2000 Peterbuilt 357 powered by a cat C12. We had a truck come into out shop complaining of no a/c. It has Freon and I have power to my pressure switches. We have a good fuse And a good relay. When you turn the ignition on I have constant power on my 30 and 86 post of my relay but it doesn’t cycle with a good relay. If you create a ground the compressor clicks on but it shows to have a good ground already. I went through and cleaned all my engine, body, frame, and battery grounds. When putting the battery grounds back on we noticed the arcing. That’s what started the short to ground search. Isolated it to the cab by unplugging all the firewall plugs and grounds. Hook a volt meter to the cab and to the negative battery with the battery grounds off we are showing 11-12.4v. I unplugged each fuse and relay with no change except a 6v drop on a roof fuse. But it you turn the ignition, headlights, or cab radio on it jumps back to 12v. I have done a wiggle test all over the harness and tried unplugging everything I can see with no luck. I havent found a bad rub either. The starter isn’t the problem either. We had it bypassed and only power running to then cab at the moment.
2000--did you look at the back of fuse/relay distribution box?
Yes. With everything hooked up and the dash panels pulled out. I have anywhere from 1-34ohms between every wire (even the dash brackets) and pin 86 with ignition off. When ignition off the dash acts as a ground for the volt meter. It’s not blowing any fuses either.
Does the ground for the AC clutch come from the thermostat switch in the evaporator like it does from many older cat machines?
It's common for only one + wire to compressor and a local ground.
That would seem to indicate the ground between the engine block and frame is missing or failed, correct?
You would think so, One area that's often overlooked is grounds at starter to frame, starter to block. It's not hard to miss one tucked in frame rail from previous work. I have found a few like that.
We found a busted wire inside the firewall plug. Thank y’all,