Electrical Diagnosis Help

[08H3]

I want to double check with everyone and see if my thinking on this situation makes sense before I dive deeper into it. Truck is an '08 3.7 with AT. Driving home last night it died. Just as if the key was turned off. DIC, lights and everything still work. Literally just as if you turned the key off. Truck would not restart. No attempt at cranking. Nothing at all. Just as if there was no battery installed (though the lights, DIC, etc. still worked.) Looked at it this morning and found fuse 5 (BAT/IGN) was blown. Looking at my Mitchell and the GM service manual, it looks as though this circuit feeds through the ignition switch and then on to the BCM. Mitchell states that if something controlled by the BCM has a short, that will potentially blow fuse 5. I disconnected all of the harnesses to the BCM and the fuse still blew. To me, that means the problem has to be somewhere between the ignition switch and the BCM. I should add that the fuse does NOT blow when the key is left in the OFF position. Only when moved to one of the other positions. It should also be noted that I had a new switch on hand that I swapped in. That did not make a difference. So that brings me back to thinking the problem has to be in the wiring between the ignition switch and the BCM.

If I am missing something, let me know. I'd just like to know I'm not overlooking something stupid before I remove the entire dash from the truck which does not look to be an easy job. Thanks in advance.

 

[Jeepwalker]

Your thinking is sound. This is going to take a methodical approach to testing circuits.

Have you inspected the harness visually (in case a rodent chewed wires)? I've had a couple times in my days where rodents had chewed the underside of a harness or wires (other vehicles, not the H3).

Does the manual give a method of ohm-testing the BCM pins for shorts? Another thing you might consider, is (and it might be too early at this point), is, there is the option of sending the BCM in for inspection via the GM parts department. They used to, and I think still do, have a testing program and 'should' be able to test it for you. I did that on a vehicle years ago. But obviously you want to ensure all wiring circuits are good first.

 

[08H3]

I think I have this figured out. I'll post what it ended up being once it's all back together and I'm certain it's fixed. Suffice it to say, there is a reason all the service manual diagnostic charts start with "Check for aftermarket accessories..." P.O. left a nice hack job on the wiring for me to clean up.

 

[08H3]

Everything is back together and better than before. A previous owner did an absolutely terrible job adding an aftermarket radio and backup camera. Backup camera power was taken directly from the wire from the ignition switch to the BCM. That cable at the back of the truck was run through a hole cut in the floor under the "HUMMER" trim plate in the cargo area. No grommet. No conduit. No protection at all, so it rubbed through and shorted out. That of course caused the IGN fuse to pop, killed power from the ignition to the BCM and then from the BCM to the PCM. Result. A brain dead truck that left me stranded. I unhooked the backup camera for now since I think it got fried in the process, but I did put the wires in place, this time being powered off the AUX PWR 1 circuit which I converted to RAP.

 

[Jeepwalker]

Good job finding that. There's a lot of questionable work out there.