The control unit reads the signal sent by the external air temperature sensor every minute (sampling). If the value read by the control unit has decreased in relation to the value taken during the previous sampling, the control unit adopts the new value regardless of vehicle speed.
If the value read by the control unit has increased in relation to the value taken during the previous sampling, the control unit adopts the new value only if the vehicle speed has been higher than 18.5 MPH for at least one minute.
With the key in the POS.1 (RUN) position and the external temperature between +2oC (36o F) and +1 OoC (50o F), the control unit will maintain the same condition as that before the key was turned to the POS.O (OFF) (if the compressor was enabled then the control unit will send the command to the relay; if it was disabled, then the control unit will not send the command to the relay).
NOTE: "Compressor enabled" means that the air conditioning control unit sends out the command for it to be enabled (+12o Volt). For the relay and subsequently the electromagneticjoint to be enabled, the following conditions must be satisfied:
The electronic control unit adjusts the temperature of the air sent to the passenger compartment to a temperature which will guarantee that the desired temperature is maintained regardless of the external temperature. Consider a journey which first incorporates extremely cold areas and then extremely hot areas. Imagine that the desired internal temperature has been set on the display.
The electronic control unit will scan a map programmed in its own memory and find a variable correction, dependent on the external temperature, which will allow the internal conditions inside the vehicle to be maintained at the desired level; this is made possible by activating the actuator controlling the hot/cold air mixing flap as required.
The characteristic curve, shown above, demonstrates the changes in the correction factor which the control unit applies to the temperature value set on the display.
If a temperature is set to 23oC (74oF) and the air conditioning system, through the NTC sensor, registers an external temperature of -1 OoC (14o F), the electronic control unit will modify the set temperature by an amount equal to the correction factor (+3oC) (5.4oF) from the map programmed for the corresponding external temperature.
In this example, with the external temperature at -1 OoC (l4oF), the control unit will regulate the air temperature sent into the passenger compartment considering not the 23oC (74oF) set on the display but 26oC (79oF) obtained after the correction linked to the external temperature.
The automatic air conditioning system electronic control unit controls the position of the system's hot/cold air mixing flap in
order to maintain the passenger compartment at the desired temperature. The position of the flap is adjusted on the basis
of the external temperature value, the desired temperature value and by the difference between the desired temperature
and the actual temperature in the passenger compartment. All these variables are memorized in maps from which the
percentage opening value can be obtained. The sum of the opening percentages obtained from the various maps produces
a total percentage opening signal, corresponding to the flap actuator signal in order to achieve the correct air mix.
With reference to the above curves, if a temperature of 23oC (74oF) is set on the display, with an external temperature of -1 OoC (14o F) and an internal temperature in the passenger compartment of -1 OoC (14o F), the total flap opening percentage would be equal to 155% which means that the flap would be positioned to let in the maximum amount of hot air possible. As the internal temperature increases, for example to 27oC (81 oF), the total flap opening percentage will decrease.
Another parameter adjusted by the electronic control unit so that the air conditioning system meets the requirements set by the user is the electric fan speed.
The signal which adjusts the speed of the electric fan is also obtained from programmed maps on the basis of the external temperature and the difference between the internal temperature and the corrected required temperature (Ti). The sum of the two values obtained from the maps produces the final command which must then be sent to the electric fan's electronic regulator.
The voltage supplied by the electronic control unit, which varies between 0 and 5 V, acts as a reference for the speed adjustment circuit. On the basis of the voltage at its input, the regulator adjusts the current circulating in the fan's motor so that the fan can operate at the required speed.
The correct distribution of mixed air is obtained by activating a motorized flap whose angular position, determined by the
electronic control unit, allows the air to be uniformly distributed throughout the passenger compartment.
The electronic control unit compares the corresponding mixed air flap opening percentage value with a programmed map
and sends a signal to the air distribution flap actuator which produces a specific angular position (opening) of the flap itself.
The system uses the straight line corresponding to the function which has been set (AUTO or ECON).
