AC trouble - How do you check this circuit?

There are a number of reasons why your compressor will not turn on. You can have refrigerant in there and if it's not enough or too much the compressor won't run. It has to be exactly the right amount.

With the engine off you can manually put 12 volts directly to the compressor and see if your clutches lock up. If you have bad high or low pressure switches your compressor won't come on.

Also that year truck has a control module that is below where the circuit breakers are. At least mine is a 95 and I had that module on there. Some of the later trucks had them on the firewall I think but you'd have to look into your system to see what you have but that module controls the fan circuit/ AC and it's problematic they often go bad. There also is a temperature probe in the cab that can go bad and cause problems.

I think to get to the whole circuit breaker you have to take that centerpiece off that has all the markings for which breaker is which.

I would look into the wiring diagram but going from memory, and it's been a few years, I think the electricity first goes to the high pressure switch so if you turn your AC on and you test the wiring there you can see if you're getting electricity from the cab.

I would double check that with your wiring diagram.

 

The plastic just a cover for the breakers. No need to take it off to test with a test light. Like Dino said, I’d try jumping the pressure switch. Unplug it from dryer, jump the plug. See if it turns on. If so, low side of switch is bad. Replace switch. The other switch on drier with pigtail is fan switch. Pressure switch doesn’t have pigtail. You can change switch without losing any Freon. I think there’s a constant power wire running from starter maybe? Cant remember. Follow the 2 wires from compressor, inspect for chafed/broken wires. I’d leave the fuse box alone. Unless necessary. I had one 3 prong breaker on my last Truck that the plug went bad. I had to twist one of the prongs and plug it in. It worked, Lol. Considering I’ve heard a lot of problems with Freightliner wiring, and haven had hardly any problems. I’d leave it alone. It’s a can of worms.

 

Freightliner wiring is pretty easu. Check power at thermistor first, then at low pressure switch. You can jump both to verify quickly

 

Yes.

 

Grab a test light and test for power at the compressor. High pressure switch and low pressure switch.

You say you got power at the compressor. So there's either a ground problem or the compressor itself isn't completing the ground.

 

I’d replace both on the drier. The drier has schrader fittings, you won’t lose any Freon. Just a slight poof when they pop off. If the problem still exists, keep the old ones for spares, or future diagnosis. Hold off on the solenoid for now. One thing at a time, so not to get lost in the process. Please update progress. Hope it’s not 100 degrees where your at.