SECTION 22: HEATING AND AIR CONDITIONING
PA1621 Maintenance Manual All Series | Section Revised November 2017
67
and transmit their heat to the water passing
through the heat exchanger.
The heat is thermostatically controlled and
operates intermittently, i.e. the switched-on
times of the burner vary depending on the heat
requirement. The water temperature depends
on the setting of the built-in water thermostat.
The water circulation pump remains in operation
as long as the preheater is operating, even in
the regulation intervals and during the delayed
cutout of the switched-off preheater. The pump
can also be operated independently from the
preheater by means of an appropriate circuit.
The preheater can be switched on at any time,
even during the delayed cutout period. Ignition
takes place once this delay time is over.
When the preheater is switched off, the fuel
supply is interrupted. The flame goes out, and
at the same time a delayed cutout of some 2.5
minutes begins. The combustion air still flowing
flushes the remaining combustion gases out of
the chamber and cools off the hot parts on the
exhaust side of the heat exchanger, while the
water circulation pump, still running, transmits
the heat present in the heat exchanger, thus
preventing local overheats. Once the delayed
cutout time is over, both the combustion air
blower and the water circulation pump switch off
automatically.
A cutout will take place in case of any failure of
the preheater.
8.1 PREHEATER FUEL FILTER
The preheater fuel filter is located beside the
preheater above the rear wheelhousing, behind
the vehicle’s L.H. side rear fender.
MAINTENANCE
Replace preheater fuel filter every 50,000 miles
(80 000 km) or once a year, whichever comes
first.
8.2 OPERATION
Switch on the preheater. The operation indicator
lamp comes on and the preheater motor and
circulating pump begin to run. After about 10-25
seconds the solenoid valve opens and fuel is
sprayed into the combustion chamber. At the
same time, the electronic ignition unit produces
high voltage (8000 V) and the mixture of fuel
and air in the combustion chamber is ignited by
the spark on the ignition electrodes. The flame
is indicated by the flame detector, then the
electronic ignition unit stops producing high
voltage and combustion continues by itself
(spark on electrodes is required only to ignite the
flame). At this moment, the preheater is working
and producing heat.
If the preheater is switched off by the on/off
switch, the solenoid valve interrupts fuel supply,
combustion stops and indicator lamp turns off.
Combustion air fan still blows air, cleaning the
combustion chamber of any fumes and cooling
down the combustion chamber. Coolant
circulation pumps coolant, making a purge cycle
for approximately 2-3 minutes, thus protecting
the preheater against overheating.
If the preheater is not switched off by the on/off
switch, the control thermostat will switch off the
preheater when coolant temperature reaches
165° ± 6°F (75° ± 3°C) and turns it on at 154° ±
9°F (68° ± 5°C). During this time, the preheater
(combustion) is off and the indication lamp and
coolant pump are on. Combustion air fan blows
air for 2-3 minutes and then turns off.
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