M939 Wiring /Lighting issue

[texczech]

The local rural fire department has a M939 with Lighting issues- Headlights, tail & turn signals don't work. Where can I get a repair manual to cover this problem/diagnosis or any quick tips on where & how to check? Thanks!

 

[Mullaney]

The local rural fire department has a M939 with Lighting issues- Headlights, tail & turn signals don't work. Where can I get a repair manual to cover this problem/diagnosis or any quick tips on where & how to check? Thanks!

.
The TM's (Technical Manuals) are posted here. Free download. TM9-2320-272-24-4 has electrical diagrams with the full truck electrical on page 819. That page is attached below.



The turn signal (SignalStat) will generally last forever - but everything out on the back of the truck goes through it - so that is a good place to look. Reading your post a second time mentions headlights, so that points more at the 3 Lever switch... Has anyone made changes to the lighting recently? Like maybe adding or upgrading to LED's possibly?

 

[simp5782]

With all of them not working it's more than likely a bad light switch

 

[texczech]

Great info to start off with.... I will be getting a look at the unit sometimes next week. Trying to get prepared. Thanks !

 

[GopherHill]

Might want to insure that personnel understand how to use the 3 lever switch.
Check in the dash 10 manual you can download I think.

 

[Mullaney]

Might want to insure that personnel understand how to use the 3 lever switch.
Check in the dash 10 manual you can download I think.

.
Good reminder @GopherHill
Here are the easy version of "Cheat Notes" for the 3 Lever Switch.



Brake lights for example DO NOT operate if the lights aren't in the "Service Drive" position texczech .
.

 

[GopherHill]

Very cool, great picture.

 

[Micmada]

Make sure the ground/bounding is good. I play with HAM radios and I have the same truck and they tend to develop rogue current through the frame from one piece of electric to the other.