Heater
Heater
The heater controls the temperature, volume and distribution of air supplied to the distribution ducts as directed by selections made on the ATC (automatic temperature control) module control panel. The heater is installed on the vehicle center-line, between the fascia and the engine bulkhead. The heater consists of a casing formed from a series of plastic moldings. Internal passages integrated into the casing guide the air through the casing and separate it into two flows, one for the LH (left-hand) outlets and one for the RH (right-hand) outlets. Two drain outlets at the bottom of the casing are connected to overboard drain hoses in the sides of the transmission tunnel.
The heater incorporates:
^ A blower.
^ A blower motor control module.
^ A heater core.
^ Control doors.
^ Control door motors.
^ The thermostatic expansion valve and the evaporator of the A/C (air conditioning) system.
^ The evaporator temperature sensor and either one (manual system) or two (automatic system) heater temperature sensors.
Manual Heater
Automatic Heater
Blower
The blower is installed between the air inlets and the evaporator, and consists of two open hub, centrifugal fans powered by a single electric motor. Operation of the electric motor is controlled by the ATC (automatic temperature control) module via the blower motor control module (voltage amplifier) installed in the outlet of the RH (right-hand) fan.
To produce the seven blower speeds the ATC (automatic temperature control) module outputs a stepped control voltage between 0 and 8 V to the blower motor control module, which regulates a battery power feed from the CJB (central junction box) to the blower. The control voltage changes, in 1 V steps, between 2 V (blower speed 1) and 8 V (blower speed 7). If the control voltage is less than 2 V the blower is off.
Blower Motor Control Module
Heater Core
The same heater core is used in both the manual and automatic heaters. The heater core is internally divided into two separate halves, with separate coolant inlets for each half and a common coolant outlet. On the manual system, the two coolant inlets are connected to a common feed from the single coolant valve. On the automatic system, each coolant inlet pipe is connected to a feed from a separate coolant valve.
Heater Core Assembly
Control Doors
Control doors in the heater control the source of inlet air and the distribution and temperature of outlet air.
On both the manual and automatic heaters, a fresh/recirculated air door is installed in the air inlet on each side of the heater. A stepper motor drives the LH (left-hand) fresh/recirculated air door and a Bowden cable transmits the drive from the LH (left-hand) to the RH (right-hand) fresh/recirculated air door. On the automatic system, a ram air door is installed inside each fresh/recirculated air door. A stepper motor drives the RH (right-hand) ram air door and a Bowden cable transmits the drive from the RH (right-hand) to the LH (left-hand) ram air door.
Each side of the heater contains separate distribution doors for the footwell, face level and windshield. The related doors on each side of the heater are installed on common drive spindles. On the manual heater, the distribution doors are driven by Bowden cables connected to a cam mechanism, which, in turn, is driven by a stepper motor. On the automatic heater, each set of distribution doors is driven by a separate stepper motor.
On both the manual and automatic heaters, a blend door is installed below the face level registers. The blend door is driven by a Bowden cable connected to a thumbwheel on the center face level registers in the fascia, and allows the temperature of face level air to be modified with cold air direct from the evaporator.
Fascia Center Face Level Register
On models with an automatic climate control system, the heater incorporates an additional blend door for the air directed to the rear passenger face level register. This allows the temperature of rear face level air to be adjusted independently from the temperatures selected on the control panel of the ATC (automatic temperature control) module. The blend door is driven by a stepper motor controlled by a thumbwheel on the rear passenger face level register. The blend door is also used to close off the rear passenger face level register when maximum air output is required for the front outlets, e.g. when defrost is selected.
Rear Passenger Face Level Register
Control Door Motors
Two types of electrical stepper motor are used to operate the control doors in the heater. A conventional 500 Hz stepper motor operates the recirculation doors on the manual and automatic systems. On the automatic system, five bus controlled 200 Hz stepper motors operate the ram air, distribution (windshield, face level and footwell) and the rear face level temperature control doors. On the manual system a bus controlled stepper motor operates the distribution doors cam mechanism. All of the stepper motors are controlled by the ATC (automatic temperature control) module. None of the stepper motors are interchangeable.
Typical Control Door Motor
Each bus controlled stepper motor incorporates a microprocessor and is connected to an M bus from the ATC (automatic temperature control) module, which consists of three wires making up power, ground and signal circuits. The microprocessor in each bus controlled stepper motor is programmed with a different address. Each M bus message from the ATC (automatic temperature control) module contains the address of an individual bus controlled stepper motor, so only that motor responds to the message.
None of the stepper motors incorporate a feedback potentiometer. Instead, the ATC (automatic temperature control) module determines the positions of the doors by using either their closed or open position as a datum and memorizing the steps that it drives the individual stepper motors. Each time the ignition is switched on, the ATC (automatic temperature control) module checks the memorized position of the stepper motors against fixed values for the current system configuration. If there is an error (e.g. after a power supply failure during operation or after replacement of the ATC (automatic temperature control) module), the ATC (automatic temperature control) module calibrates the applicable stepper motors, to re-establish the datums, by driving them fully closed or open before re-setting them to their nominal position. A calibration run can also be invoked using T4.
When any of the control doors are set to fully closed or open, the ATC (automatic temperature control) module signals the related stepper motor to move the appropriate number of steps in the applicable direction. To accommodate build tolerances and wear, and to ensure the doors are held in the selected position, every 20 seconds the ATC (automatic temperature control) module signals the stepper motor to move an additional 10 steps in the relevant direction.