Originally Posted by dchamberlain

This is a question I had for a long time and I finally tested it with a clamp ammeter.

The voltage regulator in the alternator is turned on by the I circuit. In a single alternator setup, that circuit goes to the instrument cluster and is part of what controls the battery warning light.

The instrument cluster does not apply voltage to the I circuit, thus not turning on the alternator, until the glow plugs have shut off.

The test I did was to put the clamp ammeter on the B+ wire coming out of the alternator. I then started the truck cold and observed little voltage (less than 5 volts) for about 90 seconds. Then I observed it go up to about 90 amps.