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...

[rpaoness]

Internet wisdom seems to indicate at the ABS system and the HVAC share an ignition switch terminal. I replaced the ignition switch with an Amazon Dorman early last year due to several continuous-cranking sessions. Since GM discontinued the switch options are somewhat limited.

https://www.rockauto.com/en/partsearch/ ... m=15242754

[rpaoness]

I suppose I could try the "standard" brand one next to see if it's better. Or get another Dorman...

[rpaoness]

Playing with the key seems to make the issue come and go, so I guess I’ll try a new ignition switch again. Spring feels weak anyhow. Now which one…

[kevm14]

Doesn't everything ultimately share some kind of connection with the ignition switch? It seems like you'd have more than just two symptoms if that feed was really intermittent. I guess I could try another look at the power distribution diagrams. The ones I posted showed different fuses so I stopped there.

[kevm14]

JL4 is traction control.

And lookie what I found.

It does appear that this is the culprit. The diagram didn't make it obvious but looking back at the ones I posted first, the ECBM and HVAC blower control both shared a "RUN" feed which is a common power source (despite different fuses). I didn't understand the meaning of these labels before. Here are the two diagrams that show what the "RUN" circuit actually does.

04 G3500 IGN A Fuse, START, ACCY-RUN-START and RUN Bus Bars.pdf

04 G3500 CRANK, IGN 0, TBC ACC, FR WPR, BRKS and HVAC Fuses.pdf

04 G3500 ignition switch connector.pdf

So yeah, it's HVAC and the EBCM. Plus some aux relay that may not be connected.

Replace your ignition switch and don't use Dorman electrical parts. They are junk. Try eBay for NOS. Or if that doesn't pan out, consider Standard Motor Products (not sure how much better than Dorman it is, or if it is even different).

I tried eBay and no luck for NOS. There are whole ignition switches with keys for $100 but they are used. Might just try Standard Motor Products on Rockauto. If that doesn't last, try more desperate measures I suppose. Or just keep warrantying them (3 yr/36k for regular inventory). Or try the $11 closeout item with 30 day warranty.