GMC Troubleshooting Questions? Ask a Mechanic for Answers
Three things that will commonly cause this, a bad ground, excessive voltage drop in cables, or an ignition switch. the ignition switch can usually be checked by when it occurs bang on your steering column and see i it goes away. voltage drop you need a voltmeter to check, putting it to DC volts hook one lead to one end of A battery cable and the other to the other end o the same cable, then crank the engine and watch what it spiked up to, .2v is a fail. do this to all battery cables, negative, positive to starter, and positive to fuse block. while doing the negative check the engine ground and follow the small lead down to the frame and check its ground. also check the ground to the back of your driver side cylinder head. let me know what you find
is it relatively easy to duplicate the condition as far as devices turning off? i would remove the column cover behind the steering wheel and expose that ignition switch so you can tap it if devices start going haywire and see if it goes away, also good to check there for any aftermarket devices wired to ignition in that harness.
thats normal as it has to charge higher after startup to replenish, if you can get someone to check codes that can help but no worries if not, and a good battery tester would not hurt because a bad cell will do bizare things and i have seen brand new batteries fail before. i would first try and duplicate it though and tap on that ignition switch, also if that does nothing before you restart it try unplugging the alternator electrical connector while running. but lets see if we can duplicate it and start there.
good i just wanted to see if there were any u-codes present. looks like we are okay there. since the problem seems to be under higher load IE your lights etc were on lets try that voltage drop test again under heavy load,remove the inj1 and inj2 fuses from the underhood fuse block then hook up your meter again and crank away on it, this way it will not start and the starter will be putting a high load on the cables and we will have more accurate readings
you will have to try and stick your meter where you can see it while cranking.
the other location ive seen a lot of water issues is the IP fuseblock, behind the glovebox on the far right, I would remove it completely open it and dry everything clean it all, blow out the connectors with air and put dielectric grease on everything one sec and ill get you the procedure and some pictures
sorry wrong post ignore that last one