Mechanoid

I have a problem with the tacho on my Xantia. It is fine when the engine is cold but after about 10 miles the tacho starts to jump around then stops working. If I let the engine cool down, the tacho starts to work again then repeat as above. It seems to be classic speed sensor failure that is brought about by the normal heating up of the engine BUT where the hell is the sensor located on the engine? I can't see it anywhere. Cheers for any help :rolleyes: Mech.