M923A2 Power Control Box Issues

[todds112]

So while working on the truck (M923A2) the other day I was running it off and on for a few hours. Shut her down then tried to restart it and it wouldn't start. Turn main power on, gauges came up. Turned start/run to run and ABS tested OK. Turn switch to start...nothing. No click or anything. Suspected the PCB right away. Rapped it with a plastic mallet and few times and yup, she fired right up.

Started the truck the next day and it started right up without issue. Then I noticed something odd: The heater blower worked without the power being turned on, both high and low speeds. Weird, none of the gauges or anything else is on that I can tell. Looking at the P2P info, it looks to me like that shouldn't happen. It would make sense to me that if one or both of the solenoids in the PCB are sticking it might cause these issues.

Figure I better replace the PCB before something gets fried like the ABS ECU.

I found a good deal on a NOS one and ordered it. It's odd though they are referring to it as a "Light Control Box". I verified its the same part number and NSN. Then I saw in the parts manual they call it that too. So I've seen it referred to as a Power Protection Box, Power Control Box, and Light Control Box...weird.

I'll open the old one up after I verify the problems are solved. Might try to replace the solenoids like in some of the other threads about PCB's to have a spare.

 

[todds112]

Looking further at P2P, I don't see that the 12v for low speed goes through the PCB. Might have a bad main power switch too??

 

[Jon Giovani]

I have found that instead of rapping it with a hammer, I just shut the drivers door while the battery switch and ignition switch are on and it jumps to life and then it will start. Until I get around to testing and replacing offending part, this is my temp solution, haha.

 

[Jacked5ton]

todds112 Have you Figured out what your problem was? I seemed to have the same issue after driving it all day I stopped to get diesel and it wouldn't start. I flipped the switch to on and tried to crank it and nothing. Waited a few minutes and WA-LA and it turned over and fired right up. So, after reading I believe it's either the shut off solenoid sticking or the PCB. Any advice?

 

[sandcobra164]

Kevin,
The shutoff solenoid on your truck is on the left side of the engine and has a plunger pointing downward facing a linkage on the injection pump. Yours sounded great when I drove it the other day. The black box on the firewall may be your issue. There is a member on here named Fuzzy that sells good tested take outs for about $90 plus shipping from Texas. They are very proud of new ones and if you research, you can find threads on replacing the solenoids in the PCB with new ones for relatively little cost. I like to keep a spare one around and I recommend you do the same.

 

[infidel got me]

An extra pcb box is like a spare tire-- you never know when you will need it, but its extra insurance to keep one handy! Shout out to Fuzzytoaster

 

[74M35A2]

Correct. The heater blower Lo speed routes from the battery bank through the main power switch, onward. You can also rebuild your PCB with new 24v solenoids. If replacing your PCB, the old one makes a great PCB jumper connector for emergency use if the replacement PCB flips out.