Taillights and Turn Signals Working But No Brake Lights on Trailer

Question:

My current 7 to 4 way adapter allows taillights and signals to work fine, but no brake lights. Does this adapter allow brake lights on a 2 wire trailer to work on an RV with 3 wire taillights?

asked by: Paul C

Expert Reply:

Almost every trailer in the U.S. uses a 2-wire or combined wire tail light setup. This means that the turn signals and brake lights are on the same circuit and use the same bulb in the trailer tail light assembly. Because of this, virtually every trailer wiring adapter is designed to carry the turn signal and brake lights on the same pin circuit, so I would assume that this is the case for your 7 to 4 way adapter.

It sounds like with working turn signals and taillights, that the problem may be with the 7-way wiring on your RV and not necessarily with the trailer lighting itself. There is a very easy way to check using a circuit tester, like # PTW2993 or using a specific 7-way LED Tester like Pollak 7-Way to RV Socket LED Circuit Tester, # PK12741.

When using the incandescent circuit tester, connect the clip to a clean ground and check the 3 o'clock position for left turn and then for brake light signal and the 9 o'clock position for right turn and then brake light signal. If only the turn signals work and not the brake, then the wiring issue is with the RV to the 7-way. The solution if this is the issue, is to use a 4-Pole Trailer Connector, # C59496. This will include the additional wire needed.

If all the functions of your 7-way connector work properly at the tow vehicle, then the problem may be with your adapter, or the trailer itself. Use the tester to check your adapter in the same way you did the 7-way connector. Starting from the end furthest from the male ground pin, put the probe in the first slot to check right turn signal and brake, the next slot will be left turn signal and brake, and closest to the exposed ground pin will be tail lights. Another easy way to check would be to use an LED 4-Way Flat Tester like # I26. If this is working, we can rule out the adapter being the issue.

If in your question you happened to reverse things and your trailer has the 3 wire system, that would also explain the issue with the trailer lighting. If this is the case, you can use the Vehicle to Vehicle Tail Light Converter, # 118158. This will convert from a 2 wire system (a more universal trailer wiring) to a 3 wire system where the stop light is on a separate wire. If that isn't the case, please refer to the trailer wiring article I attached for wiring troubleshooting.

I've linked a couple help articles to assist.