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Help me fix my rear lights! GMC sierra

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Friggen truck, all my rear lights except for the cargo light do not work. First it was just the break lights, but the turning and always on lights were fine. I pulled out the rear right light housing to check to see how the brake bulbs were, they were fine, and put it back in and now all the lights don't work!

Here's the really really weird part.

I unplugged the harness to the rear lights and checked my turning light speed, and it blinked really really fast, indicating that I had a burnt out bulb. But when I plugged in the harness the turning light indicator began to blink slowly again, like nothing is wrong, but none of the lights are working!

*** is that all about?

If the lights aren't working, why does the turning indicator retain the same speed as if they are working?

I checked the harness and the jackass kid that I bought it from had done something down there, he had a couple wires grounded to the frame or something cause they are stuck in a bolt for a poor connection.

I tried keeping the wire back on the frame (ground) but the lights still do not work.

Fuse inside the driver cab is good, I checked it with a volt meter..

What the hell went back in 2 minutes?

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Friggen truck, all my rear lights except for the cargo light do not work. First it was just the break lights, but the turning and always on lights were fine. I pulled out the rear right light housing to check to see how the brake bulbs were, they were fine, and put it back in and now all the lights don't work!

Here's the really really weird part.

I unplugged the harness to the rear lights and checked my turning light speed, and it blinked really really fast, indicating that I had a burnt out bulb. But when I plugged in the harness the turning light indicator began to blink slowly again, like nothing is wrong, but none of the lights are working!

*** is that all about?

If the lights aren't working, why does the turning indicator retain the same speed as if they are working?

I checked the harness and the jackass kid that I bought it from had done something down there, he had a couple wires grounded to the frame or something cause they are stuck in a bolt for a poor connection.

I tried keeping the wire back on the frame (ground) but the lights still do not work.

Fuse inside the driver cab is good, I checked it with a volt meter..

What the hell went back in 2 minutes?

The turn signal flasher speed has to do with the amount of current going thru it, it just so happens that if a bulb blows not as much current passes thru the flasher and it cools off faster, closes the contacts faster and this results in a faster indicator speed. If there is another load going thru the flasher but that load is grounded or passes thru another consumer before it reaches the bulb it will slow the flasher back down to normal speed.. It sounds really weird what you have going on and will be difficult to diagnose without looking at it and knowing what other BS has been wired in..

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Thanks for your reply, I'm starting to understand how this all works.

I did some more checking around and found that the fuse that was installed by the idiot kid that owned this truck before me, was a 30 amp and not a 15 like it supposed to be.

The fuse had obvious heat marks around the plastic, but the fuse was still in tact....

Because he used a fuse that was designed for something of a higher amperage, and because this circuit has wires that can only handle 15 amps, there is a short somewhere along the wiring harness to the rear lights.

The fuse that is in question is

"park lps"

It is the only fuse that is suspect and it must be related to the rear lights.

Anyways I now have to try and find the short along the whole truck from the cab to the rear lights.. Keep in mind that the harness is wrapped and it will be pretty damn near impossible to find the short..

****!!!!!! :thumbsup:

Are you serious?????? Wow, I want to smash this kids nose in, what a ****tard, I can't believe some people can be so friggen stupid... It's good that I only paid 900$ for the truck, but seriously I might be looking at a new truck if an electrician wants to charge me up the nose for this job.

A new truck because of a short... wow.

If someone can help me here, maybe it's not as bad as it seems. If the fuse took the load and the wires did not, where is the most likely place that the plastic on a wire would melt off? Or could it be anywhere?

Thanks so much for the help!

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does it have a trailer hitch? I have seen aftermarknet POS converter boxes that go along with hitches take out the rear lighting.

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does it have a trailer hitch? I have seen aftermarknet POS converter boxes that go along with hitches take out the rear lighting.

It has a plug in on the bed for a trailer, I found the optional harness that comes with the truck tucked in behind the wheel. The lights were wired into it so I checked the circuits.

All the connectors for the rear light grounded out with the optional harness attached, without it attached, only the ground grounded out!

Success, so I thought, so I took it out and hooked up the rear lights with just the main harness running to the front.

I got my reverse lights back, turn signals back, but still no brakes, OR running lights...

Because the running lights and the brake lights (they use the same bulb) do not work, it's not an individual switch like the brake switch on the brake pedal for example, two separate circuits.

The fuses check out, I don't know what it is.

