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OBD2 plug has power but not communicating.

Started by grumpypanda, December 13, 2021, 12:55:52 PM

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Me and my local garage are trying to locate the source of a red airbag warning on a late Mk1 Alhambra TDI S. Three different allegedly compatible code readers have been used and none has managed to communicate with the different control modules on the car. OBD2 plug has power but not communicating.
Conclusion is that it is a car fault causing this. We do not think that it is the OBD2 socket itself.
Thoughts welcome on where to start please.

Which readers have you used? Nearly all OBD2 readers are focused on communications with the engine ECU, do you have that with any of them? If you do then you don't have a coms problem, if not then you do in which case the first thing with the older VAG cars is to disconnect any aftermarket radio.

For the SRS system, you'll need something specifically designed to talk to the VW controllers such as VCDS.
03 Ford Galaxy 1.9 TDI 115 Ghia in Spruce Green Metallic
With cream leather interior, Full Bodykit, Remapped at 145bhp, Lowered on 18's
13 Porsche Cayenne 3.0 V6 Diesel in Umber Metallic
262Bhp AWD and Factory fresh...for now!
58 Ford Transit 2.2 TDI 115 in Frozen White
With retrofitted everything except another slidey door! :)
LAUNCH X431 Pad PRO - Scanning & Coding for all makes and models done in Wiltshire in exchange for winegums! :)

There are no communications through the OBD2 port, even with the engine ECU.
The car has the original radio fitted.
I think there is one diagnostic line at the OBD2 port so is it possible that any of the connected modules could stop comms?

The older vehicles use a one wire K-Line as the diagnostic as opposed to modern twin-wire CAN-BUS. From memory, K-Line is taken to ground by the diagnostic tool to inform all control modules to listen, communication is then binary over the line with each module being addressed uniquely.

Its a pretty simple system so as a start, measure out the OBD port looking for the following: Battery 12V on Pin 16, Chassis ground on Pin 4 and Signal ground on Pin 5.
03 Ford Galaxy 1.9 TDI 115 Ghia in Spruce Green Metallic
With cream leather interior, Full Bodykit, Remapped at 145bhp, Lowered on 18's
13 Porsche Cayenne 3.0 V6 Diesel in Umber Metallic
262Bhp AWD and Factory fresh...for now!
58 Ford Transit 2.2 TDI 115 in Frozen White
With retrofitted everything except another slidey door! :)
LAUNCH X431 Pad PRO - Scanning & Coding for all makes and models done in Wiltshire in exchange for winegums! :)