I've been having an intermittent issue with my left rear brake light/turn signal not working. Usually it resolved itself so I didn't pay much attention to it. However, now, it seems to be permanently broken now, so I spent some time investigating today.

Found that there's no voltage at the turn signal harness. Removed the tow harness pigtail, no change. On the other end, hooked up a test light to the BCM outputs, both the yellow (rear left) and dark green (rear right) leads blink when the turn signal is activated.

Reading the manual, it seems like BCM fuse 5 is used for both front and rear turn signals, so I'm pretty sure I've got an open somewhere in the left rear turn signal voltage line. I had re-routed all the turn signal wires inside the conduit at the top of the roof, so going to be a fun time pulling that all out.

Curious - what's the best method for finding an open in really long wire like this? Pull the whole harness + tape and inspect for anything obvious? What if I don't find anything obvious?

Would it be better to run a new wire instead of trying to salvage/repair the old wire?

EDIT: 2013 Chevy Express 2500

 

with a tracer



but wire faults can be somewhat predictable. They're usually:

• at connectors
• spots where the harness moves (like door hinges)
• places were it makes a bend, or chafes or gets squished, or where modifications were made
• where moisture gets in
• ground points.

 

the lamp assemblies plug into a connector at the very rear of the vehicle on the column. That's the first place I would check .
The rest is a solid run to the X2 connector on the cabin fuse block.

 

Since it's an extended van, it's like a 20 foot cable run between the BCM and the only connector. Harness shouldn't move (doesn't go through door hinge or anything like that), nor should it be exposed to moisture. I don't think it's a ground point issue since it's the 12V supply wire.

However, I did re-route the entire harness inside the upper conduit along the roofline, so it's likely it's somewhere along there. But still need to find it, since that's like a 10+ foot section, and none of the other wires are giving me trouble. So the tracer should be very helpful for that. Thank you for that tip!

 

If you're not aware . Complete wiring diagrams including harness routing drawings and locations of all connectors are on gmupfitter.com

 

It was the fuse box. The two 10mm bolts holding down the fuse box were loose. I could reproduce the issue by rocking the fuse box around and the tail light would flicker. Didnt have much luck with the signal tracer, but rattled wires the old fashioned way until I found the issue