'98 K1500 - AC compressor stays on...

[gocougs]

I have a problem that my compressor will not shut off. It engages as soon as the engine starts, and does not cycle off at all. As a result, the low side line builds up quite a bit of ice on the outside, and airflow to the cabin drops off over time, as if the outside of the evaporator is freezing and blocking the air passages. After shutting the truck down for 20 minutes or so, airflow resumes to high levels, leading me to believe the evaporator blockage has melted away. I checked the connector at the commpressor to see if it was a clutch problem or if it was being commanded on all the time. Turns our the connector has 12 volts all the time, whether the ignition key is on or off.

I have not checked the pressure in the system yet. Last night, I replaced the pressure cycling switch, afterwhich the system functioned correctly; cycling and turning off and on when the cabin A/C switch is activated. But, driving to work today, the compressor returned to contunious running and things started freezing again.

Most of the compressor related threads deal with a compressor that will not engage, but mine won't turn off.

Does this sound like a freon system problem or an electrical problem?

[2000CAYukon]

What happens if you unplug the pressure cycling switch? The compressor should turn off. If it does not, then you either have a bad clutch or electrical problem (bad relay ???).

Check the voltage at the compressor after unplugging the pressure cycling switch.

You should never have 12 volts at the compressor if the ignition key is off (again, I would check the a/c relay if this is the case).

//2000CAYukon

[bamadam]

When I first got mine, someone had unplugged the switch and stuck in a paper clip to make it run all the time. Just a thought.

[777stickman]

As the "CAYukon" said there should be no voltage at the compressor clutch connector with the key off. If you're seeing voltage on the Dark Green wire with the key off, you're seeing voltage from the 10a A/C fuse in the underhood fuse block. This fuse is "hot at all times" and is the power feed to the clutch but is controlled by the "A/C Compressor Clutch Relay".

So as "CAYukon" suggested, replace the relay as it may be fried in the closed position. You can check this by removing the relay and confirm the 12v signal is gone from the Dark Green wire at the clutch.

If this doesn't help let us know and we'll try to figure out something else with the relay control circuit.

[gocougs]

A bad AC compressor clutch relay was the problem. I opened the relay and found that the contacts were pitted just like an old set of ignition points.

Thanks for the help.