DTC 16955 Implausible Brake Signal

[kasibarndoor]

Hi it's a 2000 Mk 1 Alhambra 1.9 TDI and the glow plug light flashes and the brake lights don't work. I have replaced the brake light switch and there is no change, fuse 20 is OK. Both old and new switches perform the same - 1 pr NO contacts and 1 pr NC contacts. There is only 1 live wire in the connector at 12V, reading on the internet the centre brake light and the two peripheral ones are powered from different switches (hence the four wires?). I do not have a circuit diagram so its difficult to debug, any help would be appreciated.

I also tried the new type switch and that didn't work either.

 

[kasibarndoor]

Progress?

I think some of the info about the brake switch on the internet is incorrect, my guess is one pair of contacts provides a voltage to the brake lights and the other pair provide a signal to the ECU (or ABS - Cruise Control), it is quite reasonable for these circuits to be independent as it is undesirable that the engine continues to accelerate if the brake light bulbs have blown and the driver is trying to stop, there may also be a relay in circuit which inhibits the brake lights when the ABS or ESP kicks in. There does not seem to be a circuit diagram which shows all parts of the circuit and identify test points, so I think one has to grope around in the dark a little (or a lot) until a solution is found.

 

[kasibarndoor]

Just thought about it some more, the signal to the ECU is a permanent voltage present, the brake light bulbs will not effect this, the brake light switch depression or it's failure will result in removal of the voltage and is therefore fail safe.

 

[kasibarndoor]

The fuse for the brake lights is 15 and not 20 as stated on the internet. This fuse was blown, replacing it showed that the central brake light was not working, on inspecting the loom I found a break at the connection from the body to the rear tailgate where the ground in the loom insulation had broken in exactly the same place. Now repaired.