go to fergusontractors dot org, click on resources then wiring diagrams. Use diagram listed as 2 wire alternator (it is 3 wire diagram)
Assuming you have a Delco alternator. With switch off you should have battery voltage at large output stud on rear of alternator and at #2 pin on the plug. Zero volts at #1 pin.
With Switch on, not running, you should still have battery voltage at stud and #2, but should now see some voltage at pin 1. (I've seen it start charging with only 4 volts on pin 1.)
If voltages missing troubleshoot for broken wires or bad connections. If voltages are present your alternator is probably due for exchange.
Also check that your fan belt is not loose.