If your "BATTERY" light is on while your engine is running, then the problem is usually either the alternator or the voltage regulator. This simple test will help you figure out which, without having to remove anything. It applies to 60's vintage GM cars using a points-style voltage regulator. Here is a picture of that type of regulator.
 "click for bigger photo")
Points-style voltage regulator with terminals identified
With the engine running, connect the negative lead of your voltmeter to ground (or the negative terminal on the battery) and the positive lead to the #2 terminal of the voltage regulator. If the #2 terminal is hard to get a good reading from, you can instead measure the voltage on the black wire that is in the 2-wire connector on the back of the alternator.
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back of alternator
If the voltage is under 5 volts, then the alternator is bad. (Or the belt tension is way too loose.)
If the voltage is over 5 volts, the problem is either the regulator or the wire harness between the regulator and the starter solenoid. Most likely it is the regulator. Since the regulator is so easy to remove, you should just pull it and take it in to be tested.
Dogman2 on Mar 28, 2009 said:
The Alternator will create a magnetic field if it's is working properly. The Charge light, Battery light or simple red light is grounded by the regulator when the engine is not running and the key is in the "on" position. The light, Gen., alt., battery, etc. has a 12 volt supply from the Gauges fuse. The ground is replaced by voltage supplied by the alternator, once it starts charging. That additional 12 volt supply shuts off the light. Since there is no longer both ground and voltage, but two voltage supplies.
[Reply to this comment]Anonymous on May 13, 2011 said:
You have any idea how long it took me to find an original alternator for my 68 impala. If you think I'm putting a screwdriver on the back of the alternator while the cars running.....oh forget it!
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Anonymous on Jul 21, 2008 said:
"Farmer test" While still running put screwdriver on back of alternator (watch out not to ground out terminals). If alt. is working it will stick, no sticky no worky. Get home, get it fixed.
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