> I need the pinout for the diagnostic plug in my car. I have some trouble
> with the ABS brakes, and want to retrieve the fault code. As far as I know
> a slow pulse train should be available somewhere in that plug.... Probably
> I won't need a pic to read the pulse train :-) , just a led or a scope.
> But which pin, and how do I interpret the code ?
> This seems to be a well hidden secret. !!!    Btw, it's a MB diesel, year
> 1994.

They are secrets, to lock you to the dealership for repairs. Even OBD-II
does not include ABS as part of the standard, it is manufacturer-specific.

However, diagnostic info is not always accurate. My wife's car presented
the error code "EGR vacuum valve solenoid has high resistance". Although
it seemed to function OK, I assumed it had an intermittent open, so I
replaced it and reset - same trouble code came back. I tested everything,
the valve functions in the car just as it should. I disassembled the
engine control unit, and looked at the circuitry that drives the EGR
vacuum solenoid - there was no way it could detect the current, it was
just a Darlington to ground, no voltage sensing or emitter resistor. The
positive feed was to a common bus with other outputs. A friend in the
industry suggested that the codes often are indirect and further down a
diagnostic flow chart, and as such, are not always accurate. He said that
more likely the ECU expects some change in engine vacuum or oxygen sensor
output when it triggers the EGR valve, and it wasn't seeing it. I finally
tracked it down to a stuck mechanical advance in the distributor.

So, my opinion of onboard diagnostics - not so good. Quite often the
result is a wild goose chase.

Cheerful regards,

Bob



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