How to Test a 4-Way Trailer Wiring Harness Plug


I installed a 4 way connector to a 4 way-7way adaptor on my 2000 Mercury Mountianeer. When testing the connector, the test light flashed for the left and right turn signals, but when I ed the tester probe into the 3rd female plug.and applied the brakes, the test light does not go on. Am I doing it correctly ? I assume this is for the brakes. Can you provide some guidance.

Expert Reply:

You are testing correctly for the left and right turn signals, but the 3rd female plug you are testing is the taillight circuit. If you turn on the running lights you should get 12 volt power at the connection that the brown wire runs into.

To test the brake lights at the trailer connector you need to apply the brakes and test the connectors that the green and yellow wires go into. These are the same connections that you tested for the turn signals.

Trailer wiring combines the left turn/brake and right turn/brake signal for proper operation of the trailer lights. I have included a video on trailer connector testing for you to review.

The correct wiring for the 2000 Mercury Mountaineer is the T-One wiring harness, # 118345.

expert reply by:
Robert G

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