Troubleshooting Left Trailer Brake Light That Does Not Work  

Question:

Expert Reply:

If just the left brake light on your trailer does not operate, but all other trailer light functions work properly, then this is unlikely to have anything at all to do with your Prodigy brake controller # 90885. The Prodigy does connect to the truck brake light switch circuit to get its input signal, but this does not affect the trailer's lights as they are on totally different circuits.

If you refer to the linked photo of a 7-way you can see which circuits carry which signals. The vehicle socket pin at the 9-o-clock position carries the left turn and left brake light signals. The brake controller's signal appears only at the pin at the 5-o-clock position.

If just the one trailer brake light fails to operate the problem could be a bad bulb (or damaged LED), a loose/weak ground connection on the trailer at that one light or possible corrosion on the contacts of either the truck's 7-way socket or the trailer's 7-way plug.

The best way to go is to test the vehicle and trailer separately. Disconnect the trailer from the truck and apply a grounded circuit tester like # PTW2993 to the left brake light pin at the 9-o-clock position as noted above. While you have the tester in place on the contact have a helper step on the brake pedal. You should detect a signal on that pin as long as their foot is pressing the pedal.
If you get the correct signal this suggests the problem is on the trailer side. You can check the bulbs, ground and power connection for the bad light.

You can also test the trailer directly by applying 12V power to the male 7-way plug. You will need to apply a ground connection and a +12V connection at the corresponding (3-o-clock) pin on the male trailer plug. If the light works when directly powered in this way, then this tells us that the issue is on the vehicle side. In that case check the left brake light fuse in the power distribution box under the hood.

expert reply by: Adam R

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