Audi A6 C5 Brake Light Failure (And Repair)

After a couple of weeks driving the Range after its overhaul I reverted to the Audi for financial reasons.

Found no cruise control (#firstworldproblem) and, more worryingly, a permanently-lit ABS warning light.  Testing on a quiet stretch of road confirmed that ABS was inactive.

I did the obvious checks on brake fluid and fuses without finding anything amiss. Plugging the car into a VAG-COM Diagnostic System turned up this helpful hint:

16955 - Brake Switch (F) P0571 - 008 - Implausible Signal

At this point I also discovered the brake lights were not working (should have checked them earlier, but kind of assumed the car’s brain would tell me if they weren’t working).

Some internet research into fitting Audi/VW brake light switches established that the installation process required all manner of arcane rituals except blood sacrifice to avoid damaging the new switch.  This research proved largely irrelevant as the switch supplied by Goldwagen was a newer version that doesn’t require pressing of pedals while inserting.

Installation

1. Remove knee panel under the steering wheel (spanner required)
2. Move the brake pedal to see which bits move and how they interact with the switch (may need to run the engine once in a while to relieve the pressure)
3. Unplug old switch
4. Rotate switch anti-clockwise until it releases (make note of the angle through which it turns)
5. Insert new switch and rotate clockwise until clicked into place (note that there is lug that does not rotate with the switch (see below), it is held by a notch in the mounting plate)
6. Insert plug
7. Test for brake light function

Some people seem to have issues with “delicate” switches that break/fail during installation – I did find that the first switch I installed didn’t work, but it seems it was a manufacturing defect rather than ham-fisted spannering.  Goldwagen did replace it without any quibbling though.

Diagnostic Things and Interlocks

As mentioned above, I am lucky enough to have a Ross-Tech VAG-COM Diagnostic System (VCDS) so I can read fault codes and get real-time data on what is happening around the car.

Interestingly the “switch” is actually two switches inside one housing, providing two outputs.  The “implausible signal” reported above is caused by one switch working while the other one doesn’t.

A feature of modern cars is that you are often prevented from putting the car into gear until you press the brake pedal.  This is certainly the case on my Multitronic transmission.  Luckily (!) in my case the failure was only on the side that tells the car-brain to illuminate the brake lights and standby with the ABS, otherwise I wouldn’t have been going anywhere.  What I did find though is that if you disconnect the brake switch entirely then you can put the car into gear and it works normally – except for lack of brake lights and ABS… (hey, it’s a bonus if *any* lights work on some vehicles round here).

Illustration of the locking lug:

And a relevant bit of the diagnostics log file:

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Address 02: Auto Trans        Labels: 01J-927-156.lbl
   Part No: 01J 927 156 CL
   Component: V30 01J 3.0l 5V RdW 3031  
   Coding: 00001
   Shop #: WSC 04940  
   VCID: 78F5CF7258EE81FE65-5140

2 Faults Found:
17087 - Brake Switch (F) 
            P0703 - 35-00 - Electrical Malfunction
18265 - Load Signal 
            P1857 - 35-00 - Error Message from ECU

-------------------------------------------------------------------------------
Address 03: ABS Brakes        Labels: 4B0-614-517.lbl
   Part No: 4B0 614 517 G
   Component: ABS/ESP front       3428  
   Coding: 06399
   Shop #: WSC 02325  
   VCID: 254BD606BFDCCC160E-5140

2 Faults Found:
00526 - Brake Light Switch (F) 
            27-00 - Implausible Signal
18265 - Load Signal 
            P1857 - 35-00 - Error Message from ECU

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