LilBeast

I had this posted in pre 84 section, but now that I am a bit further, I think it is more appropriate for the electrical/lighting forum.

I recently purchased a 1980 4x4. It's in very good condition overall. The main problem I'm having right now is the tail fuse blows as soon as I turn on the parking lights or headlights. So the tail lights, license plate lights, dash lights (including tach & speedometer lights), and all parking lights don't work. I know these are all related to this fuse. The headlights, hazards, and turn signals all work just fine.

I have been working on this to isolate the problem. Here's where I am at. I disconnected the plug behind the passengers seat that takes the wires under the bed to the tail end. Fuse still blows, so problem is between that and the fuse box. I disconnected the front parking lights and pulled out the bulbs, fuse still blows. I just found the relays for the headlights and tails way up under the steering wheel. With both in, the fuse still blows. I took out the tail relay, and the fuse doesn't blow (of course the lights don't work either, but the fuse still blows). I tested the headlight relay in the tail relay space, because it is the same model number, and the tail fuse blows again. I put it back into the headlight relay space, and the headlights work normally again, so the tail relay itself is not bad.

So that's where I'm at, and now I'm stuck. I don't see any obvious cuts or rubbing on metal, but all the wires are so well tied together and difficult to get to. There is no stereo at all, and no trailer connector (the wires going to the back are disconnected anyways, as I said above).

I have a copy of the schematics, and it shows that the wire going from the fuse box into the relay wiring is red with a yellow stripe. Then, coming out is supposed to be a green in one spot, and a red with a dark blue stripe coming out of the other side. For some reason, there is both a green wire and red with white stripe coming out of one side, which is not shown in the schematic. It looks stock though.

Please help... I'm new, and I'm not too good at this stuff, but I'm trying hard to isolate the problem and fix it without spending hundreds at an auto electrician. I need to fix it so I can pass safety inspection and be able to drive it legally.

Thanks for your help... two pics are attached, one of the relay spot with the tail relay out, the other of the schematic.

http://imageshack.us/photo/my-images...tailrelay.jpg/

http://imageshack.us/photo/my-images/8/schematic.jpg/

jbtvt

If the wires look good, how hard would it be to bypass the light (combo?) switch itself? Also looks like you've got a dimmer (rheostat) in that circuit, potential for trouble there. Try to isolate circuit components one at a time, fun stuff...

LilBeast

This might be some good related info... could you please explain what you mean by having a dimmer in the circuit? I'm a noob to this, and anything already done was done by a previous owner. Are you referring to the extra wire coming out after the relay... the one I saw that isn't in the schematic?

jbtvt

Just glancing at the circuit, aside from a boatload of dash etc lights, which are pretty straightforward, you should have a knob of some sort that lets you turn the dash light brightness higher or lower, like a dimmer for house lights. Should be a resistor and contacts inside. If you could pull that out and just jump the wires leading to the two contacts together it would tell you if the short is in the rheostat. Same with the combo switch, which is probably a little more involved. Just a thought, I'm not very familiar with the early rigs, and have had a few too many tonight to get that into it. Hope it's clear though, happy troubleshooting.

LilBeast

I certainly do have the dimmer knob on the dashboard. Sounds like a good thing for me to investigate. I'll do some research to find out how to "jump" the wires around the rheostat. I've done a lot of research and found people also saying to test each socket (in this case the front parking lights and no need on the rear ones since the fuse blows when the wires to the bed of the truck are disconnected), but how do you test those for a good circuit if you can't give them power? As soon as I give power, the fuse pops and kills that power (unless the relay is out, but then still there will be no power to the sockets?) I'll definitely check the rheostat though... thanks for the tip. Have a drink for me

jbtvt

Continuity testers (w/ battery inside) are easy to use to check for continuity, or ohmmeter if you know how to use one. Disconnect the car battery, attach the alligator clip on the cont tester to a body ground with length of wire, make sure your bulb works or jumper around it with alligator clips and wire, then touch the probe of the cont tester to a feed wire somewhere in circuit to test after it - eg the green feed wire out of your relay. Tester lights up you have continuity or gives resistance reading if using an ohmeter.

Above is the general procedure, you need to test the feed closer to each bulb to isolate it.

LilBeast

Thanks for the detailed explaination. I think i get it... I'm going to pep boys to pick up a continuity tester now, and I'll test all the front light sockets and plugs. I believe there are 4 of them that need to be checked (front left and right parking lights, and front left and right side marker lights). The plastic around one of the plugs looks a bit shady, but I'm not sure if that's a sign of the problem or not.
I'll be back.

SoCal4Running

Does this truck have any trailer wiring at the rear bumper?

Possible the PO hacked some wiring together to do a little towing before it was sold?

LilBeast

No, doesn't look like it. I've successfully disconnected all wires going to the back by way of plugging the plug between the cab and bed, and there aren't any other wires going to the rear, so trailer or towing plug isn't the prob. I wish it was! I am leaving pep boys now with a continuity tester. I'll update in a bit.

forcefly

I just had the exact same issue on a truck I just bought yesterday and it ended up that they wired an aftermarket sterio incorrectly. Pulled the sterio and and disconnected their wires and the lights work again without blowing fuses