I have read a few post on this but still have to ask...Is the amp guage in a '72 SS dash actually a volt guage? & What should the reading be at idle. Mine moves a little when the engine is revved but stays at close to 0 when idleing.

Is the amp guage in a '72 SS dash actually a volt guage? & What should the reading be at idle. Mine moves a little when the engine is revved but stays at close to 0 when idleing.
The gauge is really a centered millivolt gauge and reads the voltage drop on the battery charge wire. To test the gauge, with the key on but engine not running, start turning everything on in the car. Start with the highbeams, the gauge should move towards the discharge side. The more you turn on, the more it should read discharge. Now leave the highbeans on for 10-15min, then turn everything off and start the engine and run it at a fast idle, the gauge should move towards the charge side, as the battery gets charged by the alternator, it should move slowly back towards the center. Now the test above just makes a amp gauge thats works correctly move a little more than it normally would. Normally you don't see a lot of movement from the amp gauge as it only supposed to show the batterys charge rate. With the engine running and a fully charged battery, the gauge will normally stay about a needles width above the center line. Anytime the needle is on the discharge side with the engine running, (even a needles width) you are running off the battery some and the alt isn't keeping up. This can happen sometimes at idle, depending on how much is turned on, but should never happen at a fast idle and higher. Hopes this answers your question.