Tdi: 1z or AHU ECU wiring info

[ewenmaclean]

Hello,

after much tinkering I managed to understand how to make the brake light and brake pedal switch work on a TDi controlled by an ECU (fly by wire), which is functionality which apparently helps reduce fuel consumption. I am putting it here just for info since I discovered a lot of people had been having trouble wiring this up and not getting errors from VAG-Com (notably 00741). If this has already been posted and I haven't seen it then my apologies (search doesn't seem to be working for me at the moment).

All of the below pertains to a 68 pin ecu for 94-96 1z or AHU Tdi engines.

1) Pin 17 (white with red), should be earthed. If you do have a clutch switch on the clutch plate as I know it is possible to have, then this should be wired to break the earth signal. If you don't do this then the engine will cut power the first time you rev for a few seconds, and only give it back once you totally release the accelerator and start again. This can be bloody annoying.

2) Pin 20 (white with yellow) can be controlled by making a NOT gate with a 5 pin relay from the original t25 black and red wire going to the right hand side rear light cluster. This controls the brake light. For example this works:

Pin 85 - Spliced from Brake light wire
Pin 86 - Ground
Pin 87a - Sender Ground
Pin 30 - White and yellow wire
Pin 87 - unconnected

Now when the brake lights are switched, the earth is broken to pin 20. You can test it's working by revving the engine then pressing the brake with your left foot and it should cut the revs.

3) Pin 44 (black with red). If you can be arsed, this is there to test that one of your brake lights is working. You can achieve this with a simple diode (at least 30W rating or so). Attach it inline to the wire controlling both brake lights (left brake light cluster I think), with the splice off the the 5-pin relay mentioned above before the diode. Join the wire going to pin 44 to the cathode end of the diode and then you won't get an earthing signal from the relay itself.

I actually just connected pin 44 wire to earth and hope that someone will tell me if my brake lights fail!

Fault are usually noted by a blinking glow plug light - earthed through pin 48 (blue and green), making sure the glow plug relay is earthed through pin 50 (yellow and blue).

I've tested mine and it's working fine with no errors. I don't claim to be an expert in any way, but I hope maybe this might clarify the brake switch error issue if anyone else is having it.

Ewen