving on the roadway) you should have roughly a 40 degree drop, or more, as compared to the ambient air temps.

The compressor is cycling on and off at regular intervals without making any abnormal noise. Where Can I find this chart? Thanks

Being a retired GM AC Tech (just retested my ASE) I always found mine in the FSM. When people would complain about AC performance I would copy the page from the correct FSM (I think the chart was the same for all but I wanted the header to say their car) I would put a mark at the humidity point and the outlet temp showing it was in the nominal range,if it was.

None of my text books have this chart and I have 2 FSM’s at home neither have the chart

Your system is not operating well. You need pressure gauges, a clock and (temp) charts for your car to say for sure. If in recirculate the humidity will not be a factor and that is the way you should check it. If the engine-cooling fans are working right you really should approach 40 F.

Humidity will affect operating pressures regardless of recirculation mode being selected and operating pressures affect AC performance.

Plenty on the web about AC performance and humidity and the factory charts factor it in.

My Manual does say “Adjust the air-conditioning controls to maximum cooling and high blower position” I guess you could say that means select recirculation mode.

That makes no sense if the windows are closed, the system is reasonably not leaky and at equilibrium. It is a pretty well sealed system. It does not jive with my practical experience either.

Think about condensor efficiency,it is less when air is humid. You don’t agree pressures are higher when humidity is higher

Time to reach equilibrium is also a measure of performance. The less water in the air from the start the quicker you reach it.

Just to clear everyone up,

At the time I tested the A/C today the conditions were as follows:

Car stationary, Recirculate, May have been in the shade, can’t remember. Low to mid 70’s.

I would tell you the temp/dew point spread if I could, but I didn’t look. I didn’t consider it a “humid” day, really.

Something else I noticed is that when the compressor is running, the air will feel alot cooler than when the compressor is idling. Then the process repeats.


When the compressor is running the low side is moving lower (lower pressure=lower temp)until it reaches its cut-off. Then the low side starts going up until it reaches its highest point until the compressor starts again,then it starts going down again.

When you see low side pressures in the 36-41 range this is pretty close to the temp of the evaporator. Imagine blowing air across a 36 degree piece of metal,the air gives up its heat.

Some compressors dont cycle they adjust their capacity by changing their displacement. Everything to keep from freezing.

O.P. You probably need to have a manual to diagnose this thing. That high of a temp, AT THE DUCT, especially at 72 degrees outside, with the windows closed and in recirculate, indicates a problem. I am not an AC expert, but I have a set of gauges, some thermometers and a clock. My the AC diagnosis tree in my manual calls for use of all of them to measure temps, pressures, and compressor cycling. I would not put new gas into anything without going through the process. A pressure switch could be bad, you could be low on refrigerant, you could have a clog somewhere,?. N.B., you might have a stuck heater blend door. I don?t know what you mean when you write, ?I know how to DIY-Recharge?. Do you have gauges?

Oldschool, you probably misunderstand me. When I say ?approach? I mean that as you follow the temp after tuning the AC on, the temp will drop quickly at first and then more slowly. Eventually it will be decrease very little or not at all as well as it can be observed with your measuring equipment. That temp varies very little at the duct outlet when the car is closed. With a typical sedan, you might have a couple of degrees difference on a hot, sunny day compared to a cool, cloudy one.

I agree that moisture in the air has a major impact on AC systems. In my part of the world, latent heat removal is the more expensive part of running AC. It is fun to come home and turn on the AC and watch the hygrometer and thermometer. First the humidity drops fast and the temp does not change much. As the RH approaches 40%, it pretty much stops changing and the temp starts to drop fast. Auto AC systems operate with the same physics. If you close the windows and run on recirculate, the ambient RH becomes irrelevant very quickly. If one only has a thermometer, it is a pretty good way to monitor the AC system performance. I have recommended that people measure and write down that ultimate temp when they buy the car. It is an easy way to tell if they are not operating up to snuff at some later date. I would use a thermometer calibrated with ice water.

What do you all mean by the “duct?” When I took my a/c’s temperature, I was sitting in my car with the thermometer stuck in one of the center vents. Are we on the same page ?

When I say “DIY-Recharge” I mean I have a can of refrigerant at the ready, with the LOW pressure gauge and connecting hose + valve.

Anyhoo, I’m taking the car to my independent mechanic to test the system for me. Thanks for everyone’s help.

When measuring the vent output temps, the engine should be idling @ 2,000 RPM’s with a large fan blowing at the condensor in the front of the vehicle. This simulates the vehicle being driven down the road. Measuring the vent temps under any other conditions doesn’t reveal the true operating condition of the system.


Jeffmw05 , we are on the same page, my duct = your vent in my mind since the end your thermometer is in the duct.

Tester, my tests are done driving down the road so no need to simulate for me, at least. Cripes, I never took the thermometer out of the vent since the last time I had a problem. That is where it lives now.

I did locate the chart I mentioned. By the FSM System performance test. Your ambient 70F should give you 36F at 20% realative humidity. 90F ambient gives you 49F at 20% humidity,taken at right center duct.Of course windows up
They want to see these temps "in about 3 min"
GM makes a !!!Record Realative Humidity

In the shade after interior ventilated

GM likes 1500rpm doesn’t mention fan blowing at condensor(maybe they think this gives a non real world test,think idling at stop light)MAX A/C

Your readings would be OK for a 85 ambient day at 30% humidity

My conclusion,your system could be working better