Had to jog my memory banks....by looking it up on Randals site....
http://www.randakks.com/TechTips.htm#38First, the ballast resistor is the long, white ceramic thing attached to the left side of the coil assembly. Basic function: the ballast resistor is a resistor contained in the primary ignition circuit wiring that lowers voltage after the engine is started to reduce wear on ignition components. Less well understood is the fact that ballast resistors provide appropriate compensation within a circuit for external demands placed on that circuit. One example of this would be to compensate for temperature changes.
The coils, ballast resistor, points and dual condenser of a GL1000 are optimized for proper ignition function by coils that are designed to deliver full secondary circuit energy pulses to the spark plugs with primary input circuit voltage of approx. 7-9 Volts ... not the 12 Volts that you might expect.
This is so you will have "full" power to the coils during start-up...if the coils were "expecting" 12 volts they would be disappointed! The nominal 12 Volt battery voltage is reduced considerably during start-up by the heavy effort to energize the starter and spin the engine...hence the reduction to around 7V as the "normal" state of affairs for these coils (at idle). Sometimes, you might hear geezers like me refer to GL1000 coils as "6 volt" coils. Don't get hung up on the exact values - the relative values are what's important here.
Once the engine fires, the alternator (stator actually) comes online and battery voltage is regulated to approximately 14.5 Volts at normal cruise rpms (voltage is less at lower rpms). Without intervention, the coils which are optimized for 7-9 V would now be seeing primary input voltage of 14+ volts. Clearly, this would cause problems of overheating and reduced life for ignition components. This is where the ballast resistor enters the picture.
Through clever wiring, the power to the coils is always routed through the ballast resistor unless the starter is spinning - in which case the ballast is by-passed. The resistance value of the ballast resistor is calibrated to deliver the 7V or so expected by the coils at idle. During starting, the ballast is by-passed and full available voltage is routed to the coils. The beauty of this design is that you get quick starts AND long life for the components. I won't go into the theory, but the ballast cleverly compensates somewhat for the ignition requirements required by changes in temperature and engine rpm.
By Randall Washington..
