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Fig. 5.36 A simple potential divider

Fig. 5.36 A simple potential divider

In the air flow sensor, the moving probe (wiper) of the potential divider is linked to the pivot of the measuring flap so that angular displacement of the measuring flap is registered as a known voltage at the potentiometer.

Figure 5.37 shows a simplified form of the air flow sensor. The closed position of the measuring flap will give a voltage of approximately zero, and when fully open the voltage will be 5 V. Intermediate positions will give voltages between these values. