WD Catalog 2018

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10-8
Chapter 10 – Component Repair or Replacement
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Problem Activating AC Clutch Circuit.
Inspect the low pressure switch located on the air block. The switch
is normally closed. It must have air pressure to disengage the clutch.
If there is no power to the clutch proceed as follows.
1. Make sure there is power to the pressure switch—use a
voltmeter or trouble light to check.
2. Make sure there is air to the controls.
3. If there is air and power available, place a jumper across the
two terminals on the low pressure switch. If the clutch en-
gages, there is a problemwith the pressure switch or air block.
4. Remove the switch from the air block, move the lever to heat
mode. If air exhausts from the port, replace the pressure switch.
If no air is vented from the port, the air block is defective.
5. If the air block and pressure switch check out OK, inspect
AC system clutch, thermostat, Trinary™ switch, or other
pressure switches, and wiring.
CTC™ (Heater) and CTC™ II (Heater–A/C System)
The constant temperature control systems maintain a pre-selected cab
temperature by pulsating the flow of hot engine coolant through the
heater core. In the CTC II configuration the compressor is cylced to
control temperature in the AC mode. The four major components of the
system are illustrated in Figure 10-3 and described as follows:
TRUCK
AIR IN
AC MODE
PRESSURE
SWITCH
TO ACTIVATOR
AIR TO
CONTROL PANEL
E/C TO
CONTROL PANEL
CONTROL PANEL
VARIABLE RESISTOR
HOT WATER IN
FREON
COMPRESSOR
AIR
EVAPORATOR
EXPANSION
VALVE
THERMOSTAT
AIR OPERATED
WATER VALVE
AIR
TEMPERATURE
SENSOR
DEFROST
HEATER CORE
TREATED
AIR
COOL
HEAT
CONDENSER
RECEIVER-DRIER
SOLENOID
ELECTRONIC
CONTROL
MODULE
E/C
Circuit Board Assembly—The assembly is located on the heater/AC
unit. It supplies power to resistor and solenoid, and monitors air output
temperature.
Variable Resistor—Located on the control panel and used with tem-
perature control lever to select desired temperature. It translates the
desired temperature into a control voltage for the circuit board.
Figure 10-3
The CTC II system is shown
in this drawing.