AC clutch doesn't engage.

If the relay clicks when powered up. It's fine. If no click. It's dead. Or a electrical issue.

It should have 4 terminals. One is hot which transfers power to or from the compressor. Another terminal should power up with the switch. The other 2 terminals are ground. One that completes the always powered circuit. The other completes the switch circuit.

I've driven one truck that burned out compressors. It had a bad expansion valve so the low pressure was low pressure. Come January when the freon contracted. It was actually in vacuum mode. The low pressure switch wasn't shutting it off. The last winter I drove the truck I unplugged the compressor so I could use defrost without the compressor running.

When you don't fix things. Other things break. Constantly.

 

Should only have 1 Low/Hi pressure switch. The smaller switch on dryer with the pigtail type plug is the fan switch.

 

When the low side of the switch is bad, or if you want to speed up adding Freon to an empty system. You can unplug the switch, jump plug with a paperclip and engage clutch. If the high side of the switch is bad, and clutch won’t disengage. Simply unplugging it turns compressor/clutch off. It’s usually a bad pressure switch, broken wire to compressor, bad ground on compressor wire at compressor, bad evap temp sensor probe, or it’s plug/wires. or my favorite the small single plug came off the blower resistor. I’ve had that happen a few times. Not sure how it all works, but that wire does control power to compressor. Probably goes to pressure switch.Whatever’s causing your problem, it’s more than likely something very simple being overlooked. Maybe the plug itself At compressor is shorting out. Should be a short hot wire from compressor going across the front of valve cover to where it plugs into another wire going the hot on starter. The short piece gets a lot of heat. The plug at compressor gets loose. The other wire at compressor goes to the one and only Hi/Lo pressure switch.

 

I think most trucks these days have a combo hi pressure and fan switch.

As for bypassing the low to keep the compressor running. Not a good idea. Raise the idle instead. To 1100. Or 1400 if the setting allows it.

You also get a idea of actual pressure. But I have gauges.

When I charge cars. I use 2500 rpms.

 

Just curious. Why is it a bad idea to jump low pressure switch to make system take refrigerant quicker? Thanks!

 

The compressor cycles till it gets enough to keep running. About 75% charged. As mentioned above. Running it with not enough pressure burns them up.

The low pressure kicks it on at 50 psi. Shuts it off at 25 psi. By making it run continuously. It goes well below that 25 psi threshold.

Much safer to just rev the motor instead. And leave the system alone to do its job.

Semis take a long time to fill up. They run all day and night so they get more wear then cars. They also hold more then cars. Sleeper trucks hold 4+ pounds. By comparison. My car only holds one pound.