Brake light issue - Does anybody have any suggestions?

[cb750chris]

HI there,
Can anybody offer any advice, I have been looking at a diesel car with a strange problem. The brake lights act normally when the ignition is in the aux position, but when the ignition is turned on properly the brake lights stay on all the time.
I have checked at the brake light switch, and the power & ground are as expected. The purple & green to the ECU is switched as expected, but the green & purple is not (The power suggests fuse F13 is “good”, the fuses have also been checked, and there are no blown fuses in the passenger fuse box)
The brake light switch output to the light switch module, green & purple, is ~12 volts when the brake pedal is depressed. When the brake is returned to the normal position, it has a voltage of ~4 volts with the ignition in the normal position (~0.8 volts when the ignition is in the aux position). If the input to the light switch is pulled to 0 volts, then the brake lights turn off, as expected. This suggests that the brake light switch just isn’t switching properly, or something else is driving this signal up, against the brake light switch.
Looking at the routing of this wire, it is connected via header K109 to the following units:
ECU
Light Switch
Cruise Control (not fitted)
ABS unit
I unplugged the ECU, and this did not resolve the issue, unplugging the ABS unit then appeared to resolve the issue, suggesting the ABS unit may have been at fault. However after reassembling the battery box etc (with ABS unit disconnected) the problem returned, throwing doubt on the initial diagnosis of the ABS unit.
The brake light switch has been replaced to no avail, and a 2nd light switch module was also trialled before I was contacted.
Overall, I feel I need to investigate the K109 header, can anybody suggest where I can find this?
The other thing I am thinking of trying is a 3rd brake light switch.
Can anybody suggest any other places I should look next, or what else to try?
The only other thing I noticed, is that the plenum looks a bit leafy, so wondering if this could be a symptom of a previously wet ECU?
Thanks
Chris

[BigRuss]

Sounds very much like a faulty LSM had a couple that have had very similar problems
Try another LSM and see if the fault disappears.

Edit: Didn't see that a replacement had been tried earlier but it wouldn't harm to try another.

Russ

[Mickyboy]

I second what Russ has suggested check the pins on the LSM I’ve seen quite a few that have had signs of damp in there.
Mick

[bigblue75]

Only asking because quite a few people have had this problem

https://the75andztclub.co.uk/forum/s...d.php?t=241449

and the only switch bought and confirmed as working as it should is sold by Rimmers.

Hope this helps

[squire]

Originally Posted by bigblue75

Only asking because quite a few people have had this problem

https://the75andztclub.co.uk/forum/s...d.php?t=241449

and the only switch bought and confirmed as working as it should is sold by Rimmers.

Hope this helps

I agree with the above, after a year of brake lights intermittently staying on and despite numerous resetting the switch and other failed bodges, a new switch was fitted and the problem solved. Do not buy off ebay get the right switch from Rimmers as they are the only ones that work properly, there is a "how to" about changing the switch. Not the easiest task to do, but even I managed to do it and really only want to do it once!

Hope this helps and your problem is soon sorted.

[cb750chris]

Well, it looks like squire & Bigblue were spot on.
The problem turned out to be the FAI brakelight switch.
After a bit more digging, it appears that when the ignition is turned to position 2 the ABS unit starts to look for the brake signal from the switch (presumably to tell it that it now needs to behave as an ABS unit), this placed enough load on the switched line to stop the brake light switch correctly switching.

Replacement with a 2nd hand switch has resolved the problem, for the moment.

Thanks for all the replies.

Chris

[cb750chris]

Originally Posted by bigblue75

Only asking because quite a few people have had this problem

https://the75andztclub.co.uk/forum/s...d.php?t=241449

and the only switch bought and confirmed as working as it should is sold by Rimmers.

Hope this helps

Thanks, only one question - why the goodness did I not find this thread when I searched for this problem

[bigblue75]

Not sure why your search did not find that thread... Glad you are able to confirm and fix the fault though.

A related issue is the lack of closure on threads. It's amazing how many threads are posted and then forgotten by the OP - you search for the solution to a problem and over half the threads you read through just stop.