Electrical problem -- need help!

[garyfish]

A week ago I had what I thought was a "minor" electrical short underneath the dash of my '85 T4WD SR5. A cloud of white smoke appeared from underneath the dash on the driver's side, and I immediately shut 'er down & popped the hood & unhooked the negative battery terminal (with a quick-disconnect gizmo I'd installed a year earlier). Probably no more than 15 seconds elapsed between the first sign of white smoke & disconnecting the battery.

I removed the lower dash cover on the driver's side, and quickly found freshly burned insulation in a 12-gauge wire I had installed a few months back as a tap on the hot ignition wire (black with yellow stripe) to run my fog lights and other electrical accessories. Turns out I had inadequately routed and secured the tap wire (Doh!), and it had been rubbing on a sharp corner of the brake-pedal arm every time I depressed the brake pedal.

So I taped up the damaged wire, and properly routed and secured the repaired wire so it was not being abraded anymore. After this I was able to start and run the car, but it became obvious I had fried the ignition switch -- there was a short in the "off" position. There was no apparent short in the "start" and "ignition" positions, and this was soon confirmed by testing the switch according to the methods detailed in the FSM.

I did drive the car for short distances for a couple days before finding a replacement ignition switch, but I had to disconnect the negative battery terminal every time I turned the car off so I wouldn't drain the battery -- or worse -- do further damage to my electrical system. During this time I noticed that the interior dome light was acting erratically, e.g., it would come on AFTER turning the car off with the door still shut! Also, I tried using my aftermarket car stereo once, and found I had to turn the volume WAY up in order to hear it. So I knew there was some sort of a short in at least these 2 circuits... and I didn't waste any time pulling the 2 fuses controlling the "radio" and "dome" circuits from the interior fuse box (driver's side kick panel). At this point, I had checked every single fuse in the car, and none had blown. I also visually checked all exposed wires and connectors underneath the dash on the driver's side, and found no sign of melted insulation, fused wires, charred connectors, or any other evidence of an electrical short.

I finally pulled a good ignition switch from an '83 T4WD SR5 parts car owned by a friend, and thought I'd be off and running and everything would be hunky-dory. But there are problems... I noticed that the digital clock turns off every time I turn the car off, and the clock is not nearly as bright as it used to be. I haven't yet replaced the fuses in the dome and radio circuits, so I'm unsure if there's still problems there. Also, I just discovered yesterday that the 20-amp fuse in the windshield wiper circuit was blown. I don't know how long ago that happened, because it hadn't rained here for a while and I had no need to run the wipers. I replaced the fuse with a 15-amp and the wipers worked. But when I tried running the wipers last night, the fuse was again blown (I haven't replaced the fuse again, but I'm gonna need those wipers soon because a storm is forecast tomorrow night!). Furthermore, just today I noticed that if I actuate the wiper switch (with no fuse in the wiper circuit), the digital clock display disappears, and when I turn the wiper switch back to off, the digital display comes back on (still not nearly as bright as before all these problems developed, though).

Fan blower, headlights, tail lights, running lights, dash lights, cigarette lighter, and all other electrical components seem to be working fine. I have not yet had the time to test the combination switch for continuity, etc., according to the methods outlined in the FSM.

SO... I'm looking to the forum for advice on how best to proceed from here. I'm definitely inexperienced when it comes to troubleshooting and repairing 12-VDC electrical systems -- but I do have a digital multimeter (VOM) and I hope I can learn how to use it to diagnose shorts, etc.

Any help or guidance here will be VERY much appreciated!

Here's a couple threads I found with some info on electrical problems, but nothing specific enough for my situation.
viewtopic.php?f=4&t=5088
viewtopic.php?f=4&t=4607

[Petros]

What likely happened was you also melted some part of the wire bundle where all those circuits run and they are shorting together. I have seen a similar effect in other cars, a friend with an old British Lotus showed me once when he turned on the headlights the wind shield wipers came on, and other strange effects. His main bundle has melted together and was causing all kinds of havoc. Power was back feeding through circuits and energizing electric components at random.

You are lucky and only a few circuits were affected. You will have to try and physically follow each of the faulty circuit's wiring until you find the place where it is shorted, or melted through the insulation of another wire. You might start with the fuse block, remove it and inspect the back. If that is good you may have to cut open the sleeves the carry the wires to see if or where it might have melted to any adjoining wires.

Should not be too difficult but it will be time consuming, you must do it systematically. In the mean time if you need your wipers you might temporarily run a jumper wire directly from a power source to the wiper motor, unplug it to turn it off.

That will teach you to treat your electrical system so carelessly.

Good luck.

[garyfish]

You are lucky and only a few circuits were affected. You will have to try and physically follow each of the faulty circuit's wiring until you find the place where it is shorted, or melted through the insulation of another wire. You might start with the fuse block, remove it and inspect the back. If that is good you may have to cut open the sleeves the carry the wires to see if or where it might have melted to any adjoining wires.

Should not be too difficult but it will be time consuming, you must do it systematically.

I have already unbolted the interior fuse panel (inside the driver's side kick panel), inspected the back & all nearby connectors, & followed all the visible wires until they disappear deep into a bundle. I've cut most of the accessible wire-bundles apart... so far, nothing unusual has shown up.

I have not removed & inspected the main fuse block under the hood. Are you suggesting I do this? I haven't because none of the affected circuits have fuses there.

I guess one of the things I was hoping for here was some advice on how to use the VOM to track down & pinpoint where shorts are located. Any help on how to get started with this?

[takza]

I'd search online on "finding a short" and so forth...I've seen a few writeups on this.

[Petros]

I am not sure how you would use a VOM to find a short. Obviously if a circuit is grounded it will show it, but not where the ground is located. And it would not be easy to use to see if one circuit is shorted to another, except by luck to test continuity between the two systems. And finding the problem would still require tracing the wire to where the fault is located.

See if you can determine which of the circuits are open to each other, and where it is possible that such contact can occur. That will at least narrow down where you will look. A study of the wiring diagram of the systems might help, consider how your wiring failure would have affected the systems that are now not working correctly.

I once had a weird gremlin in the 4 head light system of an old Datsun 510: all the head lights stay on very dim, high beam switch made two of the 4 lights come on brighter, and turning on the light switch made them go off. It turns out that for some reason Nissan choose to run hot wires through the head lights and the on/off switch worked by grounding the circuits (a bad idea since the head lights always had a live 12v running through them). So a wire wore through and was weakly grounding them all, and I had one of two fuses out. Power was back feeding through the system. When I turned the switch "on" it only severed to put 12v at both ends of the circuit, killing all the lights.

Good luck

[garyfish]

Thanks for your input. I'm going back at it this afternoon.

Good luck

Thanks... I'll need it.

[4wdchico]

Once you get the whole dash torn apart I'd look closely a the the possibility of just swapping in in a known good harness. The diagnostic and repair of the existing mess could easily take more time and be much more frustrating...