Locating Brake Wire on Horse Trailer to Resolve Brakes Locking Up

Updated 03/19/2019 | Published 03/17/2019

Question:

Im working on the wiring on my trailer. And it has a7 blade plug. My pu has a blue wire for the trailer brakes but there is no blue wire on the trailer. Which wire controls the brakes. The trailer brakes lock up when i try and move it . can you help.

asked by: Jim M

Expert Reply:

The brake wire on a trailer is typically blue but this can certainly vary as trailer manufacturers tend to change up the colors they use at the factory and a previous owner may have also re-wired it using different color wiring. This is why it is always best to either try and trace the wiring back to its connection or test them by applying 12V power to the wire when trying to determine its function.

The brake controller output pin on a trailer's 7-Way should be at the 5 o'clock position when looking at the inside of the connector (see attached diagram). You can try opening up the connector to see which color wire is attached at this location or simply start back at the brake assemblies to see what color wire is going from them up to the front of the trailer. Otherwise you will want to apply 12V power to each of the wires (while unhooked from the tow vehicle) to see which one engages the brakes.

If the brakes are locking up whenever you are hooked up to your tow vehicle I also recommend testing the brake output pin on the vehicle side 7-Way with a circuit tester like # PTW2979. On the vehicle side this will be the 5 o'clock position on the outside of the connector. Test it to make sure it is only getting power when applying the manual override on the brake controller. If it is getting constant power then the issue is actually on the vehicle side.

If the vehicle connector tests out fine then I recommend inspecting the brake wire on the trailer once found for any exposed or pinched areas that need to be replaced. Check the connections at the brake assemblies as well and make sure the ground wires are attached to a clean bare metal surface on the trailer.

If you are still having issues please let me know and we can continue to troubleshoot.

expert reply by: Chris R