M931a1. No power to cab and no crank

[jmmay82]

Hey guys my brother has an m931a1. It is an am general 6x6. Semi Truck he uses to move equipment on site and it is dead with no.power to cab or starter any wiring diagrams or helpfull hints would be appreciated I have never worked on this truck but several cucv in the past thanks for your help in advance

 

[73m819]

The PCB

 

[Scar59]

The PCB

Yep, give the black box on the firewall (in front of driver) a percussion adjustment, not to hard. It may wake it up. Then again, it may not.

 

[jmmay82]

What does the pcb do can you bypass it?

 

[jmmay82]

Does pcb stand for printed circuit board. ?????

 

[Scar59]

PCB = Protection Control Box. It contains two relays that control the start functions and protect the starter from engaging once the engine operating.

 

[jmmay82]

Does the pcb control all the power inside the cab as well are the relays in it replaced

 

[Scar59]

Yes, the box controls all the power. The relays can be replaced w/ a commercial 24v relay. Several members have been successful and there a threads describing the job. There are threads describing how the jumper/bypass the box w/ a couple of jumper wires. This is a temp fix to get the truck moved. Best to source a PCB and replace the defective unit. Then rebuild it when time permits.

 

[jmmay82]

How do I find the threads

 

[Scar59]

Use the google search, upper right of the screen. look for "PCB relay" or other key words. Maybe a brother SS will jump in with some searches.

 

[Csm Davis]

Also check the main battery switch to make sure it hasn't gone bad.

sent from my decrepit fingers

 

[cwc]

Search "ever see inside" and you will see the jumpering instructions. Connect the two large connections together at the plug ( 4 gage wire works), and the gages should then be on. Turn on the RUN switch, then jump plug terminal B to the big jumper, either side, to actuate the starter. Just be sure the truck is in neutral, brake set.