Troubleshooting No Brake Lights on 3-Wire Rear Trailer Light  

Question:

Helpful Expert Reply:

On this light the white wire is ground, the black wire is tail light, and the red wire is stop/turn signal.

If you don't have the turn signal on this one light, the we assume that you don't have brake light on that light either. Since we have tail lights we can assume that the ground is not the problem.

Let's first determine if this problem is on the tow vehicle side or the trailer side. The easiest way is to connect the trailer to another vehicle and see if all of the signals work correctly. If you do not have another vehicle then (with the trailer disconnected) measure the voltage on the vehicle connector. I have attached a trailer wiring FAQ page that will show all the possible tow connectors and which pin is for stop signals and which is for tail lights.

If you don't have 12 volts at the stop lamp wire then the problem is on the vehicle side. You will need to trace that wire looking for cuts or damage.

More likely is that the issue is on the trailer side. Remove the light fixture and disconnect the wires. Check the voltage again. If you don't have 12 volts then you need to trace the red wire back looking for damage.

If you do have 12 volts then we need to test the bulb. You could swap lamp fixtures to see if the problem moves or connect the pig tails to a battery to see if they illuminate.

If after trying these steps you still don't have any luck then email me back again for more troubleshooting.

expert reply by: Les D

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