Cruise control not working - brake light switch to blame?

[skyegtb]

Firstly I just want to state that the issue I am having is with my C5 Allroad, not my C5 RS6, but as the cars share a lot of the same parts perhaps someone here could shed some light or thier experiences.

I recently bought a 2001 A6 C5 Allroad 2.5tdi as a 2nd car to save using my RS6 too much and other than the cruise control being inoperational it seems to be just about a perfect car. I have been using my VAGCOM to check to see why the cruise control does not work and have tested all functions on the cruise stalk and the brake switch etc and everything seems fine. I have however had an intermittent (returning) fault in the ABS, but this is very infrequent:

Address 03: ABS Brakes Labels: 8D0-907-389.lbl
Part No: 4Z7 907 389
Component: ABS/ESP allrad D57
Coding: 06355
Shop #: WSC 02325
VCID: 0912408F19A9

1 Fault Found:
18265 - Load Signal: Error Message from ECU
P1857 - 35-10 - - - Intermittent

On 2 occasions in the last 1500 miles of driving the ABS, ESP and vehicle level lights started flashing while I was driving and my boost was cut until I stopped the car and turned off / on. Am I right in thinking that that could be due to a faulty brake light switch? Is the inoperational cruise control possibly linked to this as well? For info I have checked every fuse in the fuse box, all are good, and the brake lights certainly appear to work perfectly, so I am a little stumped.

I found some info elsewhere on the web about early A6 cruise control modules developing dry joints in the solder. I have not checked my cruise module yet, but is this a know issue with the Allroad?

Looking for any advice, thanks, Angus

[V8weight]

Your symptoms seem to point towards a faulty brake light switch or mass airflow sensor.

[skyegtb]

Your symptoms seem to point towards a faulty brake light switch or mass airflow sensor.

Thanks Pat. Why do you say it points posibly towards the MAF?

[V8weight]

Thanks Pat. Why do you say it points posibly towards the MAF?

The ECU uses the MAF to calculate specified load and monitor actual load. This is factored into monitoring wheel spin for the traction control and abs as well. The ecu compares MAF g/s to wheel speeds and determines whether the wheels "hooked up" or just spinning away. Faults with the MAF will often trigger an ABS light, or the traction control light will be illuminated permanently.

[skyegtb]

The ECU uses the MAF to calculate specified load and monitor actual load. This is factored into monitoring wheel spin for the traction control and abs as well. The ecu compares MAF g/s to wheel speeds and determines whether the wheels "hooked up" or just spinning away. Faults with the MAF will often trigger an ABS light, or the traction control light will be illuminated permanently.

Very interesting. I will do some more logging when I get home and see what the MAF is doing, perhaps it could be the culprit. I will also check my clutch pedal switch as supposedly this could also be to blame here. Someone else also suggested that the entire ABS module could be on its way out, but not exactly sure how to check that unless it throws any more codes.

[V8weight]

Usually if your ABS module is faulty, you will not be able to communicate with it with the VCDS...you may also store fault codes in the instrument cluster for ABS no communication with instrument cluster.

[skyegtb]

Thanks again for the good info.