Installed new coil, now the tach acts funny

anything that keeps to back emf in the 300-400V range protects that tacho
GM used a condenser and a resistor i think. plug in loom on a plastic bracket with a radio suppression cap

jaguar and Lucas used a zener diode as per the original motorola product sheet circuit diagram for the original stand alone ignition chip that the module is based on.

both achieved the outcome of limiting what voltage was seen on the primary after the spark was triggered.