Hello. I have a 2004 F150 Supercrew Lariat (2004-2008 body style)

Here is what is happening:

If the headlight switch is in the headlight on position, the rear taillights and the license plate lights do not illuminate. The rear turn signals and brake lights still work great. All other exterior lights work fine including the instrument cluster lights.

If I turn the headlight switch to "autolamp" all of the rear lights work correctly but the instrument cluster lights do not work.

I replaced the rear tailight bulbs and checked for blown fuses, no blown fuses.

Any help would be greatly appreciated.

Thank you

 

Just wanted to give an update.

I dug into it today without any success. The rear lights are only getting between 3 and 4 volts when the parking lights are turned on. The parking light switch is putting out 11.7 volts to the parking light circuit.

I took apart the connectors in the harness going from the rear to the front and only got between 3 and 4 volts as well measured to body.

When I turn on the autolamps, the tailights turn on correctly but the front parking lights and the instrument panel is dark (no illumination). When I remote unlock the vehicle the taillights illuminate correctly either using the factory remote or the aftermarket code alarm remote.

When I turn on the normal headlights, the from parking lights come on as well, but all the rears (tail lights/license plate lights) do not illuminate.

There was mention about the vehicle security module that could be bad? I did not have time to pull the rear seat and check it.

 

They are on separate circuits except for a common ground.

There are no fuses exclusively for the rear tail lights. The license plate lamps are in parallel with the rear tail lamps. Faulty tail/brake light bulbs cannot cause both features to fail.

You have an open circuit between the front and rear of the truck in the parking lamp circuit. The 3-4 volts measured is simply "noise".

Find where the installer spliced into the brown wire and inspect that specific splice. My suspicion is that it's failed or has caused a failure in the wiring adjacent to the new splice.


If you have the EVTM, your open circuit is in the circuit segment represented schematically between splice S224 (cell 92-1) and splice S332 (Cell 92-2).

 

Can you verify what you are using at ground for the meter ?

11.7 VDC is a dead battery needing to be replaced.

3 to 4 VDC looks like a high resistance ground.

When you replaced the bulbs, did you meter the connector using the bulb ground ?

 

Noise, just like when checking for fuel pump voltage when the relay isn't activated.

The rear bulbs function normally when activated by the VSM or the aftermarket system but not by the rest of the controls. There's nothing wrong with the rear bulbs.

The front bulbs work normally when activated by the normal controls but not when the VSM or the aftermarket system is to be operating them.

The system has been effectively cut in half between those two splices.

 

ProjectSHO89,

Thank you, you were 100% correct.

It was a failed aftermarket splice where the alarm/remote start installer tapped into the parking lights. This was down in the drivers sill just aft of the kick panel.

The splice corroded and eventually the brown wire broke in two.

I used new wire to make a new splice, soldered and shrink wrapped everything.

working great.

Thanks again.

 

Even a blind squirrel sometimes finds an acorn.