Operation

Operation

Air flow through the heater assembly is directed to the outlets selected by the distribution control knob. The temperature of the air from all except the face level vents depends on the setting of the temperature control knob. Hot air is available from the face level vents only when the temperature control knob is at the maximum heat setting. As the temperature control knob is turned towards cold, the temperature of the air from the face level vents rapidly decreases to ambient (non A/C vehicles) or evaporator outlet temperature (A/C vehicles). The forward speed of the vehicle and the setting of the blower control knob determines the volume of air flowing through the system.

Air Distribution
Turning the distribution knob on the control panel turns the control flaps in the heater assembly to direct air to the corresponding fascia and footwell outlets.

Air Temperature
Turning the temperature knob on the control panel turns the related blend flaps in the heater assembly. The blend flaps vary the proportion of air going through the cold air bypass and the heater matrix. The proportion varies, between full bypass/no heat and no bypass/full heat, to correspond with the position of the temperature knob.

Blower Speed
The blower can be selected 'off', or to run at one of four speeds. While the ignition is on and the blower switch is set to positions 1, 2, 3 or 4, ignition power energises the blower relay, which supplies battery power to the blower. At switch positions 1, 2 and 3, the blower switch also connects the blower to different earth paths through the resistor pack, to produce corresponding differences of blower operating voltage and speed. At position 4, the blower switch connects an earth direct to the blower, bypassing the resistor pack, and full battery voltage drives the blower at maximum speed.

Fresh Recirculated Inlet Air
When the recirculated air switch is latched in, the indicator LED in the switch illuminates and an earth is connected to the recirculated air side of the fresh/recirculated air servo motor. The fresh/recirculated air servo motor then turns the control flaps in the air inlet duct to close the fresh air inlet and open the recirculated air inlets.

When the latch of the recirculated air switch is released, the indicator LED in the switch extinguishes and the earth is switched from the recirculated air side to the fresh air side of the fresh/recirculated air servo motor. The fresh/recirculated air servo motor then turns the control flaps in the air inlet duct to open the fresh air inlet and close the recirculated air inlet.

Fuel Burning Heater (FBH) System (Where Fitted)
The FBH system only operates while the engine is running and the ambient temperature is less than 5 degrees C (41 degrees F). With the engine running and the ambient temperature below 5 degrees C (41 degrees F), the air temperature sensor connects the alternator power supply to the ECU in the FBH unit. On receipt of the alternator power supply, the ECU starts the circulation pump and, depending on the input from the temperature sensor in the heat exchanger, enters either a standby or active mode of operation. If the heat exchanger casing temperature is 65 degrees C (149 degrees F) or above, the ECU enters a standby mode of operation. If the heat exchanger casing temperature is below 65 degrees C (149 degrees F), the ECU enters an active mode of operation. In the standby mode, the ECU monitors the heat exchanger casing temperature and enters the active mode if it drops below 65 degrees C (149 degrees F). In the active mode, the ECU initiates a start sequence and then operates the system at full or part load combustion to provide the required heat input to the coolant.

Start Sequence
At the beginning of the start sequence the ECU energises the glow plug function of the glow plug/flame sensor, to preheat the combustion chamber and start the combustion air fan at slow speed. After approximately 30 seconds, the ECU energises the FBH fuel pump at the starting sequence speed. The fuel delivered by the FBH fuel pump evaporates in the combustion chamber, mixes with air from the combustion air fan and is ignited by the glow plug/flame sensor. The ECU then progressively increases the speed of the FBH fuel pump and the combustion air fan to either part or full load speed, as required by the system. Once full or part load speed is achieved, the ECU switches the glow plug/flame sensor from the glow plug function to the flame sensing function to monitor combustion. From the beginning of the start sequence to stable combustion takes approximately 90 seconds for a start to part load combustion and 150 seconds for a start to full load combustion.

Coolant Temperature Control
When the ECU first enters the active mode, it initiates a start to full load combustion. Full load combustion continues until the heat exchanger casing temperature reaches 60 degrees C (140 degrees F), at this point the ECU decreases the speed of the FBH fuel pump and the combustion air fan to half speed, to produce part load combustion. The ECU maintains part load combustion while the heat exchanger casing temperature remains between 54 degrees C and 65 degrees C (129 degrees F and 149 degrees F). If the heat exchanger casing temperature decreases to 54 degrees C (129 degrees F), the ECU switches the system to full load combustion again. If the heat exchanger casing temperature increases to 65 degrees C (149 degrees F), the ECU enters a control idle phase of operation.

