Strange cable - what does it do?

[Morat]

Hi folks, another "special" question for you..
There is a cable on my engine that runs from HT lead 1 (or 6 perhaps) up round the front of the engine and ends in a plug that looka like No3 on THIS diagram. The engine runs with or without it plugged in. Could you tell me what it does or link me to a thread where it is discussed?
I'm pretty sure this came up recently (I'm half sure it counts revs for some reason) but I can't find the right threads

Thanks
Morat

[E30Mark]

http://www.e30zone.co.uk/modules.php?na ... 325+sensor

[Morat]

fantastic, thanks mate