Technical A250 brake lights weird

[Mervk]

I have an A250 2015 with the weirdest brake light fail symptoms, and though I have read every bit of info on faulty brake lights that I have been able to find, this defies explanation. If I turn the ignition on but do not start the motor, the brake lights work as expected, coming on and staying on whilst ever I have my foot on the brake pedal. If I then start the motor and depress the pedal, the lights come on for around 2 to 3 seconds, then go out. And they will do this every single time I press the pedal. Turn the motor off then ignition back on, and they work properly again. Anyone have any ideas? I would think stop light switch if the problem was intermittent, however I can duplicate it whenever I want. All comments gratefully received. Cheers, Merv.

 

[Communicator]

I have an A250 2015 with the weirdest brake light fail symptoms, and though I have read every bit of info on faulty brake lights that I have been able to find, this defies explanation. If I turn the ignition on but do not start the motor, the brake lights work as expected, coming on and staying on whilst ever I have my foot on the brake pedal. If I then start the motor and depress the pedal, the lights come on for around 2 to 3 seconds, then go out. And they will do this every single time I press the pedal. Turn the motor off then ignition back on, and they work properly again. Anyone have any ideas? I would think stop light switch if the problem was intermittent, however I can duplicate it whenever I want. All comments gratefully received. Cheers, Merv.

I do not have a 250 model, but I am thinking that the brakelights are operated via the BCM. It will not be happy if total bulb power does not match specification. Why the BCM is reacting to engine running is another mystery but could be a fault. BCM faults are not unknown, with replacements for earlier models being both expensive and difficult to source. I hope that there is a simpler explanation.

 

[Mervk]

I do not have a 250 model, but I am thinking that the brakelights are operated via the BCM. It will not be happy if total bulb power does not match specification. Why the BCM is reacting to engine running is another mystery but could be a fault. BCM faults are not unknown, with replacements for earlier models being both expensive and difficult to source. I hope that there is a simpler explanation.

Thanks, I hope so too. Haven't changed any bulbs, so should all be good still there. I have been poring thru the manual looking for a brake relay, as I thought maybe a relay was good at 12v, but reacting when alternator putting out 13 or 14v. Only reference I can find on one drawing is an "M051" for brake system, but that must be a reference to somewhere in the main unit.