Hvac System Operation; Control Unit - PREVOST H3-41 Maintenance Manual

SECTION 22: HEATING AND AIR CONDITIONING
eliminates odors, and finally heats or cools the
lavatory with the vehicle's ambient air.
FIGURE 69: OVERHEAD COMPARTMENT VENTILATION
SYSTEM
6.3

HVAC SYSTEM OPERATION

To operate the HVAC system when the vehicle
is stationary, engine should run at fast idle.
During operation of the HVAC system, windows
should be kept closed and door not left open
longer than necessary.
battery discharge, the HVAC system will not
operate if the battery voltage drops below 24
volts.
The vehicle is divided into two areas:
• Driver's area (driver's HVAC unit).
Passengers' area (passengers' HVAC
•
unit).
The A/C compressor starts automatically when
the two following conditions are satisfied:
1. The outside temperature is above 32°F
(0°C).
2. The passengers' area temperature is
within 7° (4°C) of the set point or higher
(if set point is 68°F for example, return
air temperature must be above 61°F, at
this moment, enough heat is available
from the engine to warm up the area
while the air conditioning will remove
moisture in air and prevent fogging up of
the windows).
NOTE
Upon starting, if the outside temperature is
above 32
°F (0°C) and then drops below
(0°C), the compressor will keep running up to
a temperature of 15°F (-9°C) to prevent
fogging up of the windows.
56 PA1621 Maintenance Manual All Series | Revised February 2019
22211
In order to prevent
32
°F
NOTE
To perform a test of the driver's section
windshield defroster, it is possible to run the
system without running the engine.
6.3.1

Control Unit

The temperature control in the driver's area is
provided directly by the DRIVER control on the
L.H. side of the HVAC control unit.
FIGURE 70: CONTROL UNIT - CENTRAL HVAC SYST
22276
The driver's unit piping is paralleled with the
passenger's unit piping.
same refrigerant and coolant, and are linked to
the same condenser and compressor, even if
they are individually controlled.
engagement of the A/C compressor magnetic
clutch; consequently, the driver's unit cannot be
operated alone in the air conditioning mode.
NOTE
The driver's HVAC unit turns on automatically
at starting of the engine (multiplex receives the
"engine running" signal) when the multiplex
system receives an "engine running" signal.
The PASSENGER control on the R.H side of the
HVAC control unit enables the selection of the
temperature in the passenger's section.
Temperature control is provided in conjunction with
a thermistor sensor located inside return duct, on
L.H. side of vehicle (Figure 2, Figure 5).
The flow of hot water to the passenger's unit
heater core is controlled by the solenoid pilot
pneumatic hot water valve which varies the
cycling rate depending on selected temperature.
A red LED indicator, located on the HVAC control
unit, illuminates when the heating mode is one. A
green LED indicator illuminates when compressor
clutch is in operation.
NOTE
It is not uncommon for both the red Heat LED
and the green AC LED to be illuminated at the
same time. This indicates that the temperature
control is requesting heat and the HVAC control
is calling for compressor
Both units use the
It requires
operation for