2007 Peterbilt 387 primary air buzzer stays on

I have a 2003. Same truck, had the same problem. There is some sort of a computer that controls the air pressure gauges located in the worst spot. Thank Peterbilt for this. It's inside the frame rail beside the transmission on the driver's side. I got one out the junk yard in Richmond and the parts guy didn't even know about it.

 

I support this statement. If you don't have air once you find low pressure switch, you found your problem

 

That's it. The crazy thing does something that I haven't seen on a truck before. When the air pressure is zero in both tanks it fills the primary first to 100 and then the secondary. Normally you would see both raise at the same time.

You may want to run the truck for about 10 minutes to see if it switchs from filling one tank to the other. Once it gets to 100 it will pause and may take a minute before it starts filling the other side.

 

I have that same problem. I think it's the engine computer. J1939. I just replaced the gauges with an air line going direct to the tanks.