'85 GMC Micro-Bird - Wiring issue, HELP!

[kthelen]

Hello!

Well, here's the story... I was poking around behind the switch panel (above the driver's head), pulling out mouse chewings and various other debris. I had a metal tool in one of my hands. I should have known better... but even so, I managed to do the unthinkable: short the hot rail to ground. More specifically, my tool was touching the stud that connects to the main B+ wire. :shock:

I didn't notice until my tool was already becoming welded to the rail. It was at that point that I pulled it off, and (after a moment) checked to see just how hot the wire got. It wasn't hot at all, so I figured it was a semi-safe assumption that the bus wasn't about to go up in flames.

But then, the bad news: when I went to turn the dome light on (as a quick test), I got nothing. Same for the heater, etc. Then it got worse: I put the key in the ignition and turned it on... no buzzer, no accessories. I tried to start the bus, but again, nothing. In fact, all I *did* have was the parking lights and headlights.

So I began investigating, looking for a fuse or fuseable link in the B+ wire I shorted... but was having no luck following it back to the starter. I checked all my other fuses in the "regular" vehicle fuse box (just for good measure), and they were all fine.

By this time it was getting dark, and the bus had to be moved (I had parked it on the street, and around here you're not supposed to leave vehicles on the street overnight). I didn't have a VOM with me, so I decided to make a last-ditch effort to revive it by disconnecting and taping off the wire I had shorted, and making a temporary jumper from the battery to the hot rail. Once I did that, the accessories on the panel (clearance lights, heater fan, dome lights, etc) were working... but the ignition switch was still having no effect.

I ended up recruiting a buddy to drive my truck, and between the two of us we towed the bus off the street. But I've gotta get this figured out, and the sooner the better.

What am I missing here? I do have some auto repair skills, and know how to handle wiring (though who'd know it after this? ha ha), but I don't quite know this bus inside and out (yet).

I do have the Blue Bird body manual, and have looked at the wiring schematic. But it shows nothing between the battery and the hot rail other than that piece of wire.

Help!

 

[cnitrofumes]

GMC / Chevys typically use a fusable link at the big post on the starter or from the accessory junction block under the hood somewhere close to the battery. If your starter cable post has a smaller, 10ga. wire on it along with the fat cable that's your fusable power source....usually has a round black piece (fuse) about 1" long about 1 1/2" from the post. If there is only one fat cable on your big starter post it's under your hood. Should look the same and be connected to a post surrounded by a plastic insulator that usually has two tabs on it to keep it from being grounded.
Either way you need to have the link for obvious reasons.....you can cut it out and splice in a 10ga wire to test but be sure to replace it sooner than later.
Good luck.

 

[SHORTS]

I agree with thee previous post, gm liked to use a fuseable link between the batt + and an insulated stud on the right inner fender, if their is no fuseable link there, go to the main nengine harness where it gets power at the starter and follow it to the firewall plug usually near the steering column, if you remove the central bolt you can unplug the harness and get to it to "pin it out". usually the harness plugs were segmented and could seperated for ease of installation and troubleshooting/repair. the pins and sockets are removable from the plug with special tools, go make a friend at the local gm parts/service department or a specialized auto electric shopor try terminal supply online.

keep us informed of your progress

 

[kthelen]

This one's an oldie - just one fat wire from the battery to the starter, and the rest appears to branch out from there.

I did manage to narrow it down a bit this afternoon with my ohmmeter... the problem wire (or at least one of them) appears to be a piece of 12-ga that runs from the starter to a stud behind my battery, where it then connects with the big wire that feeds the hot bar I mentioned earlier. I was going to try and find where the partial open in my problem wire was (showed something like 7000 ohms of resistance), but then the weather turned and it started pouring cats and dogs.

I'll have to get back to it on Monday. But at least now I have a, um... lead (no pun intended).

 

[SHORTS]

also look at your body and chassis grounds, all of those wires also have 2 ends.

 

[cnitrofumes]

That stud near your battery is your accessory feed. There is probably another wire besides that lighting wire on it that feeds your original van wiring harness for ignition, lights, start, etc. Your trouble is in the wire between the starter and that stud...the fusable link wire. Run a 10ga jumper from batt positive to that stud and see if you don't get everything back.

 

[kthelen]

Run a 10ga jumper from batt positive to that stud and see if you don't get everything back.

Good to hear that - confirms what will likely be my next move. I won't have any time to mess with the bus until Sunday or Monday, but I'll be sure to let you guys know what happens (and perhaps even a few pictures?).

If I haven't already said, thanks to all! This old shortie *will* run again... as soon as I undo what has to be the biggest bone-head move I've pulled in recent memory

 

[kthelen]

Okay, here's the latest...

This morning I clipped the problem wire going to my accessory stud, taped it off, and ran a new wire straight from the battery to the accessory stud. Now the body accessories worked, but still no ignition, etc.

So I took my ohmmeter to the various wires in the loom going to the dashboard. Two of the wires in the loom were connected to B+, and one had a whole heckuva lot of resistance - about 7K ohms. So I did the ol' snip-and-splice to it as well, and voila - everything was working!

I know I really should have hunted my way back to the fuseable link that was burnt out, but I had promised someone that I'd haul some stuff for them with the bus later in the day. I'll probably go back to it later, since I hate hacking things this way... but for now, the problem is solved.

Only one little issue: the battery was pretty well discharged by the time I was done, since I had inadvertently left the clearance lamp switch on from my testing on Friday. As soon as I had the new wire run from the battery to the accessory stud, they turned on - and stayed on, though I didn't notice. I had enough juice to get it started once, and so I didn't shut the motor off for the better part of the next hour - drove about 10 miles, mostly in town. But even so, the battery didn't seem to have taken much of a charge.

I assume this will all be cured by an evening on the trickle charger. But if it happens again, I may have to check on the alternator and associated wiring.