WD Catalog 2018

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Chapter 5 – System Controls
5-8 - -
2. Vacuum Controls
Heavy trucks with diesel engines do not have a ready source of vacuum.
When vacuum components or controls are used, a separate vacuum pump
must be driven by the engine.
3. Passive (Automatic) Heater Controls
Various devices are used to control heater operation. These devices are
described below:
Water Valves
Some heater systems have an “H” type water valve design. Coolant
flows to and from the heater core through separate passages in the
valve. Means are provided to block coolant flow to the heater core
and bypass or route coolant back to the engine. A pressure valve in
the “H” opens when coolant flow to the heater is blocked. Figure 5-6
illustrates the “H” type coolant valve.
HEATER
CAB HEATER
WITH PRESSURE RELIEF VALVE
INSTALLED WITH CABLE OPERATED
WATER VALVE IN SUPPLY LINE
SUPPLY
WATER PUMP
WATER VALVE
CABLE OPERATED
"H" VALVE
RETURN
INLET
RETURN
Heater A/C (CTC II™) & Heater (CTC™) Systems
The CTC™ (Constant Temperature Control) system maintains a
preselected temperature in the cab. This is accomplished through a
controllable circuit board and variable resistor. Changes in the air
temperature activate the circuit board, which energizes or
deenergizes an air solenoid valve. The solenoid valve controls the air
supply to open and close the water valve and pulse hot engine
coolant through the heater core. In the AC mode, the thermostat is
cycled to hold a constant output temperature. Figure 10-3 shows
how the CTC II™ components work together to control heat energy
movement into cab air and maintain cab temperature.
Figure 5-6
This plumbing shows how
an “H” type water valve
may be used to allow cool-
ant to flow through or to
bypass the heater core.