DIY Fault Code Readers for older Mercs

Is it possible to improvise a fault code reader? I used to have a Ford, and I know that there was a way of poking the leads of an analogue multimeter into the socket to read off the codes (based on
the number of sweeps the needle made).
I have a '91 300 SE W126 - there is what looks like an interface socket in the engine bay. It is round with a screw-off top that hides a socket underneath with 9 pins laid out in a circular type fashion.
Mike
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