Fiesta Intermittent Brake Lights

[JonWarby]

Hi everyone,

I have an issue with my 2015 fiesta where the brake lights are working intermittently.

Either all three brake lights will work, or they wont. 

I can sit there and press the brake and see the lights come on repeatably and then they will stop working and if I keep doing the same thing they will then start working again.

When they fail all 3 fail at the same time.

The gap between when they work and dont work seems to be random. 

Since it started I have replaced both brake light switches under the pedal.

I have removed all of the lights one at a time and it makes no difference, left rear cluster, right rear cluster, central light.

It's driving me nuts.  I cant find any pattern to it.  I can just be sat there not moving pressing the brake light and it will change from a working to non working state. 

Wiggling the wires that come into the switches doesn't make any difference so I dont think it's a bad connection at the switch end.

The issue started before replacing the switches,  It's repeatable if I just press the switches manually so I dont think it's that the switches are seated correctly.

Sometimes when the brakes fail I will get hill start assist not available but not every time.

Passenger glove box fuses have all been removed and re-seated.  dashcam fuse is currently unplugged.

Anyone got any ideas before I have to give it to Ford for them to scratch their heads and charge me a lot of money for it.

 

 

 

[unofix]

The fault is either with the Body Control Module (BCM) or with the 'input' signal from the brake pedal switch to the BCM.

At this stage I would for the moment rule out a fault with the BCM as there are two separate independent outputs for the brake lights. One output is only for the centre high level light, and the other output is for the left and right brake lights.

The BCM uses the one input signal from the brake pedal to activate the outputs to both circuits. Water ingress in to the connectors of the BCM is a known problem on many models like yours and this can lead to the kind of fault you have.

You could use FORScan to monitor the signal input to the BCM. If there is no 'on' signal received by the BCM then check the wiring of the connectors for signs of corrosion and/or water ingress.

 

 

[JonWarby]

3 hours ago, unofix said:

The fault is either with the Body Control Module (BCM) or with the 'input' signal from the brake pedal switch to the BCM.

At this stage I would for the moment rule out a fault with the BCM as there are two separate independent outputs for the brake lights. One output is only for the centre high level light, and the other output is for the left and right brake lights.

The BCM uses the one input signal from the brake pedal to activate the outputs to both circuits. Water ingress in to the connectors of the BCM is a known problem on many models like yours and this can lead to the kind of fault you have.

You could use FORScan to monitor the signal input to the BCM. If there is no 'on' signal received by the BCM then check the wiring of the connectors for signs of corrosion and/or water ingress.

 

 

Thank you, that's amazing.  That gives me a few places to look.  I'll see if I can get FORscan working with my ODB II connector and check the BCM wiring.  

[unofix]

3 hours ago, unofix said:

there are two separate independent outputs for the brake lights. One output is only for the centre high level light, and the other output is for the left and right brake lights.

+++++ Correction +++++

On your model there are actually 3 separate outputs from the BCM for the brake lights. Each light has it's own feed.

[JonWarby]

I got Forscan working although my adapter is limited

When the lights come on I can see the relevant signal being received in Forscan.  When they dont come on, no signal is received.

Attached image is me pumping the brakes and the lights working and then the they just stop even though I continue to press.

This is measuring the brake on/off switch.

 

 

[JonWarby]

It's currently working for the longest it has been.

The Forscan software was really great in saving a lot of time testing and trying to use a mirror and getting the confidence that the computer wasn't receiving a signal from the switch.

After check the wiring as best as I could and not seeing any fault I replaced the black switch again. The blue one I had sourced based off of the ford part number.  The black one I went into euro car parts and asked for a switch for my car so my confidence wasn't as high and I couldn't find any useful part number on it.  Whether it was the wrong switch, although physically looking the same, or a new faulty switch I dont know.

It could just be that at the moment it's in a happy place and it's going to fail again but just tailed the car for 10 minutes on a drive and no failures which is a big difference from anything that I have had so far.  Thanks everyone for the suggestions.