Braking causes rear turn signals to switch sides

[TheNewGuy]

I just noticed a problem with my 1998 GMC Sierra. Say I'm signaling left while driving, all lights indicate that I'll be turning left, but then as soon as I apply the brake my front lights will still be signaling left but my rear signal will switch to the other side, making it appear that I'll be turning right. The same thing happens when I am turning right. Also, when the brake is applied, the turn signal begins to flash faster.

The only work I did on the rear lights after purchasing the truck was replacing the left taillight assembly and a bulb or two on the right side.

I'd really appreciate if anyone has any ideas for what the problem could be.

 

[HotWheelsBurban]

Sounds like the rear wiring harness may be hacked up. Common issue on these trucks, though your problem is not common. But usually with a light issue like this, power is getting where it shouldn't. So there's a wire that's grounding out, or hot wire touching something it shouldn't.

 

[TheNewGuy]

Well, a tow hitch was installed when the truck belonged to one of the previous owners, so it's possible they didn't do the wiring correctly.

 

[HotWheelsBurban]

Well, a tow hitch was installed when the truck belonged to one of the previous owners, so it's possible they didn't do the wiring correctly.

Highly likely, I'd start by looking over all the rear wiring harness. Betcha it's hacked up somewhere or some wires are getting together that shouldn't be.... If the spare is still in place, taking it down out of the way will make things much easier to get to.

 

[TheNewGuy]

My dad suggested looking at the ground wires, so I did. The ground wire that goes to the connection for a trailer was in 2 parts that someone had connected at some point by twisting the ends together. The wires have corroded and loosened since then. I'm going to try fixing it, but I don't have the things I'll need to do it at the moment. Do you think this ground wire would be enough to cause the problem?

 

[0xDEADBEEF]

The ground in the hitch wiring is for the trailer (lights and such), so it doesn't seem like your problem but who knows. Bad grounds can cause all kinds of problems.

 

[Das Hatt]

I just had this PRECISE issue with my truck. Turned out the ground to the frame by the drivers side rear bumper mount was a little dirty. A little emory cloth rub down until it was completely shiny. Immediate improvement.

 

[TheNewGuy]

I'll give it a try. Thanks

 

[Erik the Awful]

My dad suggested looking at the ground wires, so I did. The ground wire that goes to the connection for a trailer was in 2 parts that someone had connected at some point by twisting the ends together. The wires have corroded and loosened since then. I'm going to try fixing it, but I don't have the things I'll need to do it at the moment. Do you think this ground wire would be enough to cause the problem?

Yes.

The selection below will allow you to fix about half the wiring issues on your truck. Some people will hate on heat-shrink butt connectors, but when I was a Nissan tech, they were factory-issued with recalls. Do a tug-test on your crimp before you heat shrink, and if it passes, it's a permanent fix.
Wire Stripper/Crimper - www.amazon.com/dp/B083Z6T85C/
Heat shrink butt-connectors - www.amazon.com/dp/B07Q3JJRY8/
Heat shrink ring terminals - www.amazon.com/Haisstronica-Connectors-Heat-Connectors-Insulated-Terminals/dp/B08HK1Q4F6

If you find that you need a complete rear harness, go to Pull-A-Part and grab a Suburban rear harness. Every pickup harness I've ever seen a PAP had a bunch of hacked-up splices. The Suburbans tend to have intact wiring.

 

[TheNewGuy]

My Dad and I were able to fix the lights. The problem wasn't with the ground itself, but with the wires going to the ground. They'd been cut at some point and whoever reconnected them only twisted the ends together and put a bit of tape over them. We soldered the wires together and put some shrink tubing over them because they were nearly disconnected. We checked the lights multiple times and haven't had an issue since.