In Reply to: 1940 9N no spark posted by Luke on June 14, 2017 at 09:54:18:
It is simple, yet it is not. You want the right combination of resistors & coil primary resistance so as to yield 4.5 +/-0.5 amperes of steady state current. You can not just buy a 12/6 dropping resistor or any white ceramic resistor, because almost none sold are of a known resistance. Any fixed resistor that drops 6v at a specific current will drop twice as much voltage at twice the current and half the voltage at half the current. Furthermore, the coil primary resistance for a front mount coil can vary by 3:1 over the spread of makers and time. If you are able to accurately (& not easy) determine coil primary resistance, then it isn't difficult to calculate the additional resistance needed. Finding that value is also a problem, because auto supply stores typically don't know the resistance of what they are selling. Tractor mail order sites commonly sell the 8NE10306 & depending on source & when purchased, they vary by 3:1 (terrible!). I take a good Ohmmeter to their counter & have them bring out what they have.