Why Would Truck's Trailer Taillight Circuit Turn On/Off With Vehicle Gear Selector

Updated 05/20/2019 | Published 05/18/2019

Question:

Should the Tail and Running Lights brown wire always have power when truck is running. Seems like I lose that power as soon as I hit my brake to put into gear forward and reverse. Oddly enough this happens when I’m using my remote start. If I start the truck with the key I don’t see any power there at all. Also, on my tester I have TM, BU, BK, LT, RT and 12V. Which one of these should light up for the Tail / Running Lights? Thanks

asked by: Stephen S

Expert Reply:

Modern vehicles/trucks have interesting running light circuits. Some vehicles have day time running lights that are on all the time which would be similar to having the vehicle running lights activated. This might cause the trailer running lights circuit to be active at all times too but it depends on how closely this circuit is wired to the vehicle lights. Some vehicles will have the running lights turn on or off depending on if you are in drive or park which also could affect the running light circuit.

The circuit on your tester marked TM would be the correct one for the running lights.

You'd want to compare the trailer running lights coming on and off depending on the gear you are in to the lights of the truck as you move the gear selector. Most likely it's just how the vehicle is wired.

expert reply by: Jameson C