Fig. 1.27 Stability control; (a) understeer, (b) oversteer this they do via the CAN network as shown in Fig. 1.25. This figure also illustrates the form of output from the Hall type wheel sensors. CAN networking is covered in Chapter 2 and more details about Hall type sensors are explained in Chapter 5.

1.9 Air conditioning

Maintaining a comfortable temperature inside the passenger/driving compartment of a vehicle is a function that is normally performed by a computer controlled system. Providing heat to the vehicle interior is usually achieved by redirecting heat from the engine via directional ducts and fans. However, cooling down the interior of the vehicle normally requires the use of an extra machine-driven cooling system that will take heat from the interior and transfer it to the atmosphere surrounding the exterior of the vehicle. It is the air conditioning system that performs this function. Figure 1.28 illustrates the outline principle of a vehicle air conditioning system.

The liquid (refrigerant) that is used to carry heat away from the vehicle interior and transfer it to the outside is circulated around the closed system by means of a compressor that is driven by the engine of the vehicle. Inside the system the refrigerant constantly changes state between liquid and a vapor as it circulates.

The reducing valve is an important agent in the operation of the system. The 'throttling' process that takes place at the reducing valve causes the refrigerant to vaporize and its pressure and temperature to fall. After leaving the reducing valve, the refrigerant passes into a heat exchanger called the evaporator where it collects heat from the vehicle interior and thus cools the interior in the process. The heat collected causes the refrigerant to vaporize still further and it is returned to the compressor where its pressure and temperature are raised.

From the compressor, the refrigerant passes into another heat exchanger where it gives up heat to the atmosphere. This heat exchanger is known as a condenser because the loss of heat from the refrigerant causes it to become wet. After the condenser, the refrigerant passes through the accumulator, which serves to separate liquid from the vapor. The refrigerant then returns to the reducing valve and evaporator, thus completing the cycle.

Because the compressor takes a considerable amount of power from the engine it is necessary for the air conditioning computer to be aware of the operational state of the engine. For example, the idling speed of the engine will be affected if the air conditioning compressor is operating, and the engine ECM will normally cause an increase in idle speed to prevent the engine from stalling. To allow the air conditioning compressor to be taken in and out of operation it is driven through an electromagnetic clutch which is shown in Fig. 1.29.

This clutch permits the compressor to be taken out of operation at a speed just above idling speed and, in order to protect the compressor, it is also disconnected at high engine speed. In some cases where rapid acceleration is called for, temporary disengagement of the compressor may also occur.

Fig. 1.28 The principles of air conditioning

Clutch plate


Clutch plate

Fig. 1.29 The electromagnetic clutch


Air gap



Compressor shaft centreline

Fig. 1.29 The electromagnetic clutch

In addition to engine operating considerations, the interior temperature of the vehicle must constantly be compared with the required setting and the exterior temperature, and this is achieved by temperature sensors which are similar to those used for engine coolant temperature sensing. The following is a list of the controlling functions of an air conditioning ECM.

• Calculation of required outlet air temperature

• Temperature control

• Blower control

• Air inlet control

• Air outlet control

• Compressor control

• Electric fans control

• Rear defogger control

• Self diagnosis

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