The position of the recirculation flap is controlled by the control unit on the basis of the external temperature. This allows both external air and air taken from the passenger compartment to be sent to the air mixing system. The latter is then cooled and dried by the air conditioning system.
The command supplied by the control unit moves the flap to the RECIRCULATION or EXTERNAL AIR position without any possible intermediate positions.
Consequently, with external temperatures above 29oC (84oF), the control unit positions the flap in the RECIRCULATION position taking air from the passenger compartment. With external temperatures lower than 27oC (81 oF) the flap will be positioned in such a way to permit air to be taken from outside the vehicle.
Using a specific operating logic the automatic air conditioning system automatically eliminates problems which often arise during the winter months when the vehicle is started with the engine cold.
With external temperatures between -40oC (-40oF) and +l OoC (50oF) the electronic control unit, through the system actuators, sends air to the defrosting mechanism, starts the fan operating at top speed, positions the flap so that air can be taken in from outside and sets the air mixing system to maximum heat.
This prevents cold air from entering the passenger compartment. In fact the air can not be heated by the heater matrix incorporated in the engine cooling system due to the excessively low temperature of the coolant itself. However when the control unit registers an air temperature in excess of 45oC (1 l3oF) at the heater matrix's outlet, the system starts to function in automatic mode in accordance with the program installed; however the system will gradually increase the supply to the fan by 1 ampere per second; in this way the speed envisaged by the electronic control unit can be reached without sharp changes in the fan's speed.
Due to the high current absorption by the starter motor, it is advisable to exclude other accessories during the engine start- up stage so as to avoid excessively overloading the battery.
If it is not fully charged or during the cold season, it may prevent the engine from starting if the current absorption of the starter motor which is already very high is further increased by high-level loads such as heated rear window, main beam/ dipped head lights, windscreen wipers, the air conditioning compressor electromagnet, air conditioning electric fan. The ignition houses an auxiliary contact which eliminates this problem.
The operating principle is that of cutting off the supply to the starter motor electronic control unit which prevents any further commands from being sent by the automatic air conditioning system.
When the power supply is returned to the system the control unit will automatically enter the commands selected on the basis of the temperature set on the display and the temperature registered by the sensors, with the exception of the fan command, which will be operated at the lowest speed for 60 seconds. Once this period is over the top speeds will be reached gradually by increasing the supply to the fan by 1 ampere every second, until the speed required by the air conditioning system is reached. '
In order to adjust the mixing and distribution of air within the automatic air conditioning system, the electronic control unit monitors two step motors using digital signals which, in turn, activate the airdistribution and mixing flaps using mechanical transmission devices. The use of the step motor as opposed to the traditional DC type with position transducers is determined by the fact that it interfaces perfectly with the digital control techniques which are now commonly used in the more advanced electronic control units.
The clockwise and counter-clockwise rotation is achieved by sending signals with logical sequences to the windings which enable magnetic fields, capable of moving the rotor in a series of graduated steps, to be obtained.
From the control example shown below, it is easy to see how the rotor moves in steps which coincide with each current transition in the various stages.
If the rotation direction needs to be reversed, simply return to the previous situation of currents in individual phases.
The sensor used to monitor the passenger compartment air temperature has a resistance with a negative temperature coefficient (NTC) produced using semiconductor technology; for this reason if the temperature of the sensor increases as the passenger compartment air temperature increases, there will be a decrease in the resistance value.
With a view to improving and making the passenger compartment air temperature more accurate, the air around the
sensor is taken from the passenger compartment and blown onto the sensor by a small fan.
The mixed air temperature sensor also consists of a resistance with a negative temperature coefficient (NTC).
The sensor position is such that the mixed air temperature is accurately measured.
The external air temperature sensor is made up of resistance with a negative temperature coefficient (NTC) produced using semi-conductor technology; for this reason, if the temperature of the sensor increases as the passenger compartment air temperature increases, there is a decreases in the resistance value.
The sensor is positioned in the external rear view mirror.
