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Battery light means alty isn't charging at all. If it charges it sends opposing voltage on that circuit, so nothing flows, so that way the light shuts off.
check for, on the alty harness.. (I forget the wires exactly)
-12v coming in
-a low voltage coming out... whatever that may be
Next go to the voltage reulator (unless that's in the alt.. some GMS have that, I forget if the alero does)... sometimes in the PCM (chrysler mostly.. I think.. but I don't remember)... sometimes just a box. Search wiring diagrams online to hopefully find it.
Check to make sure you have a bit of voltage on the pin coming from the alt going into the regulator (regulator just controls current flow in field winding on alt). If there is a little voltage on there, back probe into it, and ground that wire BRIEFLY***!!! The engine should bog a bit, and system voltage should spike (like 16V or up.. which is why you only do it BIREFLY).
If it spikes when you ground the wire going to the regulator, the regulator's bad.
If it doesn't have 12v coming into the alt.. you have a problem with the circuit that feeds it.. time to test around more.
If you have 12V coming in, and 12v coming out, and 0v at the regulator, you have an open circuit between the alt and regulator.
If the field circuit all tests out OK, you can be pretty sure the alt's bad.
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