No one else has suggested this, so I will. It could be that one of the internal diodes are bad. This would cause the output of the alternator to drop (at low rpm) output; it would only be generating power for 2/3 of each rotation. Check the diode pack, have it checked on a tester that can verify allthe diodes are working, or hook it up to a scope.

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My truck is a Frankenstein creation
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How would I go about hooking it up to a scope?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

How can you see a bad diode with a scope, since there are 2 others there as well?

You can take it to the same parts store you bought it from and have it tested I bet. Most kragens etc have alternator testers

You hook up the scope to the output of the alternator. Most automotive alternators are built as 3phase generator; so require 3/6 diodes. The output has a strong ripple on it at the alternator and you can see each of the diodes working. If one is bad, there will be a timed dead or flat spot in the ripple that you can pick out.

Of course, the easiest thing to do is go to the autoparts store and have them test it...

remember, I'm a physicist/engineer/programmer and can't help being a little geeky sometimes (just whip out your scope and the laptop to analyse the signal...)

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My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Jopapa,

You worry too much! I've been working on electrical systems for over 30 years and I can tell you that many systems go down to 12v at idle. It is more important that it doesn't go over 14.4 when charging.

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Chuck Rivers, Webmaster