Posted by Brian in Oregon on August 19, 2000 at 19:29:36:
In Reply to: Re: Alternator question. posted by Merrill Fish on August 19, 2000 at 19:21:59:
First, Merril says the larger pulley on the alternator will run the alternator faster. Actually, a smaller alternator pulley spins the alternator faster, assumming the crank pulley remains the same.
At idle, the voltage output of an alternator often drops, unless small diameter pulleys are used. Even then it may not spin fast enough to put out rated output. This is why lights appear dimmer at idle, even on some vehicles with the battery hooked up.
You need a voltmeter to read the voltage across the battery. You can get suitable ones from, say, Radio Shack. Check the voltage across the terminals. They normally are just on either side of 12 volts (assumming a 12v system). The alternator should bring that up to about 13.5 volts. I prefer a limit of 14 volts. At 15 volts the batteries are being ruined.
You can also buy a volt gauge and use it as a test unit, if that makes it easier for you.