Charging system question

If it is idling quite slow, it may not show additional voltage at dead slow idle. But a slight increase in idle speed should result in a voltage increase to about 14. VR is temperature compensated. It should read different when hot or cold. "Warmed up" is nominally 13.8---14.2 with battery normalized

Measuring voltage at the alternator output terminal might show you if there is a harness problem, such as a bad connection in the bulkhead / firewall connector

"The quick and dirty" test for a no charge situation (check belt tension) is to jumper power to the alternator field terminal after disconnecting the green field wire, then run the engine, increase RPM and see if battery voltage starts to go up