Problems with PCB box on m939

[Cobra2401]

Hi. I'm having a problem with the pcb box. It doesn't start. When you turn on the battery switch the selinode for the battery activates. But nothing when the starter switch is flipped. I have all power to lights and marker lights as well to the dash lights. I checked out the wires for the pcb box located behind the washer fluid. I made a list of the wires that have constant power, power when you turn the battery switch on and power when you turn the start switch on.


Not sure if the wires that are say "off" don't have power to them until the truck is running? Or if some or all should have power when you turn the battery and or ignition switch on? Any help would be great. Thanks

 

[Cobra2401]

I just checked the start switch. I don't have any power to this either

 

[patracy]

499 should have 24V when starting. That is from the transmission safety switch. Check your shifter and the wiring to the shifter. They'll be three wires. One is power inbound (49 The other is outside the switch to the PCM 499. 566 is not used. 74 is outbound to the starer solenoid. 5 should be hot as well. That's output from the PCB. That goes to supply power to the breakers. That's energized with the PCB being powered up and the solenoid closed. Power comes in from 459 and 81 for that. I can only say check the ground to make sure the solenoid is working. Otherwise the PCB is bad (or that solenoid is actually). I've got a spare one listed in the classifieds if you end up needing one.

 

[Cobra2401]

When would 499 have power? The 3 wires that plug into the back of the shifter have no power currently. I just checked them

 

[Cobra2401]

The three wires going to the shifter have different number tags on them. 2 @585 and 1@40

 

[patracy]

Sorry meant to say 499 should have power when starting. 498 will only get power when starting from the starter switch. 499 is the output from the neutral safety switch. The other lead is voltage from the lighting for the gear indicator.

 

[patracy]

View attachment 498731

Also consider downloading the TM's and the P2P program from the TM section of the site, it's wonderful for troubleshooting.

 

[cwc]

I posted some troubleshooting aids for the M939 PCB a week or so ago:

http://www.steelsoldiers.com/showthread.php?113634-Ever-see-inside-the-Protective-Control-Box/page3

Here is a picture of the neutral safety switch on the driver's side of the transmission. I had not noticed the masking tape labels on the M925; looks like somebody had some issue in the past. The connectors are a little hard to work with. For troubleshooting you can use the old trick of sticking a straight pin through and using a meter to check for voltage. Careful if you jumper it...I had a mishap once like that, something about a 67 Plymouth and a garage door! It's easy to do it and forget it, and never get around to fixing it right.