Nothing electrical works

[Tattood08]

I can usually figure these things out by lurking the forums but I'm stumped today. 1990 suburban, 5.7 TBI stock except exhaust, 700r4, 4x4, battery Napa AGM 7 months old 12.6v. I've had zero electrical problems over my year of ownership. Yesterday drove it all over San Diego running around with no issues. Starting from my last stop I was turning it over as my son shut the pass door and it dies, completely. No lights, no fuel pump nothing, completely dead electrically. I've been working this all day.
With the bulkhead connector off:
Batt, Alternator, Starter, distribution next to brake booster, #2 pin on connector all 12.6
Grounds from battery to truck and engine are good. Cut out fuseable next to starter and hard-wired it (I know)
With the bulkhead connected they drop to 8-9 volts?
I can't find any damaged or grounded wires in the dash, and no blown fuses. Disconnected the headlight switch and ignition switch with no change. The courtesy light circuit is bus connected to the the tail, horn and clock fuses (always hot) and will put a ground at the alternator so I disconnected it but, everything is still dead. Could the ECM cause this? Sometimes the door open key in will whine real weak but that's it.

I have the gmc wiring diagrams thanks to someones post, but I'm stumped. I'm sure there is something I'm missing, its easy to get lost in the wiring.

 

[nutt7]

These are tough. There are two (I believe) separate power wires supplying the cab. One from the starter and one from the alternator. I would start by reconnecting the wire you disconnected. Sometimes you can get a ground signal through the filament of a bulb.

The alternator wire feeds some of the circuits (I think it's the dome, back up, ecmb, horn, and I think headlights). The starter one feeds the rest of the 12v constant circuits. Try one at a time and see if its corresponding circuits work. If everything, like EVERYTHING, is off and nothing is hot or blown up/hardcore shorting, maybe the battery took a dump. Not sure how often this happens though. You could obviously have it checked out. Or maybe disconnect the wires mentioned and carefully short the starter with a tool and see if it cranks. If it doesn't, the battery or starter wire may be toast.

I had mine shut off while driving once. All lost power except the circuits I mentioned above. My meter even showed power into the cab with everything else disconnected. That 12v dropped to 1v when I tried to apply a load though. Turns out the wire and or fusible link was allowing a tiny bit of current through, enough to power my meter to 12v, but that's it. As soon as I connected anything to it, it died. I think years of heat soak killed that wire. It took forever to diagnose this. A simple fast way to check is to jump a fused 10-12 gauge wire straight from the battery to the fuse box or distribution block.

You could also pull all the fuses and apply them one at a time with your meter on if you think there is a short. Keep us updated

 

[Tattood08]

The ground was definitely coming back through the courtesy lamp ckt. All the ckts on that side are bussed together. I've tried with it connected and disconnected. I also went through all the door switches and dome lights to make sure they weren't dragging down the ckt. I haven't tried shorting the starter so I can try that. I will try running my own 12V to the firewall distribution as well to rule out the starter wires even though they don't look damaged and I get 12v there. I'll take the battery to napa. If it fails it's under warranty anyway. I tried the one at time with fuses and with items attached to the ckts. It has to be grounding somewhere, I just haven't found it yet.

 

[diesel4me]

I've had that happen when battery cables that looked good outside,had like 3 strands left inside actually making a connection,and surrounded by that white grunge corrosion--like nut7 said,I'd get meter readings or a test light to light up,fooling me into thinking everything was ok,but as soon as any real draw was put upon the cable it wouldn't let enough current to pass to work anything..the starter would "try" to go clunk when I turned the key once,then nothing--and none of the lights or accessories would work..

On one truck I found someone had snipped off the side terminal cable end and spliced on one from a top post cable about a foot away,so it was hidden under the battery tray..found a taped up area and cut the tape off,all that was in there was that powder and a few strands all corroded to death..to get home I had to peel more insulation off both ends of the cable and twisted them together with pliers,and I was lucky the cable was long enough to still reach the battery..(and I had duct tape to put over the splice!)..

 

[Tattood08]

Ok. I know it's somewhere in that primary starting/ignition ckt. I'll pull all those wires out and scrutinize. Thanks for the advice. Hopefully I'll get her up and running tomorrow.

 

[1975Jimmy]

Had this happen to me once and the simplest thing fixed it. I too was scratching my head at first trying to figure out what the hell was wrong. So, I started at the battery and was planning to make my way to the starter and then the bulkhead. When I began unbolting the battery's negative cable where it connects to the alternator bracket for ground I noticed the lights came back on. So, I unbolted the cable connection, cleaned it and reconnected it. It looked fine upon inspection, but sure enough that connection was the problem. The corroded ground connection just couldn't handle any kind of load.

Of course, I don't have an ECM or anything in my old rig, so you could have bigger issues, but I think it's unlikely. My theory on this kind of thing though is if it worked yesterday, it probably ain't going to just go bad overnight, but crazier things have happened. I hope it's something simple.

 

[6872xtc]

Yeah, don't forget to check the grounds from the battery post, to the engine and the body, and engine to body. Just don't forget the grounds, they usually don't have any corrosion protection.

 

[Tattood08]

Took a break yesterday and worked on the bike instead. So I have a battery that has 12v but only provides 2a of current. My starter is also totally dead, fails every test. Glad I spent hours tearing apart my engine and dash. I knew it was something simple, almost always is. I just couldnt believe it would be the battery. She started first time every time. Thanks for the help guys.