using the diagnostic plug on W124?
 

The diagnostic plug on a Benz seems like such a convenient way to find a mysterious fault. Yet you need a special tool that costs way too much for my student wallet. Does the diagnostic plug deliver a digital output that is so amazingly difficult to decipher that one needs an expensive machine, or does it output analog signals that can be read with a normal multimeter? And if so where can one find a sheet containing the output values and their corresponding faults?

Try a search above - for fault code reader, etc. - you'll be able to read for several days.

Analog or digital depends on model/year, of course...

Diagnostic plug or OBD-II port? I have an OBD-II code reader (digital) that I got on eBay for about $69. It works wonders on a couple of my cars, and has paid for itself a few times over in letting me identify issues and repair them myself, rather than going to the dealer.

Cheers, John

No 124s were OBDII compliant.

Most of the same readers will pull OBD-I codes as well, though I don't know how far back that system went. It's been a long time since I owned a pre-1996 car. I also don't claim to know what the standards were outside the United States -- I'm sure plenty of places still don't care a hoot about OBD stuff, though manufacturers are likely building it in by default at this point.

Cheers, John