Okay I replaced the crank sensor last night. Decided to drive it to work this morning. I am now sitting on the side of the road waiting for a tow truck. Same symptoms as before. Just suddenly cut out with warning. Tachometer dropped to 0.

I managed to get it started a couple of times but it dies after a few seconds later.

I have used a timing light on each of the ignition leads in turn and determined there is no ignition. There are no fault codes. I had been driving for about 20 minutes so engine was at operating temp.

What to check next? Ignition coil? Ignition amplifier?

Check power to the coil, 12V at the + side with the key on - this will be fine though best to check. There is a slim chance it will be power to the ignition system (EZK, coil etc, from a fuse). You said the tach died straight away so it sounds like no signal to coil (as that is what drives the tacho). Next is to check if there is a signal getting to the ignition module/igniter/amplifier. Not sure how to do this on the side of the road. It sounds a lot like the ignition module, or maybe (super rare) the EZK failing when hot.

Getting closer though. Also check the spark output from the coil lead, not sure this is easy on a 740 though unplug from the dist cap and rest 1/2 to 1cm from something metal, like a strut top bolt or intake manifold and then crank it over. You will know if there is spark.

Have you done anything with the radio suppression relay and it’s wiring?

740’s suffer from this failure.

Do you have an injector test lamp?

I just got it started again. Engine has cooled a fair bit though. Thinking this is a heat related issue. Something is getting hot and failing.

Tow truck should be here shortly so I'm not going to drive it.

Test everything at home, the tow costs and time lost add up. Swap the modules out for known good ones?

I did some digging around after getting towed home. Of course it started and ran fine while sitting in the driveway.

On a whim I replaced the ignition power stage with a used spare one from my stash. So far I've taken it for 2 roughly 10 minute drives with about a 5 minute break after each. Now I've just got back from about a 20 minute drive. On all 3 drives she ran perfectly and no hint of any issue. I am hopeful this is problem solved but not yet confident enough to say that with certainty. I'll see how she goes over the next couple of days.

Thanks everyone for the input so far. Really appreciate it. I will provide an update in a day or two if all goes well.

    Brad Nice catch. Guess the things are getting to the age now where the old igniters are starting to fail.

    Wisdom suggests a spare igniter ought to be carried in the tool kit from now on, just in case.

    Rock Auto has them, at a not indecent price.

        Major Ledfoot

        The Bosch part number on the old power stage is 0 227 100 124. It's still available from a number of places in Oz too surprisingly. It appears to be used by many different makes including Peugeot, Porsche, Citroen among others.

        If this problem is indeed fixed I'll get a new one for it so I'll still have a spare in the stash.

          Brad good to hear,

          i think i replaced mine early on in ownership but cant recall.

          Did you put thermal paste on the back of it?

          FWIW if you convert it to wasted spark you bypass the factory power stage module 😛

            nugget_940 I didn't put any new thermal paste on the replacement one but I'll do that this afternoon. Thanks for the reminder.

            I've considered going to wasted spark on my 940s. Just haven't got around to it yet.

              Glad you had a quick resolution time.

              The thermal paste going off is what kills them, they lose the ability to transfer the heat out of the module into the heat sink and overheat. Then something in them fails. Re-applying fresh paste every few years is a good preventative measure.

                dmc
                I imagine wiring and connections need attention too?

                    Ex850R

                    Only on the radio suppression relay

                    The connectors tend to come out of the relay often on those

                      8 days later

                      Looks like the 740 is fixed. It's been driven a few times since replacing the power stage and there have been no breakdowns. Happy Days.

                      I also replaced the thermal paste on the back for good measure. Don't really need another breakdown.

                      Is it possible to mark this as solved?

                      @Brad yup, I highlighted the solution already, you should see a ✅ and the answer in green at the top of the thread.

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