Troubleshooting Trailer Lights That Fail Once Vehicle Headlights Are Turned On

Question:

I have a 1987 Dodge Dakota with a trailer that I just replaced the lights on with a set of LEDs. The trailer lights work fine without the truck headlights on theyre actually better now than they were before I replaced them, but if I turn the headlights on I get nothing at all at the trailer. The truck lights still work fine with the headlights on. Would a load equalizer help with this scenario, or should I look at my battery/alternator?

asked by: Jason L

Expert Reply:

I suggest you check the main ground for both your recently-rewired trailer and your Dakota since a weak ground is among the more common causes for the issue you describe. Since headlights/running lights remain on continuously they draw more current than stop or turn lights that are on only briefly. This is why ground issues tend to show up most often on the vehicle or trailer running light circuit.

Make sure the main ground wires for both the trailer and truck are affixed to a clean bare metal spots on the frames. If there is rust, grease, primer or paint at the grounding point there may be enough of a connection to allow those other lighting functions to work properly but not enough for the headlights/running lights.

Since LED lights use much less power than standard incandescent bulbs it is unlikely that you are now overloading the vehicle wiring unless the new lights present a total power demand that is greater than that for the original lights. Load resistors are usually required only on LED turn signal circuits.

If you have a circuit tester like # PTW2993 you can disconnect the trailer and test the truck wiring directly to see if the running light signal is present on the trailer connector when you have the lights turned on.

expert reply by: Adam R