Tail light fuse blows when headlights are switched on.
 

Hi all. I have an electrical gremlin I have been working on for the last few days and I'm getting frustrated. The tail light fuse would blow when I started the truck.
​​​​​​This one is a 1993, 4x, 22re. I removed the tail lights, license plate lights, headlights, front parking lights and the fuse still blew.
I unplugged the connection under the passenger seat, put in a new fuse and voila, the dash lights, and parking lights worked! Sweet isolated problem to the rear. I then removed the aftermarket towing light wires, plug the rear back in and YES! No more blown fuse. I go to Napa and buy some bulbs and shrink tube, get back, test it out again, and NOOO! The fuse now blows not when I start up, but whenever I switch on the lights! To top it off the problem now doesn't go away when I unplug the rear! I've jumped the dimmer switch with no luck, and traced and inspected the wiring where I can get to it, and have found no obvious problems. I'm about at my wits end. I gave up tonight, but tomorrow plan on having my Dad help me, by watching the multimeter hooked to license plate or parking light plug showing continuity. I would then jiggle all the wiring I can reach, and have him holler out when/if the meter dial drops.
Des this sound realistic? I'm no expert and any help/advice would be appreciated. I don't know what else to do. Thank you.

Did you check the switch on the brake pedal support, that turns the brake lights on when you push the pedal?

Did you check the grounds on the tail light buckets for corrosion? They're known for that. Any corrosion in any of the connections on the way back to the tail lights?

How were the aftermarket wires attached? How did you remove them? If there's any bare wires left, they may be doing the shorting.

Those are the few ideas i've gotten so far. I wish you luck.
Pat☺

What items are listed in the owner's manual as being on the tail light fuse?

I agree with the tail lights being a common source, along with aftermarket tow wiring, but unplugging the harness going to the rear should take that off the table.

The front parking lights, and dash lights I believe. It also says glove compartment light, but I don't have one.
​​​​​​I Guess I don't understand how to use the continuity function. When I test between a side of running lights and body ground with other lead I get continuity. Same with tail lights and licence plate lights. I thought it was because of the short to ground, but now I'm thinking I just don't understand what I'm doing.

Thanks toyguy! I didn't check the brake switch, it's not on the tail light circuit and the brake lights still work but I'll be heading out immediately after I post this to check it!
At the moment I have the rear lights/trailer wires disconnected via the harness plug under the passenger seat, so it something forward of that.
The aftermarket trailer wires were spliced into the harness and I just chopped it out.
I've went through the front harness a good bit. I haven't resorted to cutting it all apart to look everywhere yet, I did expose several areas, I thought could be a problem, but everything looked just dandy.

I'm trying to get my head around how to test for resistance. I only have a cheap dial multimeter, if I turn the adjustment all the way down I get right at 2 on the scale going from right to left pretty much wherever I can pick up resistance/continuity. Probing the parking lights (probe both sides of plug =2 and one side of plug to body ground =2. Headlights with one to ground wire and other to one side I get 2, ground wire to other terminal gets no reading.

I swapped out tail light relay with no luck, I've ordered cheapo switch assembly which should be here tomorrow to try. Yeah I know throwing $$ at it. I should instead use my $ to buy a better multimeter and take a course on how to use it.
I pulled the switch and it looks perfect India de and out so I'm not too optimistic.

Thanks! I'm heading out to check the brake pedal switch, and beat my head against random electronics. Something will give!

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Checking resistance is a pretty easy, for me, thing to do, even with a cheapo multimeter. What you need to know are two things: what does the dial indicate whan the test leads are shorted together, and what does it indicate when the test leads are not touching each other. When the test leads that come from the meter's ports touch one another, the needle should go all the way to the zero side of the dial. When the twp leads aren't touching one another, the needle should go to the OTHER side, or infinity, side of the dial.

That way, you can see in an instant, what the meter is telling you. Say you touch one test lead to the pin in the socket of the light bulb, and the other lead to a good ground point, and the meter needle swings rapidly to the zero end of the dial. You know that the resistance of the socket pin you're on is ground, AND that it's a good quality connection. If, on the other hand, the needle swings about 1/2 way across the dial when you touch the two same points, you have a ground connection, but there's a large resistance in the line someplace. Probably corrosion when the light's socket wire is connected to ground.

Same with measuring a wire, end to end. Touch one test lead to one end of the wire, and the other lead to the other end. If the needle swings down to the zero end of the dial, the wire is good, end to end. If it's much above the zero end of the dial, there's a high resistance somewhere in the wire's length. Maybe there's a dirty connector somewhere. Or perhaps the wire's of a connection are frayed.

Does that give a beginner's course in how to measure resistance with an el cheapo multi meter?
You are correct, though. A decent multimeter isn't all that expensive any longer. You can get an actually pretty good meter for about $30.00. You definitely should get one.
Something to remember, too, when measuring resistance with a multimeter: Make certain there's no way the two meter leads can NOT touch any points with voltage across them when testing resistance. It can burn up the resistance portion of the meter. Heck, when testing " stuff" with a mm, test the two points you're going to check with a mm set to measure voltage. If no reading, great, now check the resistance you want to verify. If the meter tells you there's voltage present, ensure you remove it before testing resistance. Pull the negative lead off the battery, for example. Or at the very least, the fuse feeding the circuit you're testing.

Good luck to you!
Pat☺

You rock 2toyguy ! Good explanation. Thank you.
I'm still confused about the tail, parking, license plate light sockets. Both sides of the socket when tested to ground slams the needle to zero? Is there not a positive(hot) wire? I was thinking they were all testing with a short to ground.

I got the cheapo headlight switch and WAA WAA, fuse still blows.

Tomorrow I'm going to check the reverse light and parking brake wiring. If nothing there I guess I start opening up the nice factory wrapped harness. Start from the front lights and work back.

So I figured out the short or maybe I should say a short. Seems every time I think I've won, I am humbled yet again.

Good news, the latest short was in the radio antenna.

Bad news after putting the truck mostly back together and attempting to start I heard a loud pop (sounded like it was near battery or fuse block under hood) then I had nothing except door buzzer.

I checked all fuses, and all good. Unhooked battery and reconnected. When I tried again I had all lights, but did not try to start it again. Worried I might fry something.

Now I'm thinking it may have been loose battery ground connection and I should be ok to try to start again.

Does this seem reasonable, anyone think of anything I'm missing? My biggest worry is frying something major.

checking all ground connections is always a good idea, no matter what the symptoms... watch out for ground wires that are missing.

maybe stick your head under the hood while dad turns the ignition key?

Thanks osv. It is all resolved! I hope! I think it was just the loose batter ground. Everything is now working again. Good golly that was a tremendous amount of work to find that short. Good thing is I have been through most of the wiring on the vehicle and fixed up everything suspicious looking.

lets hope that's it! i like to take time and just look at things on a car, both under the hood and laying underneath it... oil leaks, loose wiring or wiring that is rubbing against something, etc.