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Martin Jacques

Symptoms of a failing Crank Sensor on the M30 3.5?

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Hi all,

I have an M30 3.5 engine and auto-box (from an E32 7 series) installed in an 1988 E30 320 cab. Same engine as the E34 535i, except it has fly-by-wire throttle.

Will start from stone cold perfectly, and holds a perfect idle. After a few minutes, the engine just cuts and will not restart. Come back to the car a few hours later, and it starts and runs perfectly for the next few minutes, then dies again. The fact it dies almost instantly suggests a sensor to me. The time it takes to die is getting shorter each time.

After some searching on here and elsewhere, I am thinking Crank Position Sensor.

Before I just buy a new one and replace it, anyone disagree and think I'll be wasting my money? Can they be tested, or maybe cleaned and replaced to help diagnosis?

Also, does anyone know what it is actually called as neither RealOEM nor GSF car parts list any variation on crank position sensor. Pulse generator on realoem, and impulse sensor on GSF seem to be the closest.

Thanks for any help anyone can offer,


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The crank position sensor is a Variable Reluctor (inductive) type rather than resistive, so pouring water on it is unlikely to help :P BMW call the sensor 'Pulse Generator', part number 12141720853. These sensors are really pretty reliable as far as I can tell so I wouldn't fork out for a new one just yet...

It does sound like the cutting out is a spark related issue, but I'd check the distributor cap and rotor arm first of all, ensuring the rotor arm is fixed securely and in good shape. After that check the coil and it's connections closely. It's not unknown for coils to fail. Finally check the crank sensor is secure and with no obvious signs of damage.

I'd grab some spares from a scrapyard for testing in your situation - including another ECU if you can get one.

Hope this helps,


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Thanks Stuart,

I had the dizzy cap off this afternoon and cleaned the rotor and contacts - a very fiddly job on an E30 3.5 when the dizzy cover is merely millimeters from the radiator. Also cleaned the coil wire contacts too.

Engine ran a treat this afternoon, all up to temperature with no hiccups or hassle at all. Very happy. I hope it stays running normally now.



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