1-20. WINTERIZATION SYSTEM. - Continued
b. The winterization system is dependent on the APU being in operation since circulation of the engine coolant is
necessary to maintain the heater in operation. If circulation is stopped (the APU stopped) while the heater is ON,
continued heating of the coolant contained in the heater unit, will cause the heater to shutdown. A thermal breaker in the
heater must be reset before the heater can be restarted. The APU should always be started before the heater is switched
ON and stopped after the heater is switched OFF.
c. The APU also drives an auxiliary alternator which is connected to the truck batteries. The output from this
alternator is sufficient to maintain the batteries fully charged while the winterization system is operating.
d. The APU and winterization heater are controlled by separate start and stop controls, however, the fuel supply to
both units is delivered by a common fuel pump, (see fig. 1-43). This fuel pump is energized only when the APU key
switch is set to Prime or Run. Excess fuel supplied to the heater or APU is returned to the fuel tank.
Figure 1-43. Winterization Heater and APU Fuel and Exhaust System.