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On '85-'89 the Wht wire out of the dist. goes through a tach filter which is fastened/grounded through the top left bell-housing bolt.
My engine builder forgot to attach my tach filter and the tach bounced around till I used a jumper wire to ground the tach filter.
Since you have an L98 I wonder if your problem is similar. The filter looks like a round metal capacitor with a hole for the bolt.
On '85-'89 the Wht wire out of the dist. goes through a tach filter which is fastened/grounded through the top left bell-housing bolt.
My engine builder forgot to attach my tach filter and the tach bounced around till I used a jumper wire to ground the tach filter.
Since you have an L98 I wonder if your problem is similar. The filter looks like a round metal capacitor with a hole for the bolt.
That was what mine was until I bypassed it. Still, it wasn't reading right so I had it recalibrated and it agrees with the ECM