Alternator size

bohica is right about rebuilds, but what it "was" even coming out of a "decent" rebuilder has nothing to do with what it "is" "now."

AND he's right about the RPM label. Those figures are ALTERNATOR SHAFT RPM, not engine RPM. My old sixpack car was close to 4:1 pulley size, which means that when that ol 440 was wound up into 5-6K, the alternator was spinning 24 THOUSAND RPM. And no, they didn't last long. (Way back in 72 I had an integral Delco on my 340, long long before Al Gore invented the www, or anybody had ever heard of "Denzo" whoever he is.

It might have a brush/ slip ring problem, IE worn brushes, greasy, poor spring tension

It might have a damaged rotor, IE a turn shorted internally which LESSENS the magnetic output of the rotor

It might have a damaged stator, ......

or one or two "blown" diodes

These are 3 phase AC devices, which means that each of the 3 phases does 1/3 of the work. If you take away 1 phase, you actually cut the output MORE than 1/3 because of efficiency changes and the "awful" wave pattern this generates, but simply........................

An alternator with reduced output has nothing to do with "what the label says."


You really need to do a "full field" test as I sort of outlined in the other thread.

One end of the field is supplied with switched 12V (the blue at the field) from the "ignition run" line

So unhook the GREEN field wire, clip lead that field terminal to ground, and turn the key to "run"

Measure the voltage at the blue field wire (hooked up!!) to be sure you have "close to" battery voltage.

Make ABSOLUTELY sure the belt is OK, is tight, and not slipping

Then run the engine, and see if low RPM performance is improved. Be careful, because at this point you have no regulator, and if you rev the engine too much with light electrical load, you can raise the charging voltage too much, especially if the battery is charged and hot. (warm)

If the low RPM output seems better, you probably have a field circuit wiring problem, or a bad regulator. If not, there is probably something wrong with this alternator.