If so - Key on, engine off - does the light come on? Should be "yes."
Engine Running - bulb on then? Should be "no."
I know that sounds like "busy work" and would be obvious, but -
Reason I asked is the excite current for the alternator comes through the light. If the light's burned out, it can't charge.
The bulb check also checks not only the bulb, but the excite wiring itself.
It runs from "ignition switched 12 volts," through the bulb, to the alternator (or if it has a separate regulator, to that.)
If the above bulb checks are fine, then - Engine Off, Key Off - measure right at the big alternator terminal - should be 12 volts. If not, check the wiring between that terminal and the batt +.
Autozone testers have tested lots of defective alternators as "good." Reason is most people buy their alternator without bringing it in to be tested first. When the new or rebuilt Alternator they just bought doesn't work properly, they bring it back. Before adjusting one for warrantee, Autozone requires it to be tested first.
Naturally all their trash will test OK so they don't have to exchange it. Their test will not usually detect a bad diode, or a bad diode trio. They all will test good. Verify that by taking that same alternator to an alternator shop so they can test it - you'll see.
Last edited by RRich; 08-18-2008 at 09:58 AM.