From the wiring diagram I found, it has something to do with the tachometer. When I read it, I get 12V (battery) when the key is turned to the position right before cranking. After the engine is started and idling the voltage I read is about 14V and if I push the throttle the voltage will increase with RPMs.
Sorry for the stupid image but that's my circuit. If I read across the cap and across the entire circuit here are the readings I get:
input 12V cap 2.3V
input 14V cap 2.4V
input 16V cap 2.3V
When I rev the engine, the voltage reading across the cap is going down! It should be going up like the input voltage is. What is going on? I'm not good with caps. The cap is rated at 25V.