Troubleshooting New Trailer Tail Lights on Utility Trailer Towed By 2002 Ford F-250  

Question:

Expert Reply:

If you recently installed new trailer tail lights and you are having trouble with the brake light circuit functioning I suggest checking into the grounding for the lights. Some lights use their mounting posts to make the ground connection; others will use a separate (usually white) wire with a ring terminal; still others may use their hardware to establish ground. Either way, you need a good solid connection to bare metal.

Sometimes a weak ground may let one light function work but not another. Often the tail light will be the function that does not work since this circuit draws more current than brake lights or turn lights.

You can test the trailer on its own, independent of the truck wiring, simply by applying 12V battery power and ground directly to the trailer's two brake/turn light circuits. In trailer wiring the brake and turn signals get combined onto one wire for each side of the trailer, usually a yellow wire for left stop/turn and a green wire for right stop/turn. You can apply power to each, one at a time, to see if they work. If they work with this direct power then this tells you the problem is likely to be on the vehicle side, with its wiring or its trailer connector.

If you think the problem is on the truck, then a circuit tester like # PTW2993 will allow you to test for voltage on the truck's connector pins.

You could also have a corrosion issue on or inside your truck or trailer connectors. Discoloration on the contacts can indicate corrosion that cold block full signals from reaching your trailer lights. But sometimes the corrosion builds up inside the connectors where it can be hard to see. For a new 7-way socket for your truck you can use # HM40975. For a new 7-way plug for the trailer you can use 7-way Junction Box kit # HS381-9000 along with ring terminals # 44-5310A.

expert reply by: Adam R

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