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I am posting here ( am brand new to the forum) to try and help anyone who may have had a similar problem as I have had over the past year with my 2008 Ford F150 4x 4 . First my gas gauge crapped out. About 6 months after that I started having erratic problems with rear tail , signal and brake lights. Voltage was low , brake lights not functioning at times and voltage pulses seen at the upper brake light and at the brake switch when turn signals activated. I tried everything changed bubs, sockets, cleaned up grounds, removed all rear harnesses and connectors and checked for continuity. Just as I was about to change the Multi function switch the truck crapped out and would not start and not take a boost. Finally I had enough and brought it to our Ford dealer . Within 15 minutes they had diagonised the problem . It appears there is a wire harness connector under the floorboards somewhere that had shorted out . This was causing all my problems ;apparently water had gotten into it. I just wanted to let others know that if you are having an assortment of electrical problems with these vehicles and you do all the preliminary testing but still cannot find the problem you may want to keep this in mind. Below is the situations I was experiencing .

2008 Ford F150 Brake/Tail/Signal Light Problems

Situation 1 :

Headlight Switch in off position .

Observations:

All rear signal lights work normally .

Third Upper Light blinks faintly.

Situation 2:

Headlight switch in on position.

Observations:

Tail Lights are dim
Upper (Third) Brake Light comes on.

When turn signal switch is engaged in either direction all rear tail lights , id lights and upper (third) brake light blink fast. Front signals work as normal but they blink fast .

Situation 3 :

Headlight Switch On and Brake Applied.

Observations:

Upper Brake light works

Tail Lights and brake lights do not work.

Both Signal lights on back work but they blink very slowly. Upper Brake light does not blink at this time and seems to function as per normal.

ID Lights do not work.

Situation 4 :

Headlight Switch Off and Brake Applied.


Observations :

Upper Brake Light works as normal

Rear lower brake lights are dull

When signal lights are activated all rear signals and tail lights and Id Lamps blink fast. The signal light on the activated side is slightly brighter than the light on the opposite side.


Situation 5 :

Testing Voltage at Trailer Harness Plug with Head Lights on.

Observation:

Voltage is at 6 volts across each pin .


Other Issues:

Fuel Gauge does not work. When you start the vehicle it deflects past empty.
 
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