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Is there a definite diagnostic for an AC compressor?

2000 Silverado 4WD 2500 251,000 miles

Over the past 2 summers I’ve had 3 reasonably good mechanics try and fix the AC (starts out cold/cool then goes ambient temperature after no more than 10 minutes or so, the hotter the ambient temperature the less time it blows cool even if it does, this problem has been getting worse). Going from memory of things I don’t understand–a hose connection(s), switch, coolant leak check and more. The last suggestion was a new compressor. And here’s my problem. According to the mechanic there is no specific compressor test. So replacing this expensive part is largely an expensive, semi-educated guess.

2 questions:

  1. Would a shop that specifically works on AC’s have a diagnostic that could sniff out whether the compressor was the problem?
  2. If a mechanic does the compressor work is it ‘industry right’ that I’m on the hook for the bill even if the work didn’t solve the problem?


They can narrow the problem down to the compressor in general, yes. As to your #2, it depends entirely on the shop, and most of them won’t give you a refund for the work even if their diagnosis is bad.

The best way to determine if there’s a compressor problem is first listen to the compressor while recharging the system.

If there’s no noise from the compressor, then watch the high side pressure on the gauge set.

The gauge should be steady while recharging the system. If the gauge for the high side pressure starts to shake/vibrate, there’s a problem with the compressor.


A functional compressor is a necessary, but not sufficient condition for the AC system to work. When a compressor goes out the resulting debris it spews can damage other components in the AC system. AC pro’s can determine if the compressor is working or not with pretty good certainty, but not as easy to determine if there are other problems lurking. You shop’s experience for AC system diagnosis I expect is that the most frugal approach – the one that generates the fewest customer complaints – is start with the inexpensive stuff and work their way up. At this point they seem to be at the compressor.

I had a similar problem with a 1990 Ford Aerostar. The problem was the fan clutch (the Aerostar did not have an electric radiator fan) was slipping and not pulling enough air through the condenser. The mechanic diagnosed the problem by leaving the gauges connected while the air conditioner was on. When the air conditioning quit cooling, the pressure went way up. He then directed the air from a big shop fan into the grill. The pressure dropped and the system began cooling.


It seems that, in my instance, the most sensible next step is to get the name of a well respected AC shop in Denver from my local auto parts dealer and/or mechanic and have the AC shop confirm the diagnosis?


Btw, I just checked out the Cartalk mechanic directory search option. Unfortunately, the advanced search doesn’t allow you to key on a specialty, as in AC.

Does this occur while at cruising speed?

Is there a steady drip of condensation from the AC drain in the time that the AC is cooling?

I suggest you find a mechanic with a great deal more insight into the workings of an AC system.

The only compressor issues I’ve had is when they lock up, blow apart, or the clutch goes. No question the compressor was shot and the system had to be flushed. No reason that a compressor can’t just wear out though and not compress anymore which the method Tester suggested would confirm that.

Yes, that makes a lot of sense. They have the test equipment to verify all the temperatures and pressures are correct at all the right places in the system and that the test point pressures vary with temperature and AC control settings as they should.

Seems to me monitoring the high and low side pressures will tell if the compressor stops pumping properly.
Has the condenser been checked for blockage by leaves, bugs etc.?


Yesterday I drove from Evergreen to Costco in Denver. 70% interstate. As an aside and quite unbelievably, the AC blew not cool but cold all the way to Costco–~25 miles. The truck had not had that kind of AC cold for at least a year. I arrived at Costco gas, left the engine running and watched for the water drip for about 2 minutes–no drip. The humidity was 30% so I’m not so sure how much condensate, if any, an AC would be pulling out of cabin air? When I left Costco for Evergreen the AC blew ambient temperature all the way home–no cool/cold no drip.

There does not seem to be any relationship between speed and AC cool. There is definitely a relationship between ambient temperature and cool. If ambient temperature is above 70-75 you can forget whatever measly AC (except for yesterday morning) the unit will produce until it goes ambient.

Since the compressor would obviously seem to have been working full bore on the way to Costco does that mean the compressor is likely fine and that there is ‘something’ else causing this problem?

I’ve given up on general mechanics and my AC. I’m now in the market for an AC shop in Denver that’s got a great reputation.


Good decision.


I know zip about auto AC’s. Can ‘I’ see the condenser without too much trouble and the area that might get blocked with leaves?


The condenser looks like a mini-radiator in front of the main radiator for the engine.
The fact that it works OK sometimes rules out the compressor for me.
Compressor failures I’ve seen are catastrophic one time events.

The AC blew cool/cold house to library first trip today ~4 miles. At the library with engine running I walked around to the right side and no dripping. Same as yesterday. Current humidity 13%. Then drove to the gym ~5 miles. AC now blowing cool vs cold. Arrived at gym, no dripping.

Since the condenser is front of the radiator couldn’t I just shine a flashlight through the truck grille vs taking stuff apart to determine if I have significant bug/leaf coverage or fin physical damage?

But the ? remains-- HOW to locate the really good shop?

Angie’s list, maybe Google reviews, etc. The only solid ways to know a good shop are good first or reliable second hand experiences.

I want to take a shot in the dark and guess that your Silverado has a climate control computer that lets you set a temperature (probably not a digital readout), and uses one or more cabin sensors to adjust the temperature accordingly. With three mechanics looking the system and coming up empty, I have a feeling the electrical aspect has been ignored and very well may be the cause of the problem.

If your system only allows you select hot or cold, ignore my guess.

Best of luck!

Yeah that may be the issue. If the compressor works and the freon level is up, it may be time for the electronics to be looked at. I had intermittent AC in one of my Rivs and it was the HVAC programmer. I think I paid $25 at the junk yard. Everything seems to be $25 at the junk yard.

I just talked to my daughter in Denver. She used a place called Autolab. She thought she paid a lot for her AC problem, but felt comfortable the way they done the repair and took the time to explain to here what is wrong and what need to be done to get it fixed.

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