My '91 300SE has 205K miles on it and 3K since major tune including air and fuel filters.
In last three weeks it has occasionally stalled at idle, usually while sitting at a stop sign.
Last Friday it refused to start. Fuel pump is working and spark is at plugs.

My mechanic is not Merc trained and is at a loss. I would like to suggest he check the fuel delivery relay before questioning the Ignition Control Unit but I do not want to insult him. I live on a gravel road in rural Columbia Co. so I have limited options but he has not failed me (yet).

ragobro, please provide some more information about the no start problem. What are the circumstances surrounding the problem, including the mileage on your 1991 300SE? Engine running rough, then no start ... engine running fine, then suddenly no start. If your 1991 300SE was running fine then suddenly wouldn't start I would suspect the ignition control unit (ICU) mounted in the driver's side inner fender. Either swap yours in a similar car that runs and see if that produces a no start situation, or swap a known good ignition control unit into your 1991 300SE.

Failure to maintain ignition system components (spark plugs, distributor cap, distributor rotor, ignition wires, coil) can cause the ICU to fail so I recommend installing the ICU from your 1991 300SE into a running car to check if yours is bad. The good news is you should be able to purchase a reconditioned ICU from your local MB dealer.

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Fred Hoelzle

Your "mechanic" needs to know that the injection is continuous, hence there is no injection timing.

There is a flywheel trigger pickup for the ignition, and that signal also controls the fuel pump relay when the engine is running (though not when cranking to start engine).

First tests:
Can you hear the fuel pumps run for about 2 seconds when you turn the key to "on" (not start)?
Is there spark at the plugs when cranking?

the above post is being polite, you need a new mechanic. there is no injection timing. the crank sensor is JUST for ignition. find someone who speaks mercedes better. these are simpler cars than the newer ones. good luck, chuck.