In an effort to make the test a little more "pure" this is the procedure I used.
I removed my original idiot light bulb so my harness exciter wire is effectively not connected to the alternator.
A typical old truck wiring harness would have the ignition switch apply 12 volts directly to the ballast resistor. Not wanting to disrupt my harness, I just connected a temporary exciter wire between the 12 volt side of the ballast resistor and the alternator.
When that temporary exciter wire includes a 194 side marker type bulb in series, the system all worked as normal.
When I took the 194 bulb and socket out of the wire and connected the 12 volt side of the ballast resistor directly to the alternator with a plain wire, the engine continues to run after I turn off the ignition key.
Clearly an exciter wire needs to have either a resistor, an indicator lamp, a diode, or possibly a relay to prevent run-on after you turn off the ignition.
Mark
I'm getting ready to install my new alternator. I was thinking of doing the tests you did, but now I don't have to. Thanks.
Still, the difference between the light, a resistor, a diode or direct connection is interesting. If you replace the bulb with a resistor or diode and the resistance is nearly the same as the bulb's resistance, I don't see an issue. However, if the resistance is higher than the bulb, the alternator may "think" the battery is low and overcharge the battery. A direct connection (no resistance) may charge the battery at a lower rate.
