[Argess]

You are correct with your test....12 VDC across battery with engine off, 13.8 (or thereabouts)with engine running.

Just to be certain, try checking right at the output terminal of the alternator for the 13.8 volts. Try your voltemeter ground probe on both the alternator housing and the engine (block, intake, whatever is a good ground) when you do this test.

I know you said you checked grounds, but is the alternator case itself grounded? Painted adjustment straps etc could be a problem.

The AMP rating of an alternator is the max current that can be drawn safely from an alternator. It is usually defined by the designer based on the max current the diodes can handle without burning up. The alternator is actually a device that produces 3-phase AC and has a 6 diode bridge rectifier circuit in it.

I'm not familiar with your particular alternator, but I don't suppose there is mention of an output voltage adj in the literature that came with it?

All in all, your tests so far indicate a bad alternator.