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Trailer wiring question

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I just bought a newer pickup (2003 ford f150) that I assume was prewired at the factory for the small 4 plug trailer light. I have a small 5x8 trailer that I wired for my other pickup, which worked fine. When I plug the trailer into the pickup everything works correctly except the brakes. When I step on the brakes the tail lights go completely out. Any ideas on what I should be looking for?

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Sounds like a weak ground. the white should be screwed to the trailer. It also can ground through the ball, but that isn't consistant.

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Sounds like a weak ground. the white should be screwed to the trailer. It also can ground through the ball, but that isn't consistant.

A poor ground or a plugged pilot jet....

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I just bought a newer pickup (2003 ford f150) that I assume was prewired at the factory for the small 4 plug trailer light. I have a small 5x8 trailer that I wired for my other pickup, which worked fine. When I plug the trailer into the pickup everything works correctly except the brakes. When I step on the brakes the tail lights go completely out. Any ideas on what I should be looking for?

First, let me add to the ground comments, make sure that your ground on the trailer is clean, no rust or paint on the trailer where the wire makes contact, and clean up the wire connector where it makes contact.

Next, I'd check the bulbs themselves...it's not common that two bulbs burn out at the same time, but I have seen it happen.

If you have turn signals, you should have brake lights (same bulb and circuit on a 4-flat basic trailer wiring setup).

Next step, I'd check the fuses for the trailer circuit-this panel is found under the hood. #19 and 20 are for trailer brake and turn signal lamps.

fuse05.jpg

fuse06.jpg

If that's all good, then I'd test for power with the brake pedal depressed at the trailer hitch plugin on the green and yellow wires. Should have juice there.

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