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The Battery's main job is to supply power to the starter. Period.
Once the engine is running, the Alternator supplies power for Everything — plus a little extra to keep the Battery charged. (The Battery is just along for the ride at this point.)
If the Alternator output falls off, the Battery begins to take over (not its main job) and "running down". The fewer the electrical demands, the longer the running down will take. I once went about 40 daytime summer miles after leaving the D+ wire off the (V8) Alternator. (See item "B" below.)
The usual weak Alternator suspects are:
1 - The Voltage Regulator/Brush unit attached on the back of the Alternator
2 - Worn/grooved commutator (in the Alternator, where the brushes ride)
3 - Worn/Glazed/Loose/Slipping/drive belts (US cars have 2 belts.)
4 - Poor Voltage connection(s), Alternator-to-Starter-to-Battery +Plus Terminal
5 - Poor Ground wire connection (between Alternator body and mounting bracket)
If the Alternator output never rises above the Battery's 12 volts, it could be due to:
A - More severe versions of any of the items listed above
B - Failure to "pre-exite" the Alternator's charging circuits (requires current flow from the Battery Light, via the thin red wire, to the D+ Alternator terminal). Note that the BATTERY light must come on with Key in position II.
C - An internal failure within the Alternator itself (least likely)
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.
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