Troubleshooting 4-Pole Wiring Harness That Does Not Show Brake Lights Illuminated

Question:

I recently installed a trailer wiring package from you on my 2010 VW GTI. Everything seems to work well except that the brake lights are not observable when the running lights are on. Is this the standard for a 4-pole trailer connection or is something wrong?

asked by: Henry H

Expert Reply:

Thank you for your recent hitch and wiring harness purchase.

On a 4-pole harness like # 119250KIT the running light signals are carried on the brown wire and the stop/turn signals are combined and carried on the yellow and green wires. These are the connections you made at your taillights with the quick-splice fasteners include in the kit.

If you do not see the brake lights on the trailer you can check whether you get turn signals on the trailer. If you get turn signals but no brakes lights then this tells us that the connections are okay. If the connections were bad you would get neither signal. If the turns lights work but the brake lights do not then you could have a weak ground.

A weak ground can prevent the harness from delivering its full output. Check that the harness' white ground wire is firmly affixed to a clean bare metal spot on the vehicle. You can try sanding off any paint/primer and cleaning off any dirt/grease at that grounding point since these things can prevent solid electrical contact. The ground really needs to make bare metal to bare metal contact.

If making a new ground corrects the issue then I suggest protecting that connection with dielectric grease like # 11755 which will help keep out water, dirt, salt and contaminants that could degrade the connection over time.

expert reply by: Adam R