No power to main taillight harness plug

[Mbettez]

So I’ve searched for my specific issue all over the place hoping not to make another “no lights” thread but nobody seems to have had it and electrical troubleshooting is far from one of my best skills.

On my 2000 Chevy c3500 All 3 brake lights, rear signals, rear markers, and plate lights are all out, yet all front lights work just fine, the third cab brake light I can live without for now but everything else is obviously important.

Using a multimeter and test light I’ve verified there’s no power going to the main 4 wire plug that the whole taillight harness plugs into. All my fuses are 100% working, I’ve got power on both sides of every fuse, and I’ve changed them out for new ones just to be sure, and I swapped out the multifunction switch with a known working unit, to me this sounds like a ground issue im just not quite sure where to look, I’ve followed the harness all the way up to just under the under-hood fuse box where there’s another connection but I can’t seem to see where it goes after that.

Ive also got a 99 Yukon backup daily that I can swap parts out to troubleshoot if need be.

 

[Drunkcanuk]

So I’ve searched for my specific issue all over the place hoping not to make another “no lights” thread but nobody seems to have had it and electrical troubleshooting is far from one of my best skills.

On my 2000 Chevy c3500 All 3 brake lights, rear signals, rear markers, and plate lights are all out, yet all front lights work just fine, the third cab brake light I can live without for now but everything else is obviously important.

Using a multimeter and test light I’ve verified there’s no power going to the main 4 wire plug that the whole taillight harness plugs into. All my fuses are 100% working, I’ve got power on both sides of every fuse, and I’ve changed them out for new ones just to be sure, and I swapped out the multifunction switch with a known working unit, to me this sounds like a ground issue im just not quite sure where to look, I’ve followed the harness all the way up to just under the under-hood fuse box where there’s another connection but I can’t seem to see where it goes after that.

Ive also got a 99 Yukon backup daily that I can swap parts out to troubleshoot if need be.

There are factory service manuals available for download in the engine and performance section. They are in the sticky section. They have pretty good trouble shooting guides in them.
Sounds like a broken wire somewhere, and that can be beyond frustrating to track down, I know.

Have you tried making your own ground?
Like splicing into a known ground wire, and screwing into a clean section of the frame?
Not 100% certain that's the right thing to do, but it makes sense in my brain.

A

 

[Cadillacmak]

So I’ve searched for my specific issue all over the place hoping not to make another “no lights” thread but nobody seems to have had it and electrical troubleshooting is far from one of my best skills.

To help when troubleshooting electrical, always do a three part test to make sure you have a connection to ground with the test equipment. Using the same ground, test know power, test the item you are working on, back to testing know power. This is a pain, but you would be surprised how often a single test is false. I have worked 28 years in the commercial industrial electrical field, and when working on vehicles I have found this to be very help test procedure adapted from the AC work. Using the battery as the know source and ground is very helpful, you can use jumper cables to get to your test location. For example, your at your tail lights, you have 2 or 3 sets of jumps strung out under your truck, hook the ground on your tester to negative on the jumper, test jumper hot, test taillight hot, test jumper hot. Now switch to ohms and test body ground to jumper ground. Now you know exactly what you have and can start finding/solving the problem.

 

[Cadillacmak]

And ground the heck out of your truck, big grounds! 1/0 from battery to frame, 1/0 from battery to engine, 1/0 from frame to body, #6 or #4 from frame to bed. Steel is a terrible conductor for a really clean electrical system, so take the ground all the way back if its a sensitive component.

 

[Mbettez]

I more than appreciate all the replies. The truck has all upgraded 1/0 grounds and I’ve added in a 1/0 ground going from the rear of the frame to the flat spot behind the rear bumper, and I’ve added a 1/0 ground from the battery to the mounting tab of the alternator.

As far as the test light goes that’s a great call but sadly that’s something I’ve already tried too, I ran jumper wires from both the main battery and auxiliary battery to no avail, I actually even tried a 3rd battery under the truck with me while I tested for power just to see if anything changed but still to no avail.

As far as wiring diagrams go I’ve looked around but I haven’t found anything to directly help me figure out where to track this issue down. Thank you all again for your replies.

 

[Drunkcanuk]

There was a post here a little bit back that had a link to the actual physical locations of all the ground connections.

 

[Cadillacmak]

I more than appreciate all the replies. The truck has all upgraded 1/0 grounds and I’ve added in a 1/0 ground going from the rear of the frame to the flat spot behind the rear bumper, and I’ve added a 1/0 ground from the battery to the mounting tab of the alternator.

Try one more thing, pull your light bulbs out and see if you have voltage, if you do, even if a little, 90% chance of a corroded connection or wire.

 

[Mbettez]

Try one more thing, pull your light bulbs out and see if you have voltage, if you do, even if a little, 90% chance of a corroded connection or wire.

Currently no voltage all the way up the frame as far as I could follow