I'm trying to diagnose an idle speed problem (engine intermittently revs to about 1000rpm at idle), and the tech manual says one of the inputs to the computer is the 'engine speed sensor'. I've tested or replaced almost every other input, checked for vaccum leaks, etc. and my suspicion now falls on this sensor. However, I haven't a clue as to where I can find it, how it works, or how to test it.

Any help? The car is a '93 300e 2.8.