TM 9-2320-361-20 1-20. ELECTRICAL SYSTEMS OPERATION (Contd) c. Generating System Operation. ALTERNATOR - Supplies electrical power to operate all electrical accessories and keeps batteries charged when vehicle engine is operating. d. Heating System Operation. HOT WATER PERSONNEL HEATER - Warms interior of vehicle cab in cold weather. Hot engine coolant circulating through heater is utilized to warm air flowing through heater core. An electric motor, with two speeds, powers a fan to force outside air through heater and into cab. PERSONNEL HEATER SWITCH - Allows heater fan to be operated at two speeds to ensure comfort of personnel in cab. 1-58