WD Catalog 2018

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Chapter 5 – System Controls
5-2 - -
1. Active Controls
Active control types depend on system design, location and available
space. When a heater or AC system has been installed as a separate or
supplemental unit (perhaps in the sleeper cab), there is probably more
than one control panel location.
The integrated HVAC control panel shown and described as Figure 5-1
is mounted in the dash. With these controls, the AC and heater systems
may be turned on, the temperature adjusted, air directed and mixed, and
air velocity controlled.
HIGH
MED
LOW
OFF
RECIRC FRESH VENT FLOOR BI-
LEVEL
DEF
A/C HEAT
COOL HOT
TEMPERATURE
AIR DIRECTION LEVER
CONTROLS CAB AIR FLOW
FAN SWITCH
FOR 3 SPEED
BLOWER
CONTROL
TEMPERATURE CONTROL LEVER
FOR MODULATING WATER VALVE
When the air direction lever is in the AC mode, power is directed to the
thermostat and from there to the compressor clutch. As the AC unit
operates, the thermostat cycles the clutch on and off automatically. Some
controls have a separate heat-AC switch for this purpose.
The slide controls could be connected to push-pull cables or an electric
or air control module. With air or electric controls and the lever in the
RECIRC mode, air within the cab is recirculated through the evaporator.
In all other positions, fresh air is brought in from outside the cab. Some
control panels have a separate fresh/recirculating air switch or lever.
The fan control knob is connected to a four position switch. There are
three “on” positions to control blower speed and regulate air velocity
(CFM) in the system.
Note: Blower or fan speed affects air temperature. At lower
speeds the cab air moves more slowly through the fins
of the evaporator or heater, so more heat energy
moves into or out of the cab air than at faster air
speeds (CFM). Thus air leaving the vents or louvers
will be colder or warmer.
Most air vents or louvers in the cab may be adjusted by hand to moderate
and direct air movement. These are in various sizes, shapes and locations
in the cab.
The thermostat and expansion valve or refrigerant metering device
basic controls because they are necessary in any system. Both function
automatically after the system is turned on.
Figure 5-1
HVAC in-cab controls are
described in this illustra-
tion.