No AC...

98GreenZ · June 10, 2006

Greetings all....first time post for me, and of course, I have a problem.

All of a sudden yesterday, no air conditioning. Compressor clutch won't engage.

Vehicle: 1998 Chev. Z71 p/u

So far, I have determined the following:

1. A/C clutch relay is good (at the VCM, short pin 1 of connector C3 to ground, relay energizes & compressor clutch engages).

2. High-pressure switch is good (pin 25 of same connector shows 12v from A/C request switch).

3. Low-pressure/Cycling switch is good (switch is not shorting A/C cycling signal from VCM to ground).

From the above checks, it tells me that all the proper signals are being sent to the VCM, but it's not pulling pin 1 to ground, which would enable the clutch relay. Does this sound right?

If it does turn out to be a bad VCM, what's the best way to have it replaced? Dealer? A/C shop? DIY? (Although I wouldn't be able to program it if needed.)

Thanks in advance,

Mike

ChevyTech007 · June 10, 2006

With the above info I would say it is the PCM. I can honestly say I have never seen a PCM cause that. Its not to say it is not possible, just very unlikely. 9 times out of 10 it is either out of freon or cyl. switch. You might want to check all your PCM grounds. If it does come down to a PCM, do yourself a favor and get it done by a dealer.

98GreenZ · June 13, 2006

Thanks for the input guys......

007, I checked the PCM grounds, and all check out fine. Soooo....I'll run it in to the dealer's this week sometime and have them go over it. In the meantime, it gives me a good excuse to drive the Corvette to work.

I'll post the outcome....if I come out alive.

pm26 · June 11, 2006

I had several strange A/C problems on vehicles over the years.

99 Mazda 626 V6: The coolant overflow bottle leaked slowly on the bottom and hot coolant got into the A/C compressor clutch harness connector, located directly under the overflow bottle. This caused all kinds of intermittent A/C malfunction until the system quit. Cleanin the connector (and replacing the leaky bottle) was all that was needed to make the system operational again.

Same car: A/C relay burnt out. Fixed by myself

Same car: The A/C condenser cooling fan motor burned out. On a hot day in city traffic the coolant temperature went up and the A/C system shut down. It would start working again when I drove at higher speeds. I found the problem and fixed it by myself.

98 Nissan Maxima: : A rock thrown by a vehicle damaged the condenser and caused a slow leak. The system automatically shut down becuase of loss of refrigerant. A Nissan dealer did troubleshooting and found the problem. I checked everything else before, checked the A/C compressor clutch coil resistance. However, there was no power going to the coil. (no voltage measured at the A/C compressor clutch connector).

83 Mercedes 300 SD: The A/C compressor clutch would not engage. There was enough refrigerant in the system and the compressor was good. I took it to a Mercedes dealer and they spent hours troubleshooting it and told me I needed to replace a $ 1,200 temperature controller box. I did not believe them and did my own trobleshooting. I found that the thrermostat switch was defective. (I bypassed the switch and the compressor turned on).

From the examples above, it is necessary to perform a pressure check of the ysstem as well. New systems will automatically shut down if there is loss of refrigerant, to prevent damage to the A/C compressor. Also, leaks like the overflow coolant reservoir in my Mazda can cuase very strange A/C problems.

One thing these problems had in common was that the A/C compressor clutch would not engage.