Still issue with taillights

ERIC MCLEOD

MEMBER
2008 FORD F-150
5.4L
V8
4WD
AUTOMATIC
143,000 MILES

I was talking with Roy about his issue before on here. Replaced the indicator switch, and ordered a used tail light harness as ford discontinued the item from their parts list. Didn't think any aftermarket places carried them. I think the used part had the same issue as the original. When lights are on, running lights are real dim. Hit the brake, the 2 on the bed go out, yet the third brake light comes on like normal. But, during the daylight, brake lights work as do the turn signals most of the time. Had a mobile mechanic look at it, and he is stumped as well. Before I take it to a dealer or an ASE mechanic, is there some real sound advice I can get? When unplugging the harness, from power there's 13 volts, plugging the lights back in l, it drops to 7-9 volts. Using both the new/used harness and the original.

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

You have resistance in the wiring or a component. Basically you proved that out when you plugged in the lights and the voltage drops to 7-9 volts.

Where did you measure this voltage from?

Was this at the bulb?

When you drop voltage like that, there is basically an "obstruction" (resistance) in the circuit that requires current to get past in order for it to reach the ground. When there is only one load in a circuit, it uses all voltage except that needed to get to finish the path to ground. When you add another load like high resistance in a circuit, it takes power from the load so that the current can still get to ground. You only have 12 volts in the circuit so instead of the bulb getting 11.8 volts of it like normal, it now only gets 7-9 because something else is stealing 3 -5 volts. More than likely this is a connector or a wire, or a loose ground.

If it was then we need to start measuring resistance on each wire from the bulb to the end of the circuit. I would try to break it down to wiring portions so measure from the bulb to the central junction box. The light green/orange wire and orange/light blue wire should have about a half ohm on them from end to end. Then move to the same wires but from the central junction box to the multi function switch.

Lastly you need to make sure the ground is clean and tight plus ohm out that wire as well. Again, you should have less then a half ohm on this ground wire.

I doubt that a used harness had the exact same issue as yours so I would think the wiring issue is still on the truck.

I suspect your issue is in the highlighted wiring from the multi function switch back because that is where you are seeing the issue. The high mount third light is not effected because it has its own wiring from the switch to the bulb. I highlighted that as well just to point it out.

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