Truck/trailer Light Problems

59ih340 · October 3, 2008

I have an '03 GMC that I really have not been able to get the trailer lighting to work properly. I don't think it's the trailer I have not had problems towing it with other vehicles.with the trailer hooked up:running lights appear on normal (lights off if I hit the brakes all the lights on the trailer come on; with the left turn signal on ALL the lights flash(Lturn,Lbrake,Rbrake,L/R front turn sig.on truck,all trailer lights),Right turn signal works normal unless headlights are on Then the Brake Controller also comes on ...Unhook the trailer and the truck goes back to "normal" whatever that is.Any ideas? I really need to use the truck/trailer together but can't like this.is it the turn signal relay (the $100 one)I really need some direction on this one Thanks for any ideas.

Zembonez · October 4, 2008

Are you SURE the trailer wiring is OK? If so... who installed the trailer wiring plug on the truck? There is obviously something crossed up somewhere. Hard one to troubleshoot in here I'm afraid.

59ih340 · October 4, 2008

Are you SURE the trailer wiring is OK? If so... who installed the trailer wiring plug on the truck? There is obviously something crossed up somewhere. Hard one to troubleshoot in here I'm afraid.

The trailer wiring is the same as it's been I bought the trailer brand new it's onlt been used a handfull of times and I have used it with 2 other trucks since I've had it-No problems.The wiring on the truck is from the factory(2500hd tow package). If that helps.

csheatz · October 4, 2008

wow, you turn the right turn signal on and the brake controler comes on? thats a new one! That is just ephed up! Id be pissed to.

I would say its in the truck wiring, are you the first owner of the truck? Some may have had some wiring done to use a pickup bed camper or a 5th wheel, and some how screwed up the wiring either when it was done or when they "tryed" to put it back together before selling it. I would check the wires going into the plug at the back of the truck. That is where i would start and work to the front from there!

c4cruiser · October 4, 2008

Have you checked the individual contacts at the truck 7-pin connector? Use a circuit tester or lamp to verify that you have power at the tail lamp pin, the left and right turn signal pins, the 12v power lead, the ground lead (need an ohm meter here). Does the test lamp flash at each of the turn signal pins? Does the lamp come on when the brake pedal is depressed? What does it do when both a turn signal is on and the brake is depressed?

Check the trailer harness for a good solid ground from the white wire to the frame of the trailer. Sometimes corroision or rust at that connection will cause problems. If the trailer lights work properly when connected to another truck with the same type of connector, then you can most likely leave the trailer wiring alone.

Having the brake controller "come on" is strange. That sounds like a backfeed issue in the wiring. I agree that you should check for extra wires, another harness, or possibly some wires shorting inside the plastic harness cover.

ianfaber1 · January 25, 2010

I currently have an 03' Silverado and am having a similar problem. But mine is the opposite, when I have a trailer hooked up my left turn signal works fine. But, when I do not have a trailer hooked up and I turn on my left turn signal my 4 ways come on. At first I could just hit the 4 way button on for a second and the left turn signal would work fine. It has gotten worse over the last month or so. I am currently looking for any other suggestions. I will check the ground for the trailer lights.

700 sportsman · January 25, 2010

Ian, I'd start by checking to make sure you have a good ground on the trailer, then I'd go pin by pin with a test light on the truck's connector, if that's not the problem, I'd say you have a wire rubbed bare on the trailer causing a short

c4cruiser · January 25, 2010

Sound like it's more of a short somewhere. A bad ground would cause the light to not work at all. A current backfeed into the turn signal system could be doing it.

grose67 · January 25, 2010

My nephew just went through a situation like both of the problems listed here. It turned out that a couple of wires had pulled loose from the multifunction switch in the column.

GPSilverado · January 25, 2010

I have had some very weird issues with triler lights over the years when ground circuit isn't working. I would check this first by taking a set of jumper cables, and connecting one clamp to the truck on a good grounded surface, and the other end of that cable to the trailer on a good ground. If the problem is still there, then definitely start at the connector on the truck, making sure that the signal is on the expected pins. If everything is good there, then the issue has to be with the trailer.

I have had issues where the truck wasn't wired to the same pattern as the trailer. I think it was my 2000 Ford that I had to rewire due to the center post being prewired for the wrong thing.