Hi, I'm having some problems with my A/C, and I was hoping someone out there could help me with some 'baseline' stats for the car.

If it's about 75 degrees outside, what should the low and high pressures be right around when the system is not running... ie, the car is shut off?

How about when the car is running and the A/C is ON full for max cool?

The other day, my system seemed a little low, but the compressor would run. Still, there was no cooling. I added a can of that stuff, R134a or whatever. The system would then cool, but it really didn't seem COLD. I seem to remember the high side up near 250psi (I guess that's the right units).

Two days later I tried it and it did not seem to cool at all again. However, I don't believe there is a leak.

Any help or ideas would be awesome!

I really don't want to shell-out for a compressor unless I am absolutely certain that's what I need.

I can rent the gauge set at Autozone. Could a kind soul outline the exact steps to take to track down the core of my problem, for sure?

THANKS!

 

This depends on the humidity, but at 75F outside, the air temperature at the vents should be between 40-55F.

The low pressure side should be 18-27 psi.

The high side should be 210-270 psi.

That is the a/c set on max cool, highest blower setting, vents with outside air, and doors and hood open. Engine rpm at 1500.

The low end is 30% humidity, the high end of the range is 80% humidity.

I am also just eyeballing the numbers.

In the future, do not blindly add R-134A. You need to read the high and low side pressures, because overcharging can cause worse problems than undercharging.

 

Yeah, I had a set of gauges on it. High side never got past 250 on that relatively humid night. I'm going to put the gauges back on it tomorrow and see what I have. I might be fixing a small leak after all.

 

OK, I hooked the gauges up today.

With the engine shut off, low side is right at 50psi.. HIGH side is also right at 50psi.

With the engine idling at about 1500 rpm, A/C ON (compressor is indeed engaged and turning), outside air, hood up, doors open, etc.. the low side is about 45psi, and the HIGH side right around 45-48psi. It's raining outside, so humidity is way high.. maybe 95%. Temp. outside is 74 deg F.

One of my original questions was, what should the low/high readings be when the engine is shut off?

Looks like I have a leak somewhere, bleah. Is there any of that R-134a stuff with dye that is visible to the naked eye? The stuff at Autozone was 20 bucks a can, and then you have to get some special glasses and a special flashlight.

Thanks!

 

Equal pressures, to me, would indicate a problem with the compressor if the clutch is actually engaging and spinning.


The high/low pressures will be the same when the a/c is turned off. The pressure should be about the temperature outside.

 

OMG thanks for this info!

I'm starting to get a clear picture of how this stuff works!

Wouldn't the pressures be the same on the low and high side if the expansion valve was faulty? What I mean is, couldn't it be either the compressor or the expansion valve?

Just trying to get my facts straight before I go blindly replacing expensive parts like compressors.

Thanks!

 

It could be the expansion valve, rare but yes.

Just me, and by no means am I an AC expert - PA and TX know 100 times what I do. From engine off to engine on and ac on - do the pressures move? If so by how much on each side.

 

Poorman:

Engine off, high and low are nearly the same... right below 50psi.

Engine running, AC on max, etc. etc.. (compressor is indeed turning), high and low are both about 45 or so..

So both sides seem just about equal, and they both drop a few measly PSI when the system is going.

 

engine off pressures are irrelevant. if you have equal pressures with engine running, A/C on and compressor engaged, 9 times out of 10 you have a defective compressor. my experience anyway.

 

If you have a view glass on the receiver/drier, check for bubbles in the stream, clear stream, or no evidence of movement. If no movement, compressor may not be pumping. If movement, then expansion valve seems more likely.

A pressure of 50 psi suggests either low temp at time pressures were taken, or low charge. As PAH said, pressure w/ system off should approximate air temp in degF.

good luck