Any idea what this " toggle switch " is to control ?
If he says the alt won't charge the batteries, has he been running the engine?
If he is jump starting the rig and just leaving it run, it would take days for the alt to build up any charge 5 year dead batteries.
If the toggle is for transmission control , an Allison trans controller won't do a thing without good battery voltage.
Alternators , unlike generators, require power to make power, if you spin a 12v generator , it will produce 12v+ power. If you spin an alternator attached to a dead battery ( say 7v of power in the battery) you would get 8.5>9 volts out of the alt. the alt windings have to be energized in order to function properly; the higher the winding power the higher the alt output.