Jaguar Repair Problems? Ask a Jaguar Repairs Expert.
It is unlikely the 12 cylinder tachometer worked ok with a V8 engine.
You now need a V8 tachometer, for which the factory tachometer will need to be removed and calibrated for a V8.
What distributor, amplifier and coil are you using?
I would say the tachometer is not designed to be wired to a coil negative.
The RPM signal would come from the ECM.
So engine RPM is read by the ECM via the crank position sensor.
The ECM then tells the Tachometer how much RPM to display.
More than likely the Tachometer is now faulty.
It would have been inaccurate because the pulses per rev would be different.
The V12 would have been EFI. The ECM is the EFI computer.
Obviously now, no longer in use.
With your MSD, it could have ground on the negative if it has an 6AL amplifier.
Can you tell how many wires leave the distributor?
Purple and green; or red,orange and black?
What colour wires on your coil, positive and negative?
Do you need any further assistance?
I hope I was of some guidance for you.