2001 Peterbilt 379 ac help please

Oh I see, you have the later 01-05 panel. That would be your AC relay that is clicking, so control side is all good then (Pressure switches and control switch.)

Somewhere between the relay and the compressor it's not getting power.
I've even had more than one compressor with a bad clutch out of the box In the last few years.
Check with a test light right at the compressor and see if it is getting power.

 

Also check that one terminal where the relay plugs in is a key switch power. With the A/C turned off.

 

When you hit your A/C button it trips the relay, the relay then sends the key switch power out through the pressure switches and then to the compressor clutch.

 

Jump across the relay and see if the clutch kicks in. You can do it with the truck off, key on. It's easy to hear.

I would agree, if the relay clicks when you turn the dash switch on, all switches and wiring to them checks out.

On the earlier 379's, power from the relay went straight to the clutch, best I remember anyway.

 

Well crap, I ain't looked at a relay in a minute but IIRC 87 to 87a