In my experience the DTCs can be pretty helpful...I do have a fair amount of
experience but only with GM vehicles.  Others might not be so easy. Usually
when I determine what the code is, it makes it MUCH easier to track down the
problem. As for looking at the pulse train, it won't be quite that easy,
definately not with an LED, unless you can see pulses at 8192 baud. :) A scope
won't make it much easier, as the pulse train is a serial stream of data,
basically one byte of useful data (RPM, temperatures, sensor voltages, trouble
codes, etc.) every four or five bytes if I remember correctly.  Your best bet
might be to log a few seconds of the datastream through a serial port, then try
to find the datastream documentation and decode it. I could help you if it was
GM, but I don't know where to point your for an MB. It does help that it is a
'94 though...

ALSO, on many (most?) vehicles, the ABS computer is a completely separate device
from the drivetrain control module. Talking to the PCM may or may not give you
any useful data on your ABS system.

FYI, many auto parts stores (Autozone if you have them in your area) will read
the code & give you trouble codes for free, because they want you to buy their
parts. Mechanics and dealers will charge from $25 to $100+ for the same
information...

HTH,

Steve Ruse

Quoting Bob Blick <bblick@sonic.net>:

> > 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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