Rev counter not working properly


I have a 1998 fourtrak which I bought two years ago and the rev counter only works at certain revs
I have been told that the pick up is on the fuel bump which I have replaced and still the same if I pull
the green plug that connects it to the vehicle wiring apart the rev counter is still working at certain
revs so maybe that isn't the pick up this problem has beaten auto electricians I have also replaced
the instrument cluster and had the alternator and regulator tested and working ok .
Does any one have a wiring diagram that shows the rev counter wiring with out a diagram I'm not
going to sort this .
Many Thanks .