low air buzzer problems

After the air pressure is low enough for the buzzer to come on, have you put a test light on the buzzer wiring to see if you have power there?

 

Get you a copy of wiring diagram and run the circuit to find your problem. With light coming on I would think you may have a bad ground for buzzer.

 

If the light comes on, obviously your low pressure relay was working. Both the light and the buzzer work from the same relay switch. Its the buzzer. Its under the front gauge cluster, they're about $80 new or $10 at a bone yard.

BTW, ask the guy who sold you the sending unit how the light came on with a bad sending unit.

**edit**

Oh...

 

Dealer should have this big book with diagrams in it. When I had a shop they'd let me look and make me copies of need pages.
I think peterbilt may have a toll free help line that could supply it too.
You may have a partial diagram on the inside of the fuse panel cover.
The wires should be numbered or colored and that may help you trace the problem.
Every circuit has to have a common and ground, so you know the light circuit is complete.
So I'd start backwards at the buzzer and work my way back troubleshooting it and make sure buzzer is good to start with.(new means nothing)