The Saab automatic climate control or ACC was designed to regulate cabin temperature and ventilation by just setting one adjustment, the temperature. The ACC system is then to take in account outdoor temperature, sun intensity and cabin temperature to fulfill the drivers request. The ACC can also be run in a ECON mode this selection simply keeps the A/C compressor from running saving on fuel. On the early 9000 1986-89 we had a two button system or ACC1 with no off switch this was not a popular design and Saab has used a system utilizing a off button since 1990 called ACC2. Both systems have self diagnosis, although the ACC2 system needs a Tech2 for code retrieval. The ACC1 fault codes can be retrieved simply by simultaneously pressing the Auto and Vent buttons. We will completely go over this self diagnostic procedure. Over the years the theory of operation has been similar, only sensors and software have been improved. On the 9-3 and 9-5 we find the same basic operation just with better software technology to improve performance.
system- 1986-1989 2-button control
I will explain how it works. I will break this system into 3 categories
sensors, control and components.
Sensors and Location:
Inside the cabin/indoor Sensor there is a small fan that is used to draw air across a thermistor this is of the NTC type. A thermistor is simply a resistor that changes resistance according to it's temperature, and an NTC thermistor specifies that the resistance goes down or closer to 0 the hotter the temperature. One common problem with the cabin sensor is that a ball of lint can build on the thermistor acting as insulation and giving inaccurate temperature results. You can snap the cover off the front of this sensor and clean this away very easily. To test the thermistor you can unplug it and check it's resistance against the current temperature. This can be done across pins 3 and 2 on the back of the Cabin sensor with an OMH meter.
Mixed air sensor and Ambient air temp sensor these sensors are both thermistors. The mixed air sensor is of the NTC type and measurers the air temp as it leaves the AC evaporator and heater core before it enters the Heater ducting. The ambient air temp sensor Is of the PTC type it's resistance goes up with higher temperature. On the ACC1 system the ambient air temp sensor sits on the outside of the fan blower housing this way it can measure the air just before it enters the Heater/ fan housing. The ambient air temp sensor of the ACC1 is of the PTC type so it's resistance goes up with higher temperature. On ACC2 the ambient air temp sensor is a NTC type and is shared with the in Dash EDU temperature read out.
Solar Sensor On the ACC1 the solar sensor is a single photo cell that sends back a voltage to the control unit indicating sun intensity. The control unit in returns changes the fan speed to compensate. ACC2 has a more complex sensor it is made up of 5 solar cells and a microprocessor. It can determine sun intensity and angle of the sun. It transmits this information back to the control unit with a digital signal. It is also fitted with a filtered lens cap that only allows infrared light to penetrate.
(This article is meant to educate a consumer,or as a guideline for professionals. You can cause serious damage to your vehicle and/or cause yourself injury. Only those qualified should attempt repairs. I do my best to assure that the above info is correct but take no responsibility for any damages incurred.)
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