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Brief description

Brief description

The main function of the ACC-system is to control the climate control unit in order to ensure that comfortable conditions are achieved as soon as possible after starting, and then to maintain these conditions. The ACC-unit consists of a panel with a display and buttons as well as an integrated control module. Normally, the control module automatically controls the following functions:

Temperature

The control module supplies current to a stepping motor that turns the air-mixing damper. The air-mixing dampers blend cold air that has passed the evaporator with warm air that has passed both the evaporator and the heater box.

The present temperature at head level is calculated for the front seat passenger and driver. Input values include:


• Cabin temperature

• Outside temperature

• Time since engine was last started

• Time since start

• Mixed-air temperature

• Fan speed

• Sun radiation


The calculated temperature is compared to the temperature set on the ACC unit display. The set temperature is corrected slightly with respect to outside temperature so that the experienced temperature matches the set temperature. As soon as there is a difference, the mixed-air temperature must be increased or decreased. The control module turns the air-mixing damper until the requested mixed-air temperature is achieved.

When the lowest mixed-air temperature is requested, the control module closes the coolant flow to the heater box via a vacuum-operated valve to provide max cooling effect (applies only to certain markets).

Air distribution

The control module supplies current to a stepping motor that turns the air distribution damper.

The air is distributed to the defroster, floor/rear door windows or dashboard vents/rear centre vent. The air distribution control is dependent on the requested mixed-air temperature. Defroster/floor is selected at a high value, floor/dashboard or dashboard is selected at low value. For cold starts, defroster is selected at the beginning.

Fan speed

The control module controls a fan control that supplies current to the fan motor. The fan current is at its lowest when the present mixed-air temperature matches the requested temperature. The current increases as soon as cold or heat is needed. For cold starts, the value 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 current to a direct current motor that turns the air recirculation flap.

Recirculation is selected 30 seconds after start if the outside temperature is high and the requested mixed-air temperature is low (that is, the cabin has to be cooled significantly).

Then, recirculation is selected in the following three cases:


• At extremely high outside temperatures

• At high outside temperatures if the travelling speed is low (capacity of the A/C-system decreases)

• At high outside temperatures if coolant temperature is high (the engine risks being overheated)


A/C

The control module requests A/C-compressor cut-in if the outside temperature exceeds 0°C and ECON is not selected.

P-bus and I-bus

In the Saab 9-3, less control modules are connected to a bus-system when compared to Saab 9-5.

The two powertrain systems EDC and the diesel pump are not connected to the EDC I-bus. These systems require much faster communication so that no delays are noticeable, for example, at torque limitation when shifting gears.

Therefore, they are connected to a separate P-bus (Powertrain bus). The P-bus has a communication speed ten times that of the I-bus.

The P-bus is also connected to the MIU (main instrument unit). The MIU is responsible for ensuring that information available on one bus also is available on the other bus.