2004 G3500 Electrical Gremlins

[kevm14]

Here are some references.
G300 is where the EBCM is grounded.
G102 is where the HVAC blower fan is grounded.
S102 is a bundle that grounds a bunch of things that ultimately ground at G102.

The internet says G102 may be located in front of the starter on the block. Couldn't find the location in the FSM. Attaching what I did find.

I don't know if this has JL4 or not so I attached both.

[rpaoness]

Chasing an intermittent electrical issue - on what seems to be like random (damp?) days the box truck will have an ABS warning light and no HVAC fan operation. Sometimes it will work with the fan on high and the light will go out, sometimes not. My first inclination is a shared-ground problem.

I did look under the drivers side for the chassis ground wire on the cab mount - it is there and appears to be intact.

Need to look for what appears to be the under-the-passenger exhaust manifold ground or back of block and see if that's green and growing stuff or its OK.

[kevm14]

See my other thread with PDF schematics. They share neither a ground nor common power source. The interaction could be triggered by fluctuating system voltage. Try varying the voltage with headlights or other load if you even have anything.....

EDIT: See new post #1. Merged topics.

[rpaoness]

The only counterpoint I have that would contradict random bus voltage fluctuations is that it seems as though it works "best"/most often with the fan on high, and more intermittently on the lower fan speeds.

[rpaoness]

Seem to be "worst" when it has been unused for a week or more. Fan works most often on high/ABS light is off. very low, low, and medium fan speeds seem to work less often. Seems to wake up after some miles and possibly chassis flexing. For example, high-only worked very repeatably on the way to Littleton NH from Lunenburg. Fan would shut off/ABS light would come on in any other speed. On the way back, lower fan speeds worked.

Still need to crawl under it and hunt grounds...