Quote:
Originally Posted by Douglas.Sherida
Once the alternator is spinning (in the presence of the priming current) it generates its own self sustaining magnetic field and the higher voltage current it produces somehow shuts off the idiot light.
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Just FYI, if you were to look at an electrical print (at least for a GM car) you would see the light does work exactly as you said, and the reason the light behaves this way, is that the circuit is designed with (depending on make and year) either a diode or a given amount of resistance in it, and basically works by either reversing electrical flow so the diode blocks it off, or providing a less resistive path when the alternator is excited. (electricity will always flow on the least resistive path).
I agree it sounds like you have an issue with not properly charging, maybe that mysterious drain only when running is from it trying to "excite" the alternator, as that circuit would only be enabled in the igntion switch in the run position.
Here's another test idea to try. Because this is a diesel, once running there should be no difference in electrical load in the run position whether running or not. If they use the resistive system to operate the light, you will see voltage whether or not current actually flows on that circuit. So, grab an ammeter and measure load on the battery with it in the run position not running, then do the same running, that will tell you if you really have a sizeable short causing the drain. You might see a difference from the alt trying to charge it, to eliminate that doubt you might do a 3rd test with the alt disconnected and running. Just some more ideas.