[RichardJ]

Steeveedee posted,
>> I know that the system works, because when I hook up a low battery it reads about half way up on the "C" side of the meter, <<

A voltmeter will not tell you that.

"0" volts is a voltage reading. It may not be what you think you need to see as a reading, BUT it is telling you that the current being used by the truck is being replaced by the output of the Alt and is maintaining a fully charged battery.

If you periodically check the Ammeter, just as you do your other gauges and suddenly it starts reading higher than normal, it may be time to start shopping for a new battery.
In my experience, when the Ammeter starts reading, even a needle width higher each day, you have somewhere in the neighborhood of 2-3 weeks before that battery is history.

A voltmeter will not tell you that.