M939a2 intermittent starting problem

[Blazer1987]

Hi everyone.. I'm new to this site and I'm not sure if I am posting this in the right place. I recently purchased a m939a2 from government liquidation. I am so happy to finally own one of these trucks. I am having a problem with it starting intermittent. The battery switch and the control box is acting up I think. I put new batteries in the truck and when I was hooking them up I put wire#569 on the 12 volt side of the system like it says to do and the truck won't start. The ignition won't come on, none of the gauges work or anything. If I hook up wire 569 to the 24 volt side of the battery everything works fine? I don't get it. I have wiring diagrams and the manual for the truck and I can't figure it out it's driving me nuts. I know that wire 569 is supposed to be for the heater motor low speed. But it must also have something to do with. The ignition too. The control box looks replaced and new. I would just leave the wire hooked up to 24v but I don't want to fry anything. Any ideas?

 

[tractors0130]

Sounds like the PCB (power control box) might be acting up. Sometimes you can tap them with a hammer with the master switch on and they will make contact and start working for a while.

 

[Suprman]

I dont know the numbers but the 12v tap I believe is for ABS. You will have a + and - from the slave connection and then the mail + and - going from the batteries to the starter. Did you do 2 or 4?

 

[Artisan]

There is a diagram in the battery compartment showing the #569 placement yes?

All batterys and #569 wire are wired per diagram yes?

If so then perhaps check downstream to be sure it was wired correctly at the battery and heater switches.

#569 from battery (per diagram) to "C" position on Battery Switch.

#569A from "D" position on Battery Switch to Circuit breaker then on to "C" posistion on Heater Switch.

Heater switch "D" position should be EMPTY.

You should physically EYEBALL the main power and ground wires from the battery compartment
to the starter area. MANY of these trucks have wires rubbing the block (and) and insulation
wears away BIG TIME. My truck inclusive. M925A1

Additionally ALL GROUNDS should be pulled and buff ALL SURFACES to bright bare metal and
reassemble w/ combo Internal/External Star washers. (Frame then star-washer, then wire, then bolt.)

I put dielectric grease on all as well.