Tach Flutter

[navdoc101]

What would cause a tach to flutter/bounce/fluctuate ?
Its steady at idle, but fluctuates while driving, even on a smooth road.

[eurotiva]

Plug on back of cluster?

[navdoc101]

Thanks Greg.
I tried knocking on cluster when ideling with no reaction.
Might still be that, it doesn't happen all the time, just most of the time.

[WmWatt]

I think the tachometer is electronic, running off electrical signals to or from the distributor. Perhaps a loose electical connection is getting knocked about when the car is moving? It could be from engine vibration which would not depend of the smoothness of the pavement.

[Movin]

Bad ground

[navdoc101]

Thanks all