I agree with aleekat that there wouldn't be spark if the rpm sensor quit working, but take a look again at jerryc's earlier post:

1) During starter cranking, the Crank Position Sensor sends timing pulses to Ignition Control Unit (ICU = EZK or REX)

2-a) The ICU uses these CPS pulses to trigger the Power Stage (aka Ignition Amplifier), which initiates spark from the coil.

2-b) At the same time, The ICU also propagates the pulses to the FI ECU, to allow FI operation (no ICU pulses means no FI operation).


The part about no ICU pulses means no FI operation reminds me of the last time one of my 940's RPM sensors quit working (of course, during the NY State inspection on the dyno when they still had it a few years back). The car suddenly died, and the service station had no idea what was going on. It went from fully operational with no symptoms to completely dead. They insisted they had fuel pressure and spark. Only when I supplied them with a replacement sensor on a hunch did the car start and run like nothing happened.

On a more recent occasion, my newly-acquired 93 940 turbo with 75,000 original miles started stumbling and stalling out at stoplights, and losing power while driving. I was almost sure it was fuel related, since the PO had never changed the fuel filter. I had no reason to suspect the RPM sensor because the symptoms were totally different from before and all the electrical components on the car were pretty much in like-new condition. But I happened to have a spare in the trunk, and swapped it out (the old one looked immaculate). This immediately solved the problem.

This makes me wonder if the RPM sensor should be considered more timing-related than ignition-related. Sure, if it completely dies then it would probably result in no spark. But could a partial failure result in partial or out-of-synch impulses to the ECU, which provides incorrect timing or intermittent injection? Just something to consider.

Come to think of it, the only time any of my cars has had an un-driveable condition to date has been related to either RPM sensor or the power stage (I'm at about 600,000 accumulated miles at this point).

Even the youngest 780 is almost 24 years old at this point, and the original sensor would be long past its service life, regardless of miles.