I don't think that circuit does any triggering or exciting, I think it is a feedback loop to trigger the light. If the alternator is putting out, there is 12V on either side of the bulb. If the alternator quits charging, or is under-charging, the bulb comes on as the battery voltage is greater than the output of the alternator, so the potential difference triggers the light. It's just like the old Harley dash lights. Consequently, a voltmeter there would show you exactly what the voltage is doing...I've already wired my voltmeter to the accessory side of the ignition switch, which is good enough for me.

It's a motorcraft. It's original to the engine, but seems to be an oddball for an '89 Mustang.

I don't think it's necessary. I've already disconnected it, left the wire there and capped both ends. If it turns out I need it, I'll trouble-shoot more then - right now, I just want to finish the wiring. Did I mention I really hate wiring?

 

Well, that wire is needed. After I got my truck running, I checked and it wasn't charging. So, I had an old parking light socket and I isolated it in series with that green wire. When I turn on the switch, the light will slowly begin to glow dim. Once I crank, the light goes out and it charges properly. Since I have a voltmeter and I don't want to mount a light anywhere, I'll probably just replace it with a resistor of some kind...I'll start with 500ohm and go from there.

 

So current flows from the battery to the I terminal until the alternator is charging? Then it goes out because the battery and alternator are seeing the same voltage? Personally, I don't mind the lights but only if I also have a voltmeter or ammeter.

 

I guess so, Ross. I would put the light in if I had a place to put it. I don't , so I'm going to stick a resistor in there and see if that works - may try a potentiometer instead of guessing and hoping.

I do have an extra small hole on the column drop - I might be able to put a small LED there, but I don't know if the LED would have enough drop across it...who knows. Might try it.