whine. First, most inexpensive AC voltmeters donít actually indicate the correct voltage unless the AC is a good sine wave. So to measure the ripple, youíll need a meter which read True RMS. Second, based on the several articles I found, there doesnít seem to be a good agreement on how much ripple is "too much."
So Iíd probably try to find somebody that had the good AC meter. Theyíre much more common than an o-scope. If you read a few hundred millivolts of ripple, youíre probably OK. If itís several volts, then you can take the alternator
apart to check the diodes.
When you install a new alternator, measure the ripple right then. It gives you a baseline for detecting future problems.