The auxiliary air compressor supplies air for the exhaust brake, air seat, and air horns if equipped. The compressor is energized by a relay that is controlled by the auxiliary air tank pressure switch located in the reservoir. When the auxiliary air tank pressure switch is closed it supplies voltage to the auxiliary air compressor relay. The relay energizes and supplies battery voltage to the auxiliary air compressor. When air pressure builds to 724-827 kPa (105-120 psi) the switch opens and the auxiliary air compressor shuts off.