> Well, not schematically seeing the way the resistor works in a
> circuit with an electronic ignition, my only guess about this is
> that some sort of loop circuit is created by the introduction of the
In a typical GM alternator, there is a separate set of 3 diodes (called the
diode trio) that create an output isolated from the main output. This
output is used to power the regulator and field circuit, and also brought
out to one side of the warning lamp (and in this case, one side of the
resistor). The other side of the lamp and resistor are connected to the
switched side of the ignition switch.
With the engine running, when the ignition switch is turned off, the
alternator continues producing power as long as the engine is still turning
fast enough. That means that the voltage from the diode trio is essentially
battery voltage. That voltage is bled through the resistor/lamp into the
ignition circuit (which is now disconnected from the battery by the ignition
switch being off). If the current through the resistor is enough to keep
the engine running, it will do so. However, when an additional load (like
the heater) is added, the voltage drop across the resistor becomes too high,
and the engine dies.
Which still doesn't explain why John sees the idle rpm rise. I was hoping
he would say it's something like a Pertronix, and I could theorize that the
lower voltage was causing it to advance the spark ...
> I've run into something similar to this before--on buses. We had a
> problem with some buses not shutting down when the battery
> disconnect switches were thrown off, because the ignition was still
> on. That created a loop where residual magnetism in the alternator
Why do you think it was residual magnetism ? Just disconnecting the battery
will not cause the alternator to stop working.
> The only way to solve the problem was to mechanically
> interrupt field input power from the run circuit.
Which proves it wasn't residual magnetism (which alternators rarely exhibit)
since interrupting the field current caused it to shut down. Clearly it was
the field current that was allowing the alternator to continue generating
power, probably even enough to charge the battery if it was connected.
Back when I was young and foolish, I frequently would swap batteries on a
running car engine. As long as the idle rpm is high enough, the engine will
keep running forever without a battery. This was useful in the wintertime,
when we had several cars (etc) to get running and only one of them would
start. Couldn't afford decent jumper cables ...
I should add that I learned why it's a bad idea when the regulator points
stuck on my Plymouth Valiant while I was driving it to the store to buy a
new battery (the old one was completely dead). The system voltage shot
through the roof, burning out the radio & MSD ignition. No idea how high it
went, but it could easily have reached several hundred volts before the
alternator diodes popped. Most likely in my case, the MSD saved the diodes
by failing first.
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