Peterbilt 359 A/C not working, beyond stumped

A 1985 would not have an ECM. Sounds like the first thing to check would be to see if you have any refrigerant charge in the system. The compressor only works if the system pressure is within a certain range. With no or low refrigerant, the low pressure switch will prevent the compressor from engaging because there isn't enough static pressure in the system (properly charged system typically has about 70psi static pressure at room temperature. Its not a 100% accurate way of knowing if its properly charged but it tells you there is at least something in there).

 

I just re-read your post a few times. 86 should be control power, 85 control ground, 30 should be fused power and 87 should be your normally open. Basically once you switch on your circuit, 12 volt control signal should be available at 86, which grounds through 85. That circuit actuates the relay, which should send 12 volt power from 30 to 87. In most cases, pin 30 should have constant 12 volts with key on. Pin 86 should only have 12 volt power with the accessory switched on. Pin 85 should be grounded, and pin 87 should be wired directly to your accessory. I would look into why you're seeing 12 volts on the ground side of your relay control circuit as well. Without a schematic available its kind of hard to try and diagnose electrical over the internet lol.

 

I'd check to see if your getting power from your hvac on/off toggle to the A/C thermostat switch, then chase wire from there to your trinary switch on the dryer.
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