Troubleshooting Dimming Trailer Lights When Braking And Using The Turn Signal

Updated 04/18/2024 | Published 04/15/2024

Question:

I have a 2001 Suburban. All of the car lights work. When hooked to a trailer, the trailer turn signals work, but when the brake is on with the right turn signal, the left brake light goes out. If the headlights are on, with the right turn signal, the left brake light dims. This happens on two different trailers.

asked by: Doug G

Expert Reply:

Hi Doug,

Based on the symptoms you're describing it just sounds like you're dealing with a lack of power. Everything works until you start putting too much of a load on the harness. Does your Suburban have a factory tow package/7-way or did you have to install one aftermarket? Older vehicles used to pull power from the vehicle tail lights to power the trailer lights and ended up drawing too much power and causing the lights to dim. In this kind of situation you can either replace the wiring with a harness that runs up to the battery for power or you can replace the lights on the trailer with LED's to lower to demand to something the harness can handle.

Swapping out the lights will likely be both the easier and cheaper route to take and will end up being more energy efficient. Let me know what you think and we can move forward with whichever route you'd prefer.

expert reply by: Kevin C

Doug G.

4/16/2024

Thanks for the reply Kevin! Yes, the Suburban has a factory tow package with a 7 way plug. The plug itself and the trailer brake are after market. The trailer lights are already LEDs. Can you tell me more about the harness you spoke of? Thank you, Doug

Kevin C.

4/18/2024

@DougG Trailer wire harnesses used to tie into the tail light wiring and take their power from them for the trailer lights to function. This caused overloaded circuits and dim lights so manufacturers switched to running a power wire directly from the battery. With the age of your vehicle it's possible that's the same case for you but with there being a factory 7-way connector it'd make more sense for it to run to the battery as well. Most likely it's an issue with the 7-way that replacement would solve but it's not a guarantee. Have someone sit in the driver's seat and test the pins on your 7-way with a multimeter # PT89ZR for each function to see how much voltage you're getting. Each function should send 12 volts back to the trailer. If you're not then there's likely an issue with the 7-way and you can replace it with # C55774. Next you can hook up the trailer and test the voltage back at the lights to see how much voltage they get. If you're getting less than 12 volts then I recommend checking the ground first. A weak ground connection can cause dim lights or loss of function. If wire gauge is too small for the length of the run from the power source (junction box/7-way cable on trailer) to the lights then you could be seeing voltage drop and would just need to rewire with a thicker gauge. Your symptoms sound more like it's an issue with the 7-way on the vehicle though since it's happened with two different trailers.

Doug

4/18/2024

Thank you Kevin. I'll follow your advice and check the voltage at the 7 pin plug. Take care, Doug