Is the device you are asking about not a pressure regulator to control the amount of tension on the belts?
From the foggy recesses of my brain this AM...and a schematic... No air pressure on the belts, the system won't allow the alternator to be loaded, and the low air light will be on. Somewhat goof proof, eh? On my '75 MC8, the "Gen Control and Low Air switch" is plumbed into the loop to the alternator belt tensioner. So, this is where the air pressure is sampled for the low air warning, as well as allowing the alternator to come online once sufficient belt tension (air pressure) is achieved to prevent throwing them off or slipping under load. Good engineering? Two birds, one stone.happy coaching!buswarrior
...I can offer several reasons why it was setup the way it was. The coach air system use high amperage motors and when the coach is first started if the air is selected on this would put a large load on the alternator before spinup occurs. So the air tensioners needed to be tight before a large load is presented to the generator. cheers mike