101 gze tacho problem

[fraz]

We just put a 101 gze into an ae85.
I hooked the tacho up to pin 20 on the A connector which is a black wire and called IGT

It all works fine, but, as soon as the revs go over 5000, the needle just drops back to 0 and when the revs come back down it flicks back to 5000, and drops like normal.

Is there somewhere else i should hok the tacho to?

Cheers