Howdy,

I just had a very similar problem on my wife's '92 240 with Bosch ignition. Replacing the power stage (a.k.a. ignition module/amplifier) seems to have solved it. On the 240's, it's mounted on the inner fender, between the battery and the driver's side headlight. Not sure exactly where it is on the 740's.

I hear you about not liking to replace parts (sensors especially) if they're not definitely bad. On the power stage, however, from reading the Bentley, it seems that there isn't really a test for just that one part. More like "if you test everything else and it all checks out okay, then maybe your ignition power stage is faulty."

I was about to start a separate thread about this, due to some contradictory test values for the coil in the Bentley manual and the Brickboard FAQ, so there'll be some more detailed information about my car's symptoms in there. They're not exactly like yours (no high idle), but very similar. Test your coil as per the Bentley (and/or FAQs) too. A marginal coil will knock out a power stage and vise versa.

Incidentally, I replaced the Crank Position Sensor on my wife's car last year. If your's is original, take a real careful look at it as Uncle Olaf suggests. Secondly, have you checked for fault codes stored in the ECU/ICU self diagnostics? When my '92 was acting up it never threw codes, so if you're seeing a fault code or two it might point you in a different direction.

Good luck and have fun!

Best,
Afton

'68 122, '87 '88 & 92 240's, all stick shift station wagons