Trailer lights electrical problem

[Saint3448]

I'm having an odd problem with my trailer lights. I'm using a 2009 Chevy Tahoe as a tow vehicle, both connections are native 7 pin.

When I first inquired about the trailer, it didn't have any axle brakes. On request, the trailer builder (built from his home) said he would put a single brake axle on it.

When I picked up the trailer, I tested out the brake lights and signals, and everything appeared to be working correctly. (It was day time)

Here's what I've discovered:

With the headlights/running lights/any vehicle lights on, the two trailer lights do not function at all. Nada.

When I turn off the vehicles lights, the brake lights work correctly (when the brake is applied), and when I turn on the turn signal, either right or left, BOTH trailer lights begin to flash.

This has GOT to be a wiring problem, but I can't figure it out.

The trailer is wired as follows:
A brown/green/yellow harness comes out of the 7pin...
Brown/green going to one side, (same)brown/yellow to the other. This appears correct?

There is another set of brown/yellow coming out of the plug, which I can only assume is the builders way of improvising an axle brake solution. These two brown/yellow wires go into the left brake and seem to also be present on the other side.

Anyone have any thoughts before I bring the trailer to a shop?

I'm about to leave for a 1700mi military PCS move, hauling through the Rocky Mountains, so I really need to get this resolved.

 

[smokeonthewater]

Re: Trailer lights electrical problem

bad ground... or from your description NO ground... you make no mention of a ground


brown = tail
green = right turn/ brake
yellow = left turn/brake
blue = brake
white = ground or bat neg

you are going to have to trace/ replace/ add wiring to make things right

often the trailer frame is used as the ground but you will still need a ground wire from the trailer plug to the frame

 

[smokeonthewater]

Re: Trailer lights electrical problem

also all circuits on the trailer need a ground... usually the trailer brakes have the ground run along with the positive all the way to the plug and USUALLY each light fixture is grounded via a wire or a screw to the trailer frame but also sometimes a neg wire is run from each fixture to the plug or a junction box

 

[smokeonthewater]

Re: Trailer lights electrical problem

In most states you also need a break away system... for electric brakes that would be a small battery and a switch with a tether mounted on the trailer... then another (black) wire is added which is battery POS to charge the break away batt. from the switch a blue wire is tied into the blue POS wire to the trailer brakes

 

[Saint3448]

Re: Trailer lights electrical problem

bad ground... or from your description NO ground... you make no mention of a ground


brown = tail
green = right turn/ brake
yellow = left turn/brake
blue = brake
white = ground or bat neg

you are going to have to trace/ replace/ add wiring to make things right

often the trailer frame is used as the ground but you will still need a ground wire from the trailer plug to the frame

Thanks for the replies guys! Definitely sounds like a ground issue.

I'm curious though, there are 5 wires coming out from the 7pin connector, one of those MUST be a ground right? Why are there two wires going into the brake axle when all the diagrams I can find only have one going into the brake axle.

 

[Silvertip]

Re: Trailer lights electrical problem

I doubt it's a ground issue since the only lights not working are the running lights. If the turn signals and brake lights work, the running lights have a ground. I suspect the brown wire from somewhere along the route from the front to the back of the trailer is not connected or if it was, a bad connection was made.

The simple test here is to check the fuses in your truck. There are two fuse panels. One in the cab and another under the hood. Look for the tail light fuses. And by the way, do your dash lights work? Don't know if it is still true but on older GM vehicles, the signal that tail light fuse was blown was you also had no dash lights. If the tail light fuse is ok, then break out the test light or voltmeter and check for 12 volts on the connector (truck side) that feeds the brown wire in the trailer harness. If you have 12 volts there, the problem is in the trailer. Follow 12 volts until you lose it.

 

[bruceb58]

Re: Trailer lights electrical problem

It's very likely a ground for sure. The brake lights are getting a sneak path ground through the running lights. That is why when you turn on the running lights, the brake lights stop working. You just removed the sneak path.

Connect a battery jumper cable between the frame of the truck and a bare metal point on the trailer to see if it makes the trailer lights work. If it does, it shows you have a ground issue.

 

[limitout]

Re: Trailer lights electrical problem

also run a ground wire jumper across any non welded or bolted connection of the trailer along the path to the lights such as with a tilt trailer.

its a well known spot to have a bad ground connection at those areas

as to the ground must be in the plug issue it should so trace the wires out of the back and the ground will be connected to your truck frame somewhere within a foot or two of the plug and will "usually" be white in color but not always

 

[smokeonthewater]

Re: Trailer lights electrical problem

The second wire to the brakes SHOULD be ground as I mentioned in post 3 above but you also need a ground for the lights.

