Trailer Brake Lights Do Not Function When Brakes Are Applied on Tow Vehicle  

Question:

Expert Reply:

If your Tekonsha Primus IQ Brake Controller # TK90160 is effectively activating your trailer's brakes, but the trailer's brake lights only are not functioning, then the issue is within the vehicle's trailer wiring harness and not related to the brake controller itself.

Very often a bad ground connection at your wiring harness is the cause for one of the lighting functions to fail. It can happen that when the white ground wire from your harness does not make a completely solid connection to a clean bare metal surface it will only be able to pass limited current to the trailer lights, enough for some functions to operate properly but not all. Make sure the ground wires on both the vehicle and the trailer are secured to clean bare metal surfaces. Sand off paint or rust as needed as apply dielectric grease like # LT37534 to protect those connections.

Another common cause for an issue like this is a bent, corroded or damaged connector on either the vehicle side or the trailer side. Check all contact surfaces on both connectors for signs of even slight greenish or whitish discoloration; the contacts do not have to appear crusty to have a corrosion problem. Clean as required and apply dielectric grease as a protectant.

If you have a circuit tester like # PTW2992 you can test the wiring on the vehicle to make certain it is passing the brake light signals on to the trailer harness. Apply the tester to the yellow and green wires leading from the vehicle to the harness; if you detect brake signals on this input side but not on the output side then the harness is bad. If there are no brake signals going in, then the connections from the vehicle wiring to the harness may be bad; try re-inserting the connectors (if it's a plug-and-play type harness) or re-work the splices (if it is a hard-wire type).

You can also refer to the linked article which gives additional troubleshooting tips.

If you are not able to correct the problem please advise the year/make/model/version of your tow vehicle and what wiring harness you are using.

expert reply by: Adam R

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