Last weekend while driving the Cutlass I noticed that the voltage was slowly dropping no matter the speed I was driving. I charged the battery and went for another drive and while driving the voltage was down to just over 9V when I got home after about 20-30 minutes of driving.

Thinking the alternator might be the problem I took it back to where it was rebuilt and the test showed it was good. The alternator is a one wire.

Next I took the battery to the shop and when they tested it they said it was bad.

I took the battery out of my Delta and put it in the Cutlass. The test drive gave the same results with slowly losing voltage while driving.

Put the Delta battery back in the Delta and drove it and it was charging fine.

Other than the alternator or battery, what else could cause the alternator to not charge the battery on the Cutlass?

 

I think one-wire alternators are second only to scotchlok connectors in causing electrical problems. Since I assume you've correctly wired the internal regulator conversion for this car, throw that POS alternator away and simply get a stock 12SI. I like to use the 94 amp version from a 1986 Caddy Fleetwood with the Olds 307 motor.