thanks, it goes now :)

the way the factory tacho works is off induction, so there is no actual wire connections to the coil. It however requires the 12v signal when the key is set to on, to run to the tacho, on the tacho is a plastic loop, the wire needs to be fed through this loop and then looped around and fed back through, this wire then travels to the 12v positive side of the coil. an the tacho will work.

The 12v is from the old ballast resistor and is connected to the +ve of the coil. Which way do I have to wire the tacho up to? Series, parallel, and what side of the coil?

on the old setup the wire that ran to the coil -ve, I am assuming this went to the dizzy. Do you remember? if so its no longer required.

The original datsun tacho works off induction from the coils +ve supply.
The 12v supply you connected to the coil +ve, where did you get this from?
You should have gotten it from the 12v ign side of the ballast resistor.If you did this it sound like the tacho is playing up, but its hard to tell without actually seeing the car in person.

its a standard datsun tacho. the dizzy its connected straight on the coil. one black, white trace wire to +, blue wire to -ve. On the coil, there

what kind of tacho is it and what did you do to connect the ciol and dizzy?

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