Brake lights on when truck off

[N3t]

With the downpours we’ve had recently I’ve noticed a few times my brake lights (all 3) come on overnight whilst the truck is switched off. This happened a few months ago for the first time but after connecting a tow bar bike rack it seemed to reset itself. 
 

I’ve changed the brake switch behind the pedal and still the problem persists. 
 

looking under the bed behind the wheel arch I can see the main harness between the cab and bed is filthy. Is this a poor design by Ford or is something a foot here? Should the run off from between the bed and cab head straight into this harness?

Connecting a trailer/towbar meter shows a constant feed and brake lights on.
 

Any advice appreciated  

thanks 

[unofix]

The brake lights get their supply direct from the Body Control Module (BCM) so either you have a fault with that, or the BCM is getting a brake signal input from the pedal.

Since the centre brake light gets its own independent supply from the BCM and you say that all 3 brake lights are on that would suggest to me that the BCM is receiving an input signal.

[N3t]

23 minutes ago, unofix said:

The brake lights get their supply direct from the Body Control Module (BCM) so either you have a fault with that, or the BCM is getting a brake signal input from the pedal.

Since the centre brake light gets its own independent supply from the BCM and you say that all 3 brake lights are on that would suggest to me that the BCM is receiving an input signal.

Thanks for this info, will certainly look further into this. But is there a way I can discount the bcm as being where the fault lies? The lights have come on through the night after heavy downpours which says to me something somewhere is getting water in. Inside the cab is bone dry. 
 

cheers

[unofix]

If you remove the input wire to the BCM then that rules out any signal being able to turn any of the brake lights on.

If with the wire removed the brake lights turn on by themself, then the fault is the BCM.

[N3t]

35 minutes ago, unofix said:

If you remove the input wire to the BCM then that rules out any signal being able to turn any of the brake lights on.

If with the wire removed the brake lights turn on by themself, then the fault is the BCM.

Been in the footwell and traced wires coming off the brake control. Main loom heads up toward the top of the dash then appears to head towards the centre of the dash away from the bcm (bcm is by the fuse box to the right of the steering wheel right?). There’s a blue wire coming off the brake pedal loom and that heads into a loom that travels alongside the driver seat. 

Nothing like electrical gremlins on a Sunday 

[N3t]

1 hour ago, N3t said:

Been in the footwell and traced wires coming off the brake control. Main loom heads up toward the top of the dash then appears to head towards the centre of the dash away from the bcm (bcm is by the fuse box to the right of the steering wheel right?). There’s a blue wire coming off the brake pedal loom and that heads into a loom that travels alongside the driver seat. 

Nothing like electrical gremlins on a Sunday 

Been back under the rear passenger arch and the main canbus plug has a black protective cover on it…well that was not seated correctly so wasn’t properly covering the wires.  Anyway popped that off, went to town with contact clearer and reconnected the battery. So far the lights are all off! 
 

I suspect the plastic cover not being completely shut allowed water and grime into the plug and created a dodgy connection  

will let the contact cleaner evaporate and seal it properly and hopefully the gremlins stay away 🙏

[N3t]

On 3/17/2024 at 2:52 PM, unofix said:

If you remove the input wire to the BCM then that rules out any signal being able to turn any of the brake lights on.

If with the wire removed the brake lights turn on by themself, then the fault is the BCM.

Hey, the bcm got replaced by Ford after the niggles came back. Been fine all week since yet today I went through some torrential rain and flooding in the Welsh valley. Well the minute I hit the first big bit of flooding on a bend with both left tyres a fault appeared on the dash regarding brake assist and hill descent. Arrived at my destination with the spanner icon on the dash and all 3 brake lights on again!? Dodgy wiring somewhere under the wheel arches? Speed sensor? Any clue?

 

cheers