ac compressor relay control question

[hgarraway]

2000 2500 5.7, compressor relay stops activating when ac has run for a while. I have tested the relay by swapping, it's good. The pin that supplies 12v to the relay coil (#86 on the relay) always has 12v when ignition is on, the 12v to the contact on relay always has 12v (pin 87). The circuit to compressor clutch is good, It always engages if I jumper pin 87 to it (pin 30). The problem is in the circuit back to the switch from the relay coil (pin 85). This is the "ground" path for the coil. When the truck has been off for a while and the ac is turned on (with relay out) the resistance through pin/socket 85 back to ground is beteen 9 and 25 ohms. If I plug in the relay at this time the comp clutch engages as it should. After it runs a while it will disengage and when I remove it and check the resistance it is at infinity as it would be if switch were turned off or the thermostat sensed freeze and opened. It usually stays in this state until I turn off truck and let it rest. If I had a schematic of this circuit I could probably find it.

 

Have you seen this before or could you make an educated guess as to whether it could be a faulty switch or faulty thermstat? Also, do you know if this system is under computer control or just a good ole switch and mechanical thermostat? Any help would be greatly appreciated. (In an earlier post, to which you responded, I thought the problem was a bad connection in the relay socket but symptoms have gotten more consistent.)

 

Thanks in advance.

 

Hugh

[GM-Tech]

The PCM (engine control computer) controls the A/C compressor clutch relay through pin 85 on the relay. There can be any number of reasons why the PCM would shut off the compressor, but the most common problem we see is a bad low pressure cycling switch. There is no thermostat for the a/c, it determines if the evaporator is potentially too cold by pressure via the low pressure switch.

 

This is the switch that screws into the accumulator. It's a simple set of on/offcontacts that open below a certain pressure (25 psi or so) or close above that pressure. Closed the compressor is on, open it's off... assuming all other parameters the PCM wants to see to allow compressor operation are okay.

[hgarraway]

Thanks GM-Tech. That seems to have been the problem. I replaced the low pressure cutoff switch (after testing old pressure switch upon cutoff) and all is cool now.

 

You saved me a lot of work and money as I had incorrectly thought that the pressure switch close to the compressor was the cutoff and had incorrectly ruled out the switch as a possible problem.

 

I really appreciate your assistance.

 

Hugh