Charging system has been working fine until today. When I started my 78 up today, she started just fine. I noticed that the voltmeter was registering about 12 volts. Normally it is up over 14. When I turn the key on it registers about 12 volts and does not increase after the engine is started. I took the alternater off and took it to Adnanced Auto and had it checked they said it was fine, putting out over 14 volts. I did notice that when the engine was running and I openned the power windows the voltmeter went down which to me means it is running off the battery. Any recomendations on where I go from here?

Check the actual voltage AT the battery, and see what it is, both running, and not running. The gauges in these cars aren't always the most reliable from day to day.....

Thanks, Adam, I will give that a try.

With the 2 prong plug off from the Alt. their should be 12 volts (or Batt voltage) with key on, not running. But Joel would know. if this voltage was not present I think the Gen light would be on?

I believe that is cerrect, Bob. We first need to determine what the voltage is at the battery, then go to the alternator and check voltage there, and see if there is a difference. If the battery voltage is low engine running, we either have a wiring/connector problem, or a battery problem, since the alternator checked out good.
However, if the battery voltage is good, and the voltage at the alternator is close to the same, then the most likely culprit would be the gauge.

The early GM 1-wire alternators (1972-1986) will need to have the RPM raised to 1000 – 1200 RPM initially to get the alternator charging and then it will continue to charge after that. The later style GM and all Ford and other style 1-wire alternators will excite immediately when the vehicle is started. The one wire alternators cannot be used if you want to use a dash charging light. You would need to use the plug-in style regulator in order to complete the circuit to operate the light.

I believe 72 82 use the 3 wire Alt with internal regulator. if you do not have the red wire to the #1 terminal you will not get proper load handling. that is the voltage sensing circuit so as load increases the voltage is increased to handle it. The #2 terminal is for the Gen or Alt light. one wires are good for hot rods with little or no accessories