Heater System Introduction
The air conditioning control unit performs the following functions.
– Controls operation of the blower motor.
– Controls operation of the heated rear screen button.

RCH (Passenger Compartment Heating Resistors) heater

● Introduction
The direct injection (diesel) engine is designed for direct injection of fuel into the combustion chamber.
This means that the engine’s water temperature increases slowly because the cooling circuit on the cylinder head of
the engine’s upper part is small. For a structure that uses the engine coolant for heating the passenger
compartment, it becomes difficult to heat the vehicle’s passenger compartment in a short period of time in extreme
winter temperatures.
Therefore, in order to reduce the time required for heating, a heating register called RCH heater is installed at the
rear side of the heating unit, which is controlled on/off according to various information such as battery voltage,
external temperature, and engine coolant temperature.
As the RCH heater requires much power, the system operation is controlled according to the vehicle's overall
condition.
● UCH input/output signal for RCH heater control
– Input signal
(Multiplex connections) Engine coolant temperature, external temperature, power disconnect request, engine rpm, engine status, engine fan motor operation request
(Wire connections) Blower switch, battery voltage
– Output signal
(Multiplex connections) Idle speed increase request, RCH heater relay operation request, RCH heater power consumption information.
● Function
– Control according to temperature
The passenger compartment connection unit uses external temperature and engine coolant temperature input
signals to control operation of the RCH heater according to the temperature.
When the heater is in operation, the target passenger compartment temperature is fixed at 50˚C and the passenger
compartment connection unit determines operation of the RCH heater relay (1, 2, 3) depending on the engine
coolant temperature and the external temperature in order to reach the target temperature.
The RCH heater operates when the external temperature is 5˚C or below and cannot be operated when the external
temperature is 8˚C or above.
However, its operation is possible even when the external temperature is 8˚C or below during initial operation. (See
the below Table)

– Control according to power
The RCH heater requires much power. The passenger compartment connection unit uses the engine rpm, engine
status, engine fan motor operation request, and power disconnection request input signals to determine the RCH
heater relay operation (1, 2, 3). (The RCH heater does not operate when the engine is not operating.)
The RCH heater operates when the battery voltage is 11.5V or above and cannot be operated when the battery
voltage is 11V or below.
However, its operation is possible even when the battery voltage is 11V or above during initial operation.(See the
below table)

The passenger compartment connection unit uses the power disconnection request signal (maintain current status
or stop) to control the RCH heater on/off. When multiplex network with the Injection computer is not possible, it stops
the RCH heater operation.
– Operation stop control according to the blower switch
The RCH heater does not operate (turns off gradually) when the blower switch is off, and the RCH heater operates
when the blower switch is on.
– Relay control
RCH heater relay (1, 2, 3) operations are as follows:

To prevent sudden power consumption, when the RCH relays operate simultaneously (RCH heater level 2 or 3), the
relays operate consecutively at 1 s intervals.
– Engine idle rpm control
The passenger compartment connection unit uses the idle rpm increase request for efficient power control, and the
Injection computer increases the engine idle rpm.