Intermittent abs/traction control issue
 

For few months now abs/traction control lights will come on randomly, sometimes can go days without it and sometimes it'll come on every time I take the car out, but only code that comes up is 00526 - brake light switch? Brake lights work, not really sure where to start as it disables cruise control which is annoying when most of my commute is motorways + I have MOT coming up in April :tuttut:

The brake light switch may need adjusting. It has two sets of contacts which need to change at the same time as a failsafe. This problem often happens when the switch isn't quite in the right position so one set of contacts isn't changing when the ECU expects it to, just due to wear.

We had a thread about this but I can't remember what it was called. You can see it in measuring blocks in the ECU though. The switch needs to be completely compressed when the pedal is on the stop, and then be released as soon as the pedal is touched.

Great, thanks for that. I only have VCDS-Lite so not sure if I have access but I'll have a look at the sensor itself and adjust.

Mine's had 2 new brake switches in my 10 years of ownership. First one was a genuine Audi part. When that failed, I replaced it with a Meyle equivalent part (Meyle part#100 945 0000, I believe), which has proved reliable so far.

Examining the failed switches, it seems that one of the (pair) of contacts eventually starts to stick due to arcing/burning. I'm guessing that those contacts drive something inductive (gear selector release solenoid, perhaps?).

I thought the two sets of contacts did slightly different things and are triggered ever so slightly apart - a really tiny movement. IIRC one is for the brake lights and the other for the ECUs. So you could have a problem with the second set and not know as the brake lights appeared normal.

As MJ says, you can monitor these with VCDS and observe them changing as you move the brake pedal. Just can't recall offhand which exact measuring block it is.... :o

I have a suspicion mine needs adjusting again (or replacing) as I'm occasionally getting some of the weird symptoms of sluggishness that I used to get quite often (caused by the ECUs thinking the brakes are still applied, when they're not).

Should be OK for checking readings.
Delving back into my marathon thread about lack of power I found useful comments by MJ and many others. In summary:

Engine measuring block Group 66 in VCDS is the brake pedal.
The switch should have a value of 1000 when untouched.
A VERY LIGHT touch on the pedal (engine off) should change it to 1010 but the slightest bit more and it goes to 1011.
That sequence could be reversed by lifting off very carefully.
(Engine on and the servo makes it all happen too quickly)

Yes thats it +++ IIRC they have to go in the right sequence otherwise the ECU can't work out what is happening.

Sweet, thanks for the info. I'll check it after work and report back, hopefully can sort the issue +++

I needed to do a scan so after that I checked this out while also monitoring the brake light.
I can confirm that the brake lights only come on at stage 2, i.e. 1011, so at 1010 presumably the ECUs know you're just starting to apply the brakes and at 1011 know (if they get/use that signal) that the brake lights are on.
It's only a tiny bit (on my car) after 1011 that the brake pedal goes hard without the servo.

Mine only showed "Not iniated" and "active" for some reason, but even looking at that seems like when I press pedal and release, it stays active.

Didn't get a chance to adjust today but I'll make sure it sits tighter and make sure it de-activated as well when pedal is pressed.

This would also explain when cruise is on, sometimes it'll disable by itself and then start working at some point.