The clamp on amp meters that read DC and AC current use a Hall effect
IC. I bought one on Ebay for $60 over a year ago.
Most run of the mill clamp on amp meters use a transformer which will
only read AC.

On Tue, 7 Dec 2004 06:16:02 +0000 (UTC), David Lesher
wrote:

(John_F) writes:

Find or purchase a clamp on amp meter that will read both AC and DC
amps. Turn on enough loads so that the alternator is generating at
least 50% of it's rated load. Measure the DC amps and the AC amps of
the alternator output. The AC current should not exceed 15% of the DC
amps. If it exceeds 15% then you have a bad diode or stator winding.
In many cases you can not get 50% of rated output if you have a bad
diode.

Interesting approach. I'm not sure how well it will work with the
tools a GA pilot has.

A real clamp-on DC ammeter is a rather specialized tool. The usual
ones are AC -- the clamp makes a 1-turn transformer out of the wire
you are around. It may or may not read on dc-dominant paths; I can
see the trasformer getting saturated and/or only reading accurately
on balanced waveforms.

Hmm, it should be possible to use a DVM in AC mode on the
alternator output. Fair-good DVM's are really inexpensive.
A True RMS one would tell you in a second. But it would take
some experimenting to say what AC voltages would mean "yes,
you DO have a bad diode."...