The theory that adding a bulb will stop the over-run but a straight wire will not, makes no sense. We know that the bulb is used to indicate charge. The bulb must be in a two wire socket. One side is to the alternator blade terminal 1 and the other is to the ignition. (to a terminal that is hot when key is on).
When key goes to on, 12V goes from the switch thru the bulb and finds ground inside the alternator. The light lights. When the motor starts, and alternator is charging properly, 12V is sent thru the exciter wire (blade terminal 1) to the bulbs other end. Since there is no potential/no difference/no flow/no ground, the bulb goes out.
So we got that.
Now people who are supposed to be experts say use a bulb if you want an indicator. Use a straight wire if you don't. The blogosphere says some folks have run-on with a straight wire, but a bulb stops that because it has "resistance". So what?
If the motor stays running it means the coil is getting voltage. The only place to get voltage with key off (unless some kind of wire screw up) is from the spinning alternator. Voltage goes up the wire, which has a little resistance, to the ignition switch terminal which is "off".... But if that wire is on the same terminal as the coil wire, off is not off. Voltage continues thru the coil windings resistance, to the distributor, thru the points resistance to ground. SUPPOSEDLY. Some trucks have a ballast resistor. It goes thru that too. All that resistance and it still runs? But a little 1 amp light bulb that may or may not draw 1 amp at that point, but relies on the filament resistance, and that's enough extra to stop the motor? Not sure about that.
I believe various issue cause some to have run-on, some not
1. Miswire
2. Non isolated ignition switch terminals or a bad switch
3. No ballast 6V systems
4. HEI
5. One wire alternator
6. long wire runs
7. Short wire runs
The fact is, many folks run a straight wire. no problem. MAD Electrics and others say no bulb needed except for indicator.
