Troubleshooting Brake Lights not Lighting On Trailer When Using a 7 to 4-Way Adapter  

Question:

Expert Reply:

On a 7-Way and 4-Way, the brake light function is on the same wire as the turn signals. So if your turn signals are working, then the connection is made, there is just a problem that is not allowing the brake lights to function.

Most likely, there is a bad ground. First check that the ground on the vehicle side 7-Way is attached to a bare metal surface and is free of dirt and corrosion. Next, check the trailer side 4-Way ground and make sure it, too is attached properly and free of dirt and corrosion. At this point I recommend checking the pins on each connector and cleaning them.

Next, check the grounds on the trailer lights. Most trailer lights ground through the mounting studs, so you will need to check those for any dirt or corrosion. Also, if you recently added a plastic license plate holder that attaches to one of the taillights, that, too, could cause a problem.

If you check all the ground connections and determine them to be adequate, then you can move on to testing the pinning of the trailer connectors and adapters. Adapters like the Pollak 7 Pole Round Pin to 4 Pole Flat Wiring Adapter, # PK12742, are wired according to the most used pin patterns used in the trailer and towing industries. I have included a link to a trailer wiring FAQ below that explains how the various connectors are used. However, not all vehicle and/or trailers are always wired the same and a problem like the one you describe may appear.

What you can do is to check the 7-Way round connector on your vehicle for function and either re-pin it according to the 7-Way round vehicle side connector in the FAQ, or re-pin the adapter. The adapter would be difficult since it is enclosed. So it would be easier to re-pin the vehicle side connector and the trailer to the industry standard, which will also help with troubleshooting in the future. You will need a circuit tester like the Quickee # 3808.

Looking at the face of the 7-Way round and using the circuit tester, the functions should be as follows.

12 oclock position = ground
2 oclock position = 12 v battery power
4 oclock position = left turn and brake
6 oclock position = reverse lights
8 oclock position = right turn and brake
10 oclock position = tail and running lights
Center position = brake controller output

On a 4-Way trailer side connector the pins should be as follows.

The covered pin is the ground (white wire)
The next pin is tail and running lights (brown wire)
The third pin is left turn and brake (yellow wire)
The last pin, furthest from the covered pin is right turn and brake (green wire)

With the 7-Way round on the vehicle and 4-Way on the trailer pinned like explained above, and without any bad grounds or other wiring problems, the adapter will work.

expert reply by: Michael H

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