Texas Redneck

Hey everyone,

A few months ago my 99 Suburban started blowing tail light/ back up fuses and I initially thought it was a problem with the driver's side tail light because there seemed to be a loose pin on the circuit board. I replaced that circuit board and am still having the same problem. I haven't been able to figure out a pattern though as to what is causing it to blow. Sometimes it will last for several turns, sometimes for a couple of days, and sometimes barely at all. The only pattern I have been able to detect is that if it manages to make it through an entire trip it will often blow soon after I start the vehicle for the next trip. For instance, a few days ago I put in a new fuse and drove home. It was raining so I figured if it was a short somewhere underneath the vehicle then it would blow quickly. It made it all the way home and through several trips over the next couple of days. When it blew I missed it, so I replaced it with a 25 amp fuse instead of the 20 amp (it was all I had). It worked for several turns before I stopped again. When I started the vehicle the next time smoke started coming out from somewhere around the dash area. To be honest I didn't see exactly where because I was trying to get the fuse out as fast as I could. Needless to say, I haven't put a fuse back in. Any suggestions on what I should do next to diagnose the problem?

Thanks.

mulmur1

I would look at the rear wiring harness connections that are behind the bumper. If you have access to a volt meter or trouble light, it will make it easier to diagnose the problem. First look for areas in the wiring harness that have possibly worn through. If all looks okay, then check the connection block, mounted to the frame. Take off each connector and look for corrosion or signs of wear. It sounds like your wiring harness is shorting out. Please try not to use larger fuses than the circuit is rated for, as you can cause additional damage. IE smoke is really not good

If you find an area where the wire casing has worn through, then cut out this section, solder in a new piece of wire and put some heat shrink tube over the repair to keep the water out of the new repair.

If you do not find anything, disconnect the rear light wiring plugs and install a new fuse, then check for power through the terminal block. You can turn on the lights and confirm all is working with the test light, by testing each part of the connector. If the fuse blows, then your wiring problem, will be closer to the front. It you can find a wiring diagram for the socket connectors, this will make your job a little easier.

Electrical problems can be a challenge to find. I expect that the problem is in the rear harness.

Good luck with your repair.

Texas Redneck

Thanks for the reply Brad. I'll try that out tomorrow and let you know what I find.

Troy

a55bruce

+1+1 for what Brad said. Don't forget to check the trailer socket for corrosion.

Texas Redneck

So I went and got some more 20 amp fuses and it lasted all the way home. I finally got around to checking it out yesterday evening. I took the plug apart and using a test light I tested the plug. I didn't see any worn spots on the wires, but maybe I need to look longer with more light. The only place I saw any issue was at the very end of the the trailer wire. At the very end where it hangs down it had some tiny cracks in it. Since it had been wrapped around the hitch, I unwound it and wrapped it with electrical tape to see if it was perhaps cracked somewhere else and shorting out on the hitch. The fuse lasted
this morning to church and then to a restaurant. When I tried it on the way home it had blown. When I got home I put in a new fuse and it blew as soon as I turned the key to the on position. I tried another, and it also blew as soon as I turned the key to the on position. I don't recall it ever blowing while using it. If I turn on the turn signal the fuse never blows mid-turn.

Any more thought? I appreciate it.

mulmur1

There should be a second wiring harness at the rear of the vehicle that connects to the main tail lights. I would also look at this harness. Did you try leaving the trailer light connector disconnected? If you see a potential problem with it, you could disconnect the main connector for the trailer, tape it up and leave it disconnected to see if it is your problem. Since you are getting blown fuses pretty quickly, there is a wire that is shorting to ground somewhere. Unfortunately, you may have to start to trace the wires.

Try and disconnect the trailers wire, so that you isolate it and see if solves the problem. I expect that it is not the problem, looking at your pictures.

Then find the main connectors for the rear tail lights. Disconnect it and use a test light to find the reverse, turn signals and tail lights. Once you figure which wires correspond to each function, you need to trace them.

Since you are blowing fuses so quickly, if you disconnect the rear harness and it doesn't blow the fuse, when you put the vehicle in reverse, press the brake etc, the problem will be in this harness. If it still blows, you will need to trace the wire from the connector up to the front of the truck.

It is a pain to do this. Unfortunately, you will need to try and locate where the problem is in the wiring. If you have a multimeter, you could also start to look at the resistance of each wire.

Good luck and let us know how you make out.

a55bruce

This idea will help save fuses and your sanity.
I made a tester from an old fuse and an old headlight bulb. Plug this in to the fuse block. When the test light is full bright the short is present, start removing sections of the wiring harness, wiggling the wires, removing the bulb sockets until you find the short (test light will go out or dim noticeably).



This works best if the test light is a higher wattage than the normal load on the circuit you're testing. Using a low wattage test light will show almost full bright when a normal working circuit has a high load (ie. test light really bright but the car's light bulbs dim). If this happens, I'd recommend removing the bulbs on the circuit you're testing.
Hope this makes sense.

Texas Redneck

Thanks for the ideas. I went back out today and didn't find any other plug other than the one in the picture. When I disconnect that one the tail lights quit working so I am assuming that is the main one. The trailer harness is connected to that harness so I would have to cut the trailer wires off to eliminate it. I tried it with this connector unplugged and it still blew a fuse. I don't have a soldering iron, but I bought a 25 pack of fuses, so I kept putting in new fuses. Just out of curiosity I wanted to see if maybe something was happening before it went to the wire harness. Like maybe somewhere in between the fuse and the turn signal arm in the steering column. So I started the vehicle, turned the tires and tried another - it blew. I tilted and steering wheel all the way up and tried a fuse - it was fine. I tilted the wheel down - and it blew. I tried it again. Steering wheel up - fine. Steering wheel down - blew. So I took off some of the covers around the steering wheel and tried pulling on the wires. Sometimes it would be fine and sometimes it would blow. There was no consistant pattern as to which wire or which movement would make it blow. I was getting low in fuses so I thought I would try to make a test light like the one above with a tail light and some jumper wires. I got it made, but I could never get it to light up. My wife needed to go to the store so I said I would take her. I put a new fuse in and started the vehicle. Just to try it again I started pulling in the wires one by one. Nothing was happening. So I started grabbing several wires and moving them. This time I heard the radio getting staticky as I moved them but still no blown fuse. Then I heard one pop and I saw some more smoke - but it wasn't the tail light it was the brake light fuse. But the smoke seemed to linger so I disconnected the battery real quick. So now I'm really confused and I think I may have a bigger problem. I'm going to go back out and try to start tracing all the wires in the steering column and see what I find. I am confused as to why the brake light fuse blew, the turn signal didn't, and it started smoking just from me pulling and pushing on the wires in the steering column.

Any other thoughts?

Texas Redneck

I've got another wrinkle. After not finding any worn wires I put fuses back in it and they didn't blow. However, when I went to put it in Drive, I couldn't move the shift lever. Whatever that thing is that only lets you shift after pressing the brake wasn't moving. So, since I had the covers off around the steering wheel I just popped that rod off of the shift mechanism and was able to shift it into Drive. So whatever happened earlier it ended up damaging that brake/shift mechanism.

mulmur1

It sounds like you have found where the area where the problem area is for the wiring. When you move the steering wheel, it must be causing the short. You will want to try and locate the wires for the lights and pull on each one individually to try and locate what is causing the problem. It could be a wire going into one of the switches. It sounds like that the wiring harness is rubbing inside the steering column and after many years of use, has rubbed through.

Your second problem is the brake shift interlock. The solenoid may not be getting power, since you mention that the brake light fuse is blowing. The solenoid is powered off the brake light switch.

I know that this is getting frustrating, but you are getting closer to solving the problem.