Nice catch. As a former instructor I found it important to use correct terminology to prevent confusion. "AC Generator" is the standardized term the industry is going to. "Alternator" was introduced by Chrysler in 1960 and they copyrighted the term so you won't find it in most other service manuals. Regardless, whenever you say "alternator", everyone will know of which you speak and no one is going to correct you.
The voltage regulator inside the generator gets its turn-on signal voltage from the dash light circuit. Once it begins running the generator and it sees some output, it puts voltage back on that line to in effect cancel out the battery voltage, which turns the warning light off. That would imply that as far as the voltage regulator is concerned, it thinks the generator is working. If indeed it is, the output current is not getting back to the battery to keep it charged up.
Think of a garden hose with a kink in it. The water flow, (current) can't get through so you have high pressure at the faucet but little pressure at the nozzle. Voltage is electrical pressure. With a break in the wire between the generator's output terminal and the battery, you'll find the normal 13.75 to 14.75 volts at the output terminal while the engine is running but the battery voltage will stay at its normal 12.6 volts, or less. In that story the generator is trying to produce current but there's no place for it to go.
Most cars are using a bolt-in fuse in the under-hood fuse box for the generator's output wire. If yours has that, be sure those bolts are tight. Here again, the clue would be found by measuring the voltages on either side of it.
There are other potential causes of a no-charge condition but I've discounted some because the battery warning light appears to be working correctly. As for "How would I measure it", you need an inexpensive digital voltmeter. Harbor Freight Tools has a perfectly good one for less than ten bucks. I can help with how to use it but I'll wait to hear if I need to type all of that.
Friday, September 7th, 2012 AT 3:43 AM