Tail Lights Go Dim or Out When Brakes are Applied

Updated 09/09/2020 | Published 01/04/2010

Question:

I rewired my trailer and now have the following problem. Left and right blinkers work as do running lights, however when the brakes are applied on the vehicle the tail lights go out. This is small utility trailer connected with 4 pole flat plug to 2004 ford ranger with factory tow package.

asked by: Richard

Expert Reply:

It sounds like you have a bad ground connection. If all ground connections are not making good contact to bare metal, when the brake pedal is pressed and the tail lights are on, the increased amperage draw will make this problem show up.

You will need to check all of the ground connections especially where the taillights are mounted to the trailer frame, and where the light brackets are mounted if they are bolted to the frame. If a plastic license plate holder is mounted between the left side taillight and the trailer frame this could be the problem.

Once you have all of ground connections checked, if the problem persists, you will need to check for a broken pinched or frayed wire on the trailer somewhere. If this is not the case then you will need to use a circuit tester, item # PTW2993 if needed, and make sure your wire connections have been properly made at the taillights.

expert reply by: Robert G

Steven P.

9/7/2020

My 97 Tahoe when I hook up my trailer all the lights work fine until I turn on headlights then the trailer lights dim. Thanks,

Jon G.

9/9/2020

You will want to walk through the same troubleshooting steps that Bob mentions above for the running light problem. Here is a link to our wiring troubleshooting FAQ that you can check out.

Helpful Links

faq-4-5-way-troubleshooting.aspx