2.4d tach/ rev counter

[wolfboy]

Hi everyone!! I live in the uk and drive a 1997 vw2.4d t4(eurovan). I would like to run dash clocks with tach from a 1997 passat tdi. Everything works as it should apart from the rev counter flips round to full revs with the engine running. The standard alternator has the w connection on it but i think i am in need of a diode to put into the circuit to stop this situation?? Anyone have any ideas?? Many thanks, wolfboy.

[Mark(The Miser)UK]

Could it be that the incompatability is in the alternators? I took a tach out of a small lorry [why I didn't note its make I don't know but it red lined at 3.5 k... However connected to the 'W' on my Passat mk2 TD the revs were very accurate. It just meant that it was going off the clock at near 90mph.  Therefore I suggest the tach has to match the # of poles on the alternator. ( I checked the accuracy with a GTD tach that arrived a few weeks later) There may be a span adjuster on the tach :idea: