Peterbilt 379 Cruise not working

So that leads me to believe that the CC is not working because it is supposed to not be working......something like, oh I don’t know, maybe the brake foot valve is sticking.

Whatever you discover please keep us posted. I love reading this stuff to educate myself

 

If the foot brake valve was sticking his brake lights would be stuck on as well as the brakes dragging.
He can verify it is not by testing the brake switch which he has by checking it with a multimeter as well as reading the position with a laptop.

 

"If" you trust the tech with the laptop, it looks like you could have a ecm issue. That is a pretty big if.
I did replace a cat ecm once for not entering cruise mode. Not saying that is your issue however.

 

Speaking from my experience the brake lights would not come on unless it was a “heavy” stick on the valve. There is basically a dead zone there

 

Is this the air switch for the parking brake? And would that effect the cruise?


IMG-20210106-101234973

 

Can you disconnect the air line from it and see if air comes out when you release the park brake.

 

Fixed.
The switch in the picture above was the parking brake switch. Someone used an adapter to make it fit, but the adapter pushed it out too close to the dash. One of the wires was sitting on top, right between the dash and post. Over time it wore bare and kept the circuit closed.

I tested all the switches, but with the parking brake on, so never thought to see if the circuit would open. It does not show in the ecm on a laptop as a part of cruise control, so the technician did not see it either.

If anyone else reads this like I did trying to fix your cruise, the wiring diagrams in the forum are awesome.

This is the 1000th reason I hate when people jury rig trucks, take the time to get the right #### part.

 

I know this an old post but good info. Same issue with my truck.

 

This parking brake switch that was causing the problem. Is it a normally closed switch?