M813A1 "Black Box" Question

[bbbang]

I received my M813a1 the other day and it appears to be a great truck. The only issue I had was batteries (dead) which I am solving with new batteries. I have been going through the truck while it is running checking switches, etc, and have cleaned up the starter, etc.
Now to my questions:

What is the "black box" on the upper driver's side firewall in the engine compartment? When I received the truck I found that it was disconnected. I reconnected it but am not getting any electric reading to it. It appears to be connected between the starter and a switch in the cab.

Any Information on it's purpose, why it might have been disconnected and if I am not getting electicity to it at this point, how big a problem do I have would be appreciated. Thank you. Chris

 

[breelanr]

Chris,
The "Black Box" is the Protective Control Box, it prevents engagement of the starter motor if the engine is running (prevents grinding the starter).

 

[ARMYMAN30YearsPlus]

Great looking truck please post some home home on the range photos for us to drool over.

 

[papercu]

black box

Cranetruck had a great article in the MV magazine a while back about repairing them. Wayne

 

[cranetruck]

Wayne, you may be referring to the 25 amp regulator, I think the issue here is the "Protective Control Box". This box contains the starter relay (normally a separate item), the reverse polarity protection relay and some control circuitry, which monitors the engine oil pressure (lock-out switch).
I have checked into this a little and found that there is a problem with build-up of mositure, the box is not vented properly IMHO and there may also be a potential problem with the switching of the reverse polarity relay without a load applied. Power relays have a minimum switching current specified, so that some arcing can take place and "burn off" oxides on the contacts.
My findings are simply my observations and I'm not very impressed with the box, since few if any mil spec components are used.
Study the circuitry (image below) and you can see how the box replaces the normal, lone "Starter Relay" found in most systems, like the deuce, for example.