AC clutch doesn't engage.

You need power and ground to make the clutch work. I could be wrong but I think ground goes through sensors before actually reaching ground.

Now here's the thing.
You all do this and I have no idea why.

FIRST. Make sure you have at least 50 psi. Otherwise clutch won't engage. The low pressure isn't completing the ground.

If you got too much freon. The high pressure will stop it from working.

Second would be electrical.

I'm betting you're out of freon. Which means a leak. And causing the electrical to not function.

 

One wire clutch or two wire?

 

If your Freon is low that’ll cause it to not engage. Probably want to have it evacuated and weighed..

I went through the same problem and low Freon was the problem. Started out with a condenser leak and one thing led to another

 

Freightliner used power to activate, instead of ground, at least at that time. So 12V at the clutch (assuming no wiring has been done) would mean power has gone from the dash control, through the thermostatic switch, and through the low and high pressure switches, to the clutch.

So either a ground issue, or a damaged 12V supply that can't carry the load

 

So you could use a ground wire connection and see if the clutch engages. You don't need the motor running.

You could also test for power at the ground wire coming out of the clutch. Unless it's one wire and the compressor itself is grounded.

I'd still check the freon though.

 

Is this the first you’ve used the truck this year? It’s almost June. One would have thought the ac was in use months ago.
One test to do, turn on the ignition, but don’t start. Turn on the hvac blower(low setting is fine) now press the button that turns on the ac, if everything is good you should hear the click of the ac clutch. Turn the air control to defrost, you also hear the ac clutch click. The ac compressor activates when using the defrost in modern trucks.
If no clicking sound from the ac clutch, you have an issue, could be electrical, sensor, control switch, low refrigerant, etc.

 

Is there ENOUGH freon?

Anything less then 50 won't trigger it.