I do have a brake light built into the cargo light, and that is working fine, so at least people KNOW that I'm coming to a stop on the highway. But without running lights they may not know I'm there when it's dark lol, so still not safe or legal for that matter.

Kind of confused as to where to start, but tomorrow I think I'm going to clean the harness connections and start from there. If that doesn't work **** I dunno... Any pointers are appreciated.

Thanks for pointing that out though, as it was part of the problem.

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ok so you say they were grounded out....have you double checked all the fuses? if the fuses are ok then i would reccomend doing a voltage drop on the power wire to the tailights/brakelights. to do this run a long jumper to the battery and hook that to the ground side on your DMM then take your other lead and with the headlights on back probe the closest connection for the taillights. the voltage should read below .5v, if it reads anything above .5v then move the the next connection back, this works great for figuring out bad portions of the circuit. this test also works for the ground side as well, just use the negative battery terminal and the ground side wires. good luck

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the tail light panel that all three lights(per side) plug into are always burning up I cant count how many i have replaced! Also make sure you have the proper bulb many will fit and work for a limited time till they burn out or burn out wires

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Just an update.

I put this off for a while, trying to only drive when the sun was up so that I wouldn't get busted.

Finally just last weekend I went to the yard and decided to get the whole harness front to back and install it. As easy as I thought it was, it wasn't.

The harness not only controlled the lights, but it also controlled the fuel pump, and after about 30 minutes of trying to drop the fuel tank at the yard I said **** it and cut the wire.

Then I couldn't fish the harness out from behind the fuel tank cause the connector on the rear was too big to fit through, so I cut the harness in half.

Inside the cab was a special hub to connect all the harnesses from the engine bay and the rear lights etc, I could not for the life of me just disconnect the harness for the rear lights out of this cluster, so I cut it.

So today I Bought some connectors with shrink wrap built in and started to mend the harness together where I cut it. Instead of pulling my old one out, I left it in cause I didn't want to drop the tank and left the old harness with the only the fuel pump wires connected to the firewall. The other wires for the lights I cut, spliced and used the connectors.

It was LG, DG, Y and B for the rear lights, all the rest of the wires were for the pump so I left them alone. I used zip ties to connect this new harness along side the old one and ran it throughout the frame of the truck to the rear. The new harness stops at about the differential so that you can connect the final rear light harness, so I did that and pulled out the old one. Put in the new one, connected it all up and put in a new 15A fuse in my 20A slot (bought the wrong fuse, but at least they are lower and not higher).

THEY WORK!!!!

Yea, I was friggen stoked. An hour and a half in the sleet splicing and crimping and running wire through dirt and mud with my hands freezing paid off.

Both the old harness's must have just been toast, and I spent the time and did a good job where these connections are sealed really well, waterproof and then wrapped up in a nice clean bundle of electrical tape.

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Just an update.

I put this off for a while, trying to only drive when the sun was up so that I wouldn't get busted.

Finally just last weekend I went to the yard and decided to get the whole harness front to back and install it. As easy as I thought it was, it wasn't.

The harness not only controlled the lights, but it also controlled the fuel pump, and after about 30 minutes of trying to drop the fuel tank at the yard I said **** it and cut the wire.

Then I couldn't fish the harness out from behind the fuel tank cause the connector on the rear was too big to fit through, so I cut the harness in half.

Inside the cab was a special hub to connect all the harnesses from the engine bay and the rear lights etc, I could not for the life of me just disconnect the harness for the rear lights out of this cluster, so I cut it.

So today I Bought some connectors with shrink wrap built in and started to mend the harness together where I cut it. Instead of pulling my old one out, I left it in cause I didn't want to drop the tank and left the old harness with the only the fuel pump wires connected to the firewall. The other wires for the lights I cut, spliced and used the connectors.

It was LG, DG, Y and B for the rear lights, all the rest of the wires were for the pump so I left them alone. I used zip ties to connect this new harness along side the old one and ran it throughout the frame of the truck to the rear. The new harness stops at about the differential so that you can connect the final rear light harness, so I did that and pulled out the old one. Put in the new one, connected it all up and put in a new 15A fuse in my 20A slot (bought the wrong fuse, but at least they are lower and not higher).

THEY WORK!!!!

Yea, I was friggen stoked. An hour and a half in the sleet splicing and crimping and running wire through dirt and mud with my hands freezing paid off.

Both the old harness's must have just been toast, and I spent the time and did a good job where these connections are sealed really well, waterproof and then wrapped up in a nice clean bundle of electrical tape.

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