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Brief description

Brief description

The main purpose of the ACC system is to control the climate unit in order to achieve comfortable climate conditions as soon as possible after starting the car and then to maintain them. The ACC unit consists of a control panel with display and buttons as well as an integrated control module. Under normal conditions, the control module automatically controls the following functions:

Temperature, separate for passengers and driver

The control module supplies current to a stepping motor which turns the mixed-air flap in the driver's zone. Another stepping motor turns the mixed-air flap for the front seat passenger and a link arm the mixed-air flap for the rear seat passengers. The mixed-air flaps mix cold air that has passed the evaporator with warm air that has passed both the evaporator and the heat exchanger. The current temperature at head height is calculated for the passengers and the driver. The following default values are used:


• cabin temperature

• outside temperature

• time since engine last ran

• time since start

• mixed-air temperatures

• fan speed

• solar irradiance


The calculated temperatures are compared to those the driver and passengers have set on the ACC unit display. The set temperature is adjusted with respect to the outside temperature so that the perceived temperature corresponds with the set temperature. As soon as a difference occurs, the mixed-air temperature must be increased or decreased. The control module turns the air-mixing flap in question until the required mixed-air temperature is achieved. The linkage to the rear seat air-mixing flap is adjusted so that the temperature for the rear seat corresponds with the temperature for the front passenger seat.

When the lowest mixed-air temperature is selected for both zones, the control module shuts off the supply of coolant to the heat exchanger with a vacuum controlled valve so that maximum cooling efficiency is achieved.

Certain engine variants are equipped with a circulation pump for improved coolant flow through the heat exchanger. The control module starts the pump at low engine speeds and low outside temperatures.

Air distribution

The control module supplies the stepping motor, which turns the air distribution flap for the driver's zone and front seat passenger as well as the air distribution flap for the back seat passengers via a link. Consequently, air distribution is the same for the driver and the passengers.

Air is distributed to the defroster, floor/rear door windows or panel/rear centre outlets. The distribution of air depends on the selected mixed-air temperature for the driver's zone. At a high temperature (driver's zone requires heating), defrost/floor is selected and at a low temperature (driver's zone requires cooling), floor/panel or panel is selected. When starting in the cold, defrost is selected at the start.

fan speed

The control module regulates a control unit which supplies the fan motor. The air flow from the fan is at its lowest when the current mixed-air temperature corresponds with the selected temperature for both zones (none of the zones requires cooling or heating). The air flow increases as soon as one of the zones requires cooling or heating. When starting in cold weather, the temperature is determined by the coolant temperature and the outside temperature. The fan current is limited if the engine is not running.

Air recirculation

The control module supplies power to a DC motor which turns the air recirculation flap.

Air recirculation is selected 45 seconds after start if the outside temperature is high and the requested mixed-air temperature is low for both zones (i.e. both zones must be cooled down considerably).

Air recirculation is then selected in the following three cases:


• at very high outside temperatures

• at high outside temperatures if the vehicle speed is low (A/C system performance decreases)

• at high outside temperatures if the coolant temperature is high (risk of engine coolant boiling).


A/C

The control module requests that the A/C compressor be engaged if the outside temperature exceeds 5°C and ECON is not selected.



P bus and I bus


All the control modules in the Saab 9-5 are connected to a bus, except for the ABS and SRS. The bus is divided into a Powertrain bus (P bus) and an Instrument bus (I bus). Both busses are connected to the main instrument display panel (MIU). The busses are electrically separate. The P bus data transmission rate is ten times greater than that of the I bus. All the information sent by one control module is available to all the other control modules connected to the bus. MIU is responsible for making the data on one bus available to the other bus.