crankshaft position sensor.

[ah]

Has anyone had one of these go bad,if so what we're the symptoms?m20

[Brianmoooore]

Very common. Symptoms are a misfire that usually gets worse as the engine gets hot.
That's for failure of the sensor itself; not to be confused with the water pump pulley cutting through the cable, when the car may just not fire or run, with no sparks or fuel pump activity.

[ah]

Cheers Brian,can they be tested?

[Brianmoooore]

Three pins in the plug. The one the furthest away from where the lead enters the plug is just the screen on the cable, and shouldn't connect electrically to the other two in any way.
The other two should have a resistance between them of around 540 ohms.
Any significant variation from this indicates a dud sensor, but good readings don't necessarily indicate a good one, since a few shorted turns in the winding won't change the resistance much.
Wriggle the cable a little, close to where it goes into the sensor while you take the measurements, but not too hard, or you may break a perfectly serviceable one.

[Mitchen]

I have a new, genuine one for sale, if you care about such factors.

[Simon13]

i've never known or had one fail on any of my e30's most have been M20's

[Mitchen]

Not sure whether they're prone to fail, or whether it's because of their close proximity to the radiator fan..