Yes Cam, minus with respect to engine 300-350V negative going back emf when the inductive discharge ignition shuts off the current to yield a spark, say 100:1 turns ratio for the ignition coil, a 350V "spike" induces 100 times that primary field collapse and impresses 35,000 volts on the secondary winding. The primary is "charged" with positive 12Vdc, when the current is interrupted the self induced voltage is now negative at the magnetic field is now reversed. Simple, a wry smile here. The modded RGS Tacho i did was 12V input, you can modify a genuine oem tacho by removing the input capy and scaling resistance.
Any owner needs to remove the tacho wire from the ignition coil primary, contemplating a now common 12V tacho drive system. HTH j
** the iis 12V tacho drive was a tachometric stepping from 1Krpm to 10Krpm on self test position F, allowing check of tacho accuracy.