On entering the control idle phase, the ECU immediately switches the FBH fuel pump off, to stop combustion, and starts a timer for the combustion air fan. After a 2 minute cool down period, the ECU switches the combustion air fan off and then remains in the control idle phase while the heat exchanger casing temperature remains above 59 degrees C (138 degrees F). If the heat exchanger casing temperature decreases to 59 degrees C (138 degrees F), within 15 minutes of the ECU entering the control idle phase, the ECU initiates a start to part load combustion. If more than 15 minutes elapse before the heat exchanger casing temperature decreases to 59 degrees C (138 degrees F), the ECU initiates a start to full load combustion.

In order to limit the build up of carbon deposits on the glow plug/flame sensor, the ECU also enters the control idle phase if the continuous part and/or full load combustion time exceeds 72 minutes. After the cool down period, if the heat exchanger casing is still in the temperature range that requires additional heat, the ECU initiates an immediate restart to part or full load combustion as appropriate.

Shutdown
The FBH system is de-activated when the alternator power supply to the FBH unit is disconnected, either by the engine stopping or, if the ambient temperature increases to 5 degrees C (41 degrees F) or above, by the contacts in the air temperature sensor opening. If the system is active when the alternator power supply is disconnected, the ECU de-energises the FBH fuel pump to stop combustion, but continues operation of the combustion air fan and the circulation pump to cool down the FBH unit. The cool down time depends on the combustion load at the time the alternator power input is disconnected.

Cool Down Times
Combustion Load Cool Down Time, Seconds
part 100
Full 175

Diagnostics
The ECU in the FBH unit monitors the system for faults. Any faults detected are stored in volatile memory in the ECU, which can be interrogated by T4. A maximum of three faults and associated freeze frame data can be stored at any one time. If a further fault is detected, the oldest fault is overwritten by the new fault.

The ECU also incorporates an error lockout mode of operation that inhibits system operation to prevent serious faults from causing further damage to the system. In the error lockout mode, the ECU immediately stops the FBH fuel pump, and stops the combustion air fan and circulation pump after a cool down time of approximately 2 minutes. Error lockout occurs for start sequence failures and/or combustion flameouts, heat exchanger casing overheat and out of limit input voltage. The error lockout mode can be cleared using T4, or by disconnecting the battery power supply for a minimum of 10 seconds.

Start Failure/Flameout: If a start sequence fails to establish combustion, or a flameout occurs after combustion is established, the ECU immediately initiates another start sequence. The start failure or flameout is also recorded by an event timer in the ECU. The event timer is increased by one after each start failure or flameout, and decreased by one if a subsequent start is successful. If the event timer increases to three (over any number of drive cycles), the ECU enters the error lockout mode.

Heat Exchanger Casing Overheat: To protect the system from excessive temperatures, the ECU enters the error lockout mode if the heat exchanger casing temperature exceeds 105 degrees C (221 degrees F).

Out of Limit Voltage: The ECU enters the error lockout mode if the battery or alternator power input is less than 10.5 ± 0.3V for more than 20 seconds, or more than 15.5 ± 0.5V for more than 6 seconds.

PTC Heater (Where Fitted)
The PTC heater operates while the engine is running and, for a limited period, if the ignition is switched on without starting the engine.

When the ignition switch is turned to position II, the ECM energises the fuel pump relay, which, in addition to connecting power supplies to the fuel pumps, connects a power supply to the coil of the power relay in the PTC heater system. The ECM keeps the fuel pump relay energised while the engine is running. If the engine is not started within approximately 60 seconds of turning on the ignition, the ECM de-energises the fuel pump relay.

The earth side of the power relay coil is connected to earth via the contacts of the control relay. The coil of the control relay is connected in series to the heater switch, blower switch and ignition switch. While the ignition switch is in position II, if the blower is selected on and the temperature control is rotated clockwise to a position past 1 o'clock, a power supply is connected to the coil of the control relay. The control relay energises and connects the coil of the power relay to earth, which then energises and connects power to the PTC heater.

On manual gearbox models, when the control relay is energised a power feed is connected to the ECM. When the ECM senses the power feed, if the engine coolant temperature is below 85 degrees C (185 degrees F) it increases engine idle speed by 50 rev/min, to improve idle speed refinement while the additional load is imposed on the alternator by the PTC heater.