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"Question for the Board's electrical systems experts: is it possible - and if so, is it wise - to run a wire from the battery's + terminal to the back of the alternator? Or, does that create a risk of damage in the long-term"?
This information relates to my experience with the Denso 100 Amp alternator which is on both my wife and son's 940.
It may or may not apply to other model alternators.
It's possible, but not wise to make a hard connection from the battery positive to the excitaton circuit on the alternator.
The reason(s) are:
1. You wouldn't have the convenience of a charge light to tell you that your alternator is charging.
2. The excitation circuit (originating in the instrument cluster and going directly to the alternator via the small red wire) powers directly to the internal voltage regulator.
This allows the charge light to illuminate.
The voltage regulator provides a circuit to ground to excite the alternator field windings until the alternator begins charging.
After the alternator begins charging, the voltage regulator opens this circuit
and the alternator continues to provide its own field (excitation) voltage.
If you connect a wire directly from battery positive to the alternator excitation terminal, you will have a constant drain of the battery, and probably damage the voltage regulator over time.
On my son's 940, I did run a seperate wire to the excitation terminal.
I ran this wire from a "switched hot" source under the dash and on to the alternator.
Inside the car, I pulled a loop of this wire out of the dashboard, cut it, and soldered a standard 1034 12V light bulb into the circuit.
I soldered directly to the bulb terminals, hot in on one terminal, hot out on the other.
This did three things.
It gave me a "idiot light" to tell me if the alternator was charging.
It provided a dependable excitation voltage source to the alternator that would only be on when the ignition switch is on.
It provided a current limiting device (the light bulb element) to the alternator excitation circuit.
The bulb element will handle the current that is required to excitate the alternator field.
I later went further and installed a bulb holder in the circuit to avoid having
to solder directly to the light bulb.
This will make it easier to change the bulb should it ever need replacing.
hope this helps
steve
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