tail lights will turn on and off by themselves with or without key

nova3162007nova3162007 sleepy eye mnMember Posts: 3
edited January 2017 in Ford
So first its a 2008 Ford F-150 w/ 5.4 liter triton engine. it does have an auto start.

for the last few months here my bosses ford f150 has had an issue where his tail lights will turn themselves on (for up to 12 hours, or until disconnection of battery) or until they randomly decide to turn themselves off.

this can happen while the vehicle is being driven but more often happens without anyone or the key in the vehicle. so he usually woke up to a dead battery.

as of recently he purchased a battery tender that he mounted into the engine bay so he can start his vehicle every morning.

we have checked/pulled all fuses and relays we can thing to and also multiple fuses and relays in a variety of combinations but none seem to cancel out the constant signal to show as though he is in the truck holding the brake pedal to the floor.

any and all help would be appreciated


  • PF_FlyerPF_Flyer Pennsylvania Furnace, PAMember Posts: 9,372
    edited January 2017
    Perhaps the simplest solution? Sounds like it could be a shorted out brake light switch. Here's a video on how to replace the switch I found after a quick search.
  • nova3162007nova3162007 sleepy eye mnMember Posts: 3
    thanks. we removed the brake light switch entirely but the lights continued to remain lit. its also not always all 3 lights. sometimes only the running lights will kick on. depends on how it feels. we have also removed the light switch and with that out they also remained lit
  • thecardoc3thecardoc3 Member Posts: 5,595
    Does this have the trailer towing package, or was a trailer hitch and wiring added to the truck?

    The parking lights are controlled by the headlight switch as well as the vehicle security module. Are there any codes being set in any module?
  • nova3162007nova3162007 sleepy eye mnMember Posts: 3
    yes. we had to disable the towing switch, as awhile ago it started going off on its own and never stopped. so the tow controller is no longer connected via cables/wires but is still mounted in its original position.

    if im not mistaken he said recently he had the trailer power wires redone as the previous one worked loose and drug itself away on the pavement
  • thecardoc3thecardoc3 Member Posts: 5,595
    I would need to get this to act up to prove what is going on, but part of the plan would include being ready to prove if the trailer harness is involved in the failure. The trailer harness (7 pin) would have its own power supply for the trailer that would be hot all the time. If there is a copper to copper sneak circuit, that "could" power up the parking lights. Testing would have me prepared to remove that power supply, or measure any current flowing in it during the failure while trying to not disturb anything else.

    That's one possibility, a solid routine would make easy work of this no matter where the failure is. Get a schematic so that you can see just what is involved in the circuit, and from there decide the most convenient points where the circuit could be separated for testing so that you can first prove what part of the circuit is the source of the issue and then go about finding exactly where. Remember that the vehicle security module does turn the lamps on, making it accessible would make it easy to disconnect as part of the testing. It is located in the left rear corner of the cab with a supercab, or the "B" pillar in a regular cab.
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