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2008 F150 XLT tail lights

2008 F150 XLT tail lights always on. Replaced multifunctional switch, brake light switch and cruise cancel switch. Any feedback would be appreciated.

Taillights or brake lights always on?

Do you have a trailer wiring harness? If so, check it for a short.

Symptom: Stop lamps always on

Symptom: Tail/parking/license lamps always on

It seems to be the tail lights. However when I disconnect the headlight switch the lights still stay on. If I turn on the left turn signal, the lights blink on and off. When I turn on the right turn signal, the right tail light will blink once then stop. The left tail light will go out when I have the right turn signal on. I’ll check into the trailer harness. Looked at it initially and did not see any damage or loos wires.

nice troubleshooting tree above...

Quote:

Originally Posted by sjp777 (Post 6089986)

we see this sort of thing on trailer wiring all the time.
You have a broken ground wire -somewhere-.

The bulbs are designed in a way that the tail filament and the brake/turn filament share a common ground inside the bulb. Only one ground wire comes out of each bulb.

what happens next depends on where the ground for the other bulbs are spliced together, and where the broken connection is.

if you have two bulbs with the ground spliced together, and the broken wire is between one bulb and the splice, when power is applied to the "bright side"(brake/turn) of that bulb it can't get directly to ground. Instead, it will jump over to the "dim side" (tail light) inside the bulb, travel through the tail filament, and into the other bulbs that share the power connection for the tail lights.

Thanks for all the input. Found the ground problem si the harness connection between the front anback harness below the driver door along the truck frame. Working well now.