Ron. Good to talk on the phone (was it yesterday?). I know you said the brushes seemed to be fresh on the starter and that you had cleaned everything up. We talked about a possible "bad spot" on the armature which would cause the starter to fail, and will cause some people to beat on their starters with broom handles and whatever else is handy, . . . but if the armature is out-of-round the only cure is to put it in a lathe and turn it down until you have it perfectly round and concentric on the shaft. You probably know this already, but I bring it up for sake of completeness.
I'm not sure where the voltage detector fits in, but generally speaking, if one is interested in learning if a certain voltage is present there are many ways of doing so. The simplest is to tap into a resistor to get a reference at a particular voltage above ground. Another means is to employ a zener diode set for a particular voltage level; if the zener is conducting, then the required/desired voltage is present. (Note: the zener diode is reverse biased in this application.) I believe there are other, more complicated, circuits employing semiconductors like diodes and transistors, triacs, SCRs, etc. to set, detect, and regulate various voltage conditions; but my bet is that a zener diode is involved in your case.
It should be easy to test; put the desired/required voltage to it and see if it "detects" it in some way. You can also probe it with an ohm meter or, better, a digital multimeter with a diode testing function.
Just some thoughts to feed the grist mill of your mind.