1999 silverado won't cycle properly

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Joined: Sun Jun 03, 2018 6:50 pm

1999 silverado won't cycle properly

Postby kaane » Sun Jun 03, 2018 6:57 pm

I have a 1999 silverado.

I am having a weird AC issue.

It has all new parts. New compressor, new Condenser, new accumulator, new low pressure and high pressure switch, new orifice tube, fresh recharge, no leaks.

The PCM will not cycle the compressor properly, it works on and off. When I connect tech 2 scanner, I see AC request signal to the PCM, I can see the compressor cycling switch closed yet, it says AC relay command - OFF

I have tried bypassing the low pressure switch, makes no difference plus the scanner correctly shows its operating properly.

I have tried bypassing the high pressure switch also makes no difference, when I unplug it it correctly shows AC request off, plug it back in I get AC request on.

It does cycle once in a while, and the AC is ice cold when it does, it just doesn't keep running for long. Since this is a 1999, that has a fixed compressor, is there some other crazy logic that makes the PCM not engage the compressor? I was under the impression that this is a simple analog system with low pressure switch that cycles the compressor and the high pressure switch to shut it off if pressure gets too high. Is there anything else I am missing?

I checked pressures and when the compressor is off, I have 100-110 psi on both high and low. When it does cycle low side drops to 40psi, but it runs only for a few seconds and shuts off.
Posts: 26
Joined: Fri May 27, 2016 12:46 pm

Re: 1999 silverado won't cycle properly

Postby 71403 » Mon Jun 04, 2018 7:48 am

The PCM can command the compressor clutch relay off for a bunch of reasons independent of the a/c request status. Overtemp, unable idle, low voltage, open/short in relay control ckt, etc.

Are there any current or history codes to lend a clue?

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