I've recently fitted a new "old" smiths tach to my F27. The tach bounce is unreadable above 3k RPM.

It's current driven from both sides of the coil, not the greatest solution I know - but I've already cut the hole in the dash and tied it into the loom.

Could someone help with the reason or solution.

I'm sure it needs a smooth unit but not sure, help would be much appreciated!

What ignition system are you running? Those old Smiths tachometers simply don't work well with modern constant energy ignition systems.

my mate made converter for sierra dash, because did not work with modern zetec ecu.
chematic - capacitor is not needed

[Edited on 5/1/13 by fregis]

It's suited to the engine and ignition really it's powered from both sides of the coil from a Lucas distributor.

The old crossflow is coming out after this summer but would like to get it sorted, any help would be great.