 

[Silvertip]

Re: Trailer lights electrical problem

Guess I'm confused about the "two trailer lights" not working with any other lights on. This is a tandem trailer. As such it over 80 inches wide and therefore must have marker lights which there was no mention of NOT working -- just the two trailer lights. So I guess I'm wrapped around the axle with the description of the issue. I'll just bow out.

 

[MAC ATTACK]

Re: Trailer lights electrical problem

I had the same issue on a neighbors trailer. It got so frustrating I did the following;

Very first thing is to put a little grease on the ball and tow the trailer around ensuring a positive connection. Believe it or not, a weak ground from trailer to vehicle will cause light malfunctions.

Unplug the trailer from the vehicle (with trailer hitched) and use a test light to ensure the vehicle connector is providing power to all circuits when activated (turn/brakes/tail). If that checks ok then it is the trailer, if not you have blown fuses for the trailer connection.

Connect harness to trailer and check all lights. If you have any failures, re-do every grounding point to ensure good connection. I re-drilled holes and re-splice new heat shrink connectors.

If you are still having issues, use a tester to pierce the wires starting at the front of the trailer. It could be a broken or damaged wire somewhere down the length of the trailer.

Make sure to remove every bulb and check the connections for corrosion. Sometimes just removing and re-installing a bulb will fix it.

If none of you running lights are working, but the hazard/brake/signal lights are, it is your brown wire going to left/right. Could be just the main connection is corroded at the truck for the running lights or the truck has a blown trailer fuse.

Make sure to wiggle the connector to see if anything changes or blinks.

 

[MAC ATTACK]

Re: Trailer lights electrical problem

Guess I'm confused about the "two trailer lights" not working with any other lights on. This is a tandem trailer. As such it over 80 inches wide and therefore must have marker lights which there was no mention of NOT working -- just the two trailer lights. So I guess I'm wrapped around the axle with the description of the issue. I'll just bow out.

You are correct on the marker lights, but I have seen many on the road with no marker lights up front or in the middle of the trailer in the rear.

 

[bruceb58]

Re: Trailer lights electrical problem

Very first thing is to put a little grease on the ball and tow the trailer around ensuring a positive connection. Believe it or not, a weak ground from trailer to vehicle will cause light malfunctions.

If you are still having issues, use a tester to pierce the wires starting at the front of the trailer. It could be a broken or damaged wire somewhere down the length of the trailer.

Never want to rely on ground through the hitch. Needs to be a ground wire in the harness that connects the ground of the trailer to the vehicle.

Also, never want to pierce wires. That just lets in water and makes the wires corrode and then you have an even bigger issue down the road.

 

[smokeonthewater]

Re: Trailer lights electrical problem

WHS

when diagnosing trailer light issues I ALWAYS leave trailer unhitched from tow rig.... the lights MUST work without the trailer touching the rig or you have ground issues

 

[smokeonthewater]

Re: Trailer lights electrical problem

I just rewired a trailer yesterday... $37 for all new LED taillights, side markers, and wiring... For me (barring obvious damage) there are two possibilities... either it's a ground issue OR I rewire the trailer

This trailer had a problem with blowing the tail fuse in my tow rig.... there were a dozen bare spots inside the frame likely from mice....

 

[limitout]

Re: Trailer lights electrical problem

I just rewired a trailer yesterday... $37 for all new LED taillights, side markers, and wiring... For me (barring obvious damage) there are two possibilities... either it's a ground issue OR I rewire the trailer

This trailer had a problem with blowing the tail fuse in my tow rig.... there were a dozen bare spots inside the frame likely from mice....

yep, its worth it just in time saved to rewire the trailer (20 minutes) rather then troubleshoot and attempt top repair the wiring (maybe hours to track down and often a helper is needed) that might have more issues again next week.

as smoke said, if you have a good ground and you know the lights work then don't waste your time trying to fix it just rewire

 

[bruceb58]

Re: Trailer lights electrical problem

yep, its worth it just in time saved to rewire the trailer (20 minutes) rather then troubleshoot and attempt top repair the wiring (maybe hours to track down and often a helper is needed) that might have more issues again next week.

The OP does not need to rewire his trailer. I would love to see how you rewire a trailer in 20 minutes.

 

[MAC ATTACK]

Re: Trailer lights electrical problem

My intention when telling the OP to keep the trailer hooked for a sure ground was to help in troubleshooting the wires/lights. I have seen on several occasions when troubleshooting without being hooked led to unnecessary repairs. Sounds like enough knowledge on this thread to get him going.

 

[bruceb58]

Re: Trailer lights electrical problem

He can do the same thing as I said in post #7 with jumper cables. Its just a test to see if a good ground helps his issue. The hitch is such a poor ground it wouldn't prove anything.