GMC Troubleshooting Questions? Ask a Mechanic for Answers
Do you have a good quality scan tool by any chance?
OK, does it communicate with anything besides the ECM? I'd be looking to see if the BCM is communicating?
I don't think it does, either, but I'm not as familiar with that device as you probably are.
Nonetheless, let's try to pick out some things to look at. Did the turn signals stop working at exactly the same time that the engine failed to start? And you have no other gauges working on the dash either, is that right?
OK. Do you have a test light?
Turn the key on, then use the test light to touch the tops of all of the fuses, both under the hood and under the dash. I'm looking to see if you have large groups of fuses that don't light up. Make a note of which ones aren't hot, and let me know what they are.
OK. Is the maxi fuse 40A labeled IGN B hot on both sides of the fuse?
OK, that fuse is open. Replace that maxi fuse.
Are the circuits working now, or does the truck now start?
Alright, did that fuse open again?
OK, there's a short in that circuit. Let's go to the schematic.
Does the fuse blow as soon as you replace it, or when you turn the key on?
I asked you if that fuse was open again, and you said yes. So you have power on both side of that fuse now, correct?
Maybe we're missing something in the communication here.
With the fuse installed, touch the test light to the tiny metal tab on either side of this fuse. Both side of this fuse should be hot when it's installed. If it's only hot on one side, that would be because the fuse is open.
OK, good. Follow that schematic, you can see that this fuse feeds a red wire, which turns to red/white, and goes to the ignition switch. With the key ON, there should be power then coming down the brown wire at the ignition switch. Can you access the ignition switch wires inside the steering column and test to see if you have power on these circuits?
The brown wire is the wire that comes out of the ignition switch, and it's fed by the red/white wire. So, if you have power at red/white, the key is ON, and there is no power on brown, your ignition switch is no good and needs to be replaced.