My voltmeter shows
12.46 volts across the terminals of the battery when ignition is off and on.
As soon as the car starts, the voltmeter reads
1. This reading would normally be seen when measuring resistance, not voltage. The
1 would indicate an open circuit.
After the car is shut off, the reading returns to around
12.46. (Actually, a little less than that due to the voltage drop.) I am assuming this would indicate the alternator circuit is in series.
What could potentially cause these readings? Open circuit, blown fuse, ground short?