Turn signals make trailer LED running lights flash

[Matt42]

My work truck is a 2004 3500 CTD with the trailer towing package. It has an odd problem that I've narrowed down to the truck itself, and not a sneak circuit in the trailer's wiring. The trailer, as well as the drop-on cap, both have LED stop-turn-tail lights. Both the drop on cap and the trailer use the truck's separate circuit intended for trailers. (I think that it's operated by one or more relays under the hood.) When I signal for a turn in either direction, it causes the LED tail light on the other side of the trailer and cap to also flash, but only at the brightness of a tail light. The running lights (marker lights) also flash similarly.

(The LED turn-stop-tail lights use a circuit that reduces the voltage by half for the running, or tail lights for the lower light output.)

Has anyone else seen this problem? I haven't had an opportunity to plug the trailer into another truck because there isn't one available. I'm looking for suggestions! Many thanks.

 

[Prairie Dog]

Sounds like a weak ground. Have you run a dedicated ground to the cap and checked the one to the trailer? You can't rely on the clamps or the hitch.

 

[RikDavis]

Almost always weird light problems will come back to a bad ground somewhere

 

[Matt42]

Yes. I've run dedicated grounds in both cases. The trailer has a dedicated ground with multiple wires straight to a stainless steel ground bolt at the tongue. The wire connectors are eyes in which the connectors are soldered and protected with dual layer marine heat shrink tubing with heat-activated sealer on the inside. That in turn has a wire going to the 7 way Pollack connector that goes to the truck. The cap is similarly grounded. In both cases, I tested the ground system by running a 16 gauge wire straight from the common lighting ground wire to first a battery negative terminal and then to a ground point adjacent to one of the truck batteries. Doing so had no effect on the problem.

That's why I'm so puzzled. I'm hoping someone will say something like, "Oh, yeah, I had the same problem when a moth got into one of the relays and a can of bug spray fixed it," or something like that. What I'm afraid of is that I'll have to crawl under the truck and look for a place where three wires have chafed together and caused a sneak circuit on the truck. But after ten years of driving it in weird places, it's filthy down there, even though I try to wash it whenever I can.

 

[Matt42]

Double post. Sorry.

 

[ScottLewis]

If it only happened with the trailer connected, your problem is in the trailer, or in the trailer connector itself. A miss-wired plug or socket could be the problem but my guess would be a bad ground somewhere on the trailer that makes the current feed back through the running lights when signals are applied.

 

[RikDavis]

Since these are LED lights it is possible that if there is moisture and maybe trash, spider webs etc on the backside of one of the connectors that there is enough leakage for the lights to operate. Very little current is needed

 

[Matt42]

Weirder and weirder. All the suggestions so far have been things I have looked at. The problem with the LED lights on the truck bed's cap is there even when the trailer is not connected. But the spider web, dirt, and crud idea on the truck connections is one I have not yet checked.

 

[Ozymandias]

Try the Trucks connector with a standard Bulb, sometimes the TIPM doesn't like the like LEDs.

That flashing of your bulbs could be the check voltage from the Tipm that is seeking a bulb, just little impulses.

 

[Hoefler]

How is the truck cap wired into the system? Do you have an aftermarket fifth wheel/gooseneck harness that plugs into the trailer connector behind the bumper plug?? I have had 3 of those harnesses get water in them and corrode the splices and cause all kinds of weird stuff. LED's will only work when wired correctly, if the polarity is reversed, they usually don't work, so this eliminates a bad ground. I would look for a bad connection in the plug or the junction box if you have one. Sounds like a stray strand is making connection some where.

 

[Matt42]

Well, it turned out to be both strange and simple at the same time. The cap uses a logic module to make the high center brake light work. The logic module was feeding back somehow and giving the running light circuit just enough power to flash with the turn signals. As I was checking the truck's circuits, I accidentally disconnected the logic module, and the problem went away. I have a new logic module on order. It's a heavy duty version to replace the light version. If that doesn't do the job, I'll run a line from the truck's CHMSL system. The logic module, if it works, keeps me from having to crawl under and search for the right color wire to tap into.

Thanks for the ideas. You all gave me ideas to think about in the shower, which is where it all came together in my mind.

 

[RikDavis]

The wiring connection for the high center brake light ends in the wiring harness by the back bumper and is usually doubled back and taped. It was easily found on both the 99 and the 2013. Just a quick glance should find the factory supplied lead

 

[crispyboy]

Just as RikDavis said there is a specific wire for the cap brake light - no need for the module. I wouldn't let the guys at the truck cap dealership tap my trucks electrical wiring. My brother bought his truck cap at the same dealership and got the hack wiring job complete with Scotchlock connectors........ I had to go back and correct the wiring job for him. I also installed both LED lights and switches in both truck caps.