Diagnosing AC problem

greg334 · Jul 24, 2010

I don't know about that. It seems they are hunting in the dark. If they know anything about GM products, then they don't need a schematic or wiring diagram. There isn't much to these systems and not much has changed. Diagrams are gotten only two ways, through Helm who actually publishes the books and information for GM or through AllData which any shop can get the needed info online.

By passing the clutch relay brings up an issue of safety, is the HP cut off switch also bypassed?

chefdennis · Jul 24, 2010

Hell forget all of this, just meet up with Greg somewhere....

greg334 · Jul 24, 2010

Yea I would help if I didn't have a full work schedule but crap I am dealing with one small fleet, working on my own truck, and on top of all that cleaning out the mother-in-law's house while trying to get half of it into my own house.

chefdennis · Jul 24, 2010

Mother in law "stuff" in your home is one thing....Mother In Law herself in your home.....if that was the situstion at my house , id be building her a seperate apartment or staying out on the road longer.....

skyraider · Jul 24, 2010

AMonger said:
Ok, got it into a shop, and fortunately, it's a shop that has testimonials on the walls from customer who were getting rooked elsewhere and then came here and found the treatment more honest.

It's got plenty of freon. But the powertrain control module isn't grounding the AC compressor clutch. That happens normally under high or low pressure conditions, but this is happening constantly. So, they say, the PCM has to be replaced, at a cost of around 600-800 semolians.

How old is ur unit or vehicle????

AMonger · Jul 25, 2010

skyraider said:
How old is ur unit or vehicle????

2007. New to us.

AMonger · Jul 25, 2010

greg334 said:
I don't know about that. It seems they are hunting in the dark. If they know anything about GM products, then they don't need a schematic or wiring diagram. There isn't much to these systems and not much has changed. Diagrams are gotten only two ways, through Helm who actually publishes the books and information for GM or through AllData which any shop can get the needed info online.

By passing the clutch relay brings up an issue of safety, is the HP cut off switch also bypassed?

He said no. But just today, it froze up. Couldn't get any airflow through the vents. Had to shut it down and blow hot air through it to thaw it. Isn't that what happens when the HP switch is gone?

aukinet · Jul 25, 2010

If the evaporator is freezing up then the cycling switch (low pressure switch) is stuck closed, allowing the compressor to run all the time.This is assuming that the refrigerant charge is correct. Under certain conditions the compressor needs to cycle off so the evap doesn't get too cold and cause ice buildup, and thats all dependant on the low side pressures. The low pressure switch is probably on the accumulator. To be sure about everything I would recover and measure the refrigerant in the system, put a deep vacuum on it for at least an hour and recharge with the appropriate amount. Then run her and watch the pressures on the guages. Run it outside in humid conditions with the windows open and at 1500-2000 rpm.If the low side gets down below 20-25PSI and the compressor is still running, then the low pressure switch is very suspect. Or just replace the switch(they are cheap). You don't even have to discharge the system to replace it. You still need to know exactly how much refrigerant is in there and what the pressures look like. Too much guessing going on. Need to start with step A instead of C or D.

AMonger · Jul 26, 2010

Ok, I called the guy back today to see what update he was able to get. Turns out he was calling the Delco assistance line or something like that. They said it was probably the high pressure switch inside the compressor. Can't be replaced separately. They also told the shop that after the compressor is changed, the computer might need to be changed.

Wondering about this, I called another shop to see about a second opinion. They don't give free diagnostics but when I told him what the other shop is saying, he says it sounds like they're pursuing a reasonable path, doesn't sound like he's ripping me off or anything. This second shop has a good reputation in the area. That's going to annoy me if both need to be changed. But I guess it wouldn't surprise me. The AC has run a lot of hours this summer.