99 Peterbilt 379 A/C Electrical Issue

I don't know where you are but Rush Truck Centers in Mobile, AL has some electrical wizards that are just mind-blowingly good. If you have a Cat engine in it, one of the guys there has been working on them since the '80s and can do everything Cat and mechanical in general. It also helps that it's a Peterbilt dealer so there's access to everything Peterbilt.

 

There is no power signal from the switch to the relays. Pete used a ground signal. You can follow it across the top of the firewall.

My memory may be off, but

The switch send a ground to the thermal switch, then low pressure switch, then from low pressure it goes to the high pressure switch, and then to the A/C clutch relay, where it grounds the relay.

The relay then sends the signal to the A/C clutch with a different wire.

 

I have a 2000 379, I had an electrical issue with my headlights. They had it fixed in a day.