Operation

Operation

Heating And Ventilation Control Diagram:

The Heater ECU operates the heating and ventilation system to produce the conditions selected on the control panel. The system operates when the ignition switch is in position II. For operation of the automatic demist function, the engine must be running. Power for system operation is supplied from the engine compartment fuse box. The BeCM supplies the Heater ECU with auxiliary power and an engine running signal.

Fresh/Recirculated Air Selection
When the fresh/recirculated air switch is pressed, the switch LED illuminates and the Heater ECU supplies battery voltage to the fresh/recirculated air servos to drive the flaps in the air inlet housings to the recirculated air position. A second press of the switch extinguishes the LED and the Heater ECU reverses the polarity of the supply to the fresh/recirculated air servos, which then drive the flaps to the fresh air position.

Blower Control
Battery voltage is supplied to the positive side of the blower motors via the auxiliary relays. Blower speed is controlled by the blower rotary switch, which regulates the voltage of blower control signals from the Heater ECU to the negative side of the blower motors. At the oft position, the blower control signals are equal to battery voltage. Turning the blower rotary switch clockwise progressively reduces the voltage of the blower control signals, and increases blower speed, until the signals are earthed and the blowers are at maximum speed. When the demist mode is active, the Heater ECU overrides the

For diagnostic purposes, the Heater ECU monitors the blower motor's positive and negative voltages via the blower voltage feedback and blower safety inputs respectively.

Temperature Control
When one of the temperature control switches is turned the Heater ECU supplies battery voltage to the related temperature servo to drive the flaps in the heater unit to the applicable position. To change the direction of drive, the Heater ECU reverses the polarity of the supply.

Distribution
When one of the distribution switches is pressed, the switch LED illuminates and the Heater ECU supplies battery voltage to the distribution servo to drive the flaps in the heater unit to the applicable position. To change the direction of drive, the Heater ECU reverses the polarity of the supply.

Demist Mode
When the PROG switch is pressed, the Heater ECU overrides the manual selections and operates the heating and ventilation system in the automatic demist mode. A second press of the PROG switch returns the system to the previous manual settings. While the automatic demist mode is engaged, the windscreen and rear screen heaters can be operated to reset their timer start point without effecting the demist programme.

Self Tuning
Periodically, the Heater ECU performs a self tuning routine of the temperature and distribution servos, to accommodate bedding in of the flaps and their control mechanisms. During the routine, operation of the blowers is inhibited and the servos are driven through their full range to re-calibrate their flap positions. The routine is invoked at the beginning of the 1st, 10th, 20th, 50th, 100th, 500th and every subsequent 500th start up. The routine should also be invoked, using Testbook, after replacement of a temperature or distribution servo.

Diagnostics
The Heater ECU continuously monitors the servo and blower circuits for continuity and short circuits. The feedback signals of the temperature and distribution servos are also checked for plausibility at each end of the servo travel range. If a fault is detected a related fault code is stored in memory and can be retrieved using Testbook.