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Old 12-20-2014, 03:06 PM   #1
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Tail Light Short

Hey there,
I'm new to this forum and have a question I thought I might be able to get some answers to. I have a 1988 GMC 6000 school bus that I am converting to a portable living space. When I bought the bus about 4 months ago all the lights worked, and I have not had any issues with them since then. However last night I fired the bus up to move it and the tail lights were shorted out. The brake lights still work, and I've been able to isolate the short to somewhere between the tail light fixtures and the wire coupling on the inside above the windows. At this point I'm guessing the problem is in the tail light fixture itself, but I was wondering if you guys might have some insight.
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Old 12-20-2014, 04:10 PM   #2
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2 or 4 lights?
What works?

You said brake lights work, are they also park or turn signals?

Do your hazard lites work?

Dig into more details, I have 8 lights total, and the bodies of the lights(or sockets) ground to the metal body with mounting screws

Oh and welcome
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Old 12-20-2014, 07:29 PM   #3
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Thanks for the welcome. There are 2 back-up lights, to signal lights, and two other lights that serve as the tail and the brake lights (one for each side). I pulled the lights off of both sides and it was still shorting out when I turned the lights on, so it seems the problem is not with the light sockets. There are two wires running to each socket, one seems to be for braking and the other for the tail lights (I used my multi-meter to tell which was which). I then checked for resistance from the socket end of the tail light wire to the next connection, but my multi-meter said there wasn't any resistance along the tail light wires going to either side. At this point I'm stumped.
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Old 12-21-2014, 06:15 AM   #4
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When you say shorted out, is it shorted to ground causing the fuse to burnout?

Or the park lights just not get 12v to them?

Do the front park lights still work?

Do dash lights still work?
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Old 12-21-2014, 07:45 AM   #5
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If the light is for brake/tail and has two wires, when you took it out of the body it no longer has a ground. The metal part of the light fixture has to be connected to the metal part of the body. Brake light may be grounding back through the tail light wire and not the socket like it's supposed to. It's much easier to check wiring problems with a test light. Make sure your wire is grounded good (check it with a known 12v source). Then probe down into the socket where the contacts are. You have an 88 bus which probably had all metal sockets. Biggest problem area with those was where the base of the bulb made contact with the socket where it grounds.
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Old 12-21-2014, 01:41 PM   #6
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I already checked the sockets to make sure that the springs were good and that they were grounded when the bulbs were in. There is bundle of wire connections inside the bus behind the light panels, which is were the lights are wired through. When I separate the connection for the tail lights they stop blowing the fuse. However, if I connect the tail lights, but disconnect the sockets, I blow the fuse, so I'm guessing there must be a short in one of the tail light wires. The strange thing is, when I checked this last stretch o tail light wires with my multi-meter, I was able to run a current through them just fine.Honestly I have no idea what to check next. Oh, and the dash light still work.
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Old 12-22-2014, 09:33 AM   #7
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how about keeping wire connector unplugged and use multi-meter to check for a short to ground?

Use ohm function and check for continuity

Ground the black to the bus and then attached red lead to socket pin, move wire around and see if you can get continuity, if so the wire is chafed and touching body?
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Old 12-22-2014, 12:01 PM   #8
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I was able to find the short yesterday, one of the screws on the lower lights rubbed the insulation of the wire over time. However, now I'm not getting power from the lower brake lights and license plate light up to the main tail lights. I guess I'll see if I can figure this one out today.
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Old 12-22-2014, 03:32 PM   #9
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Good to hear....good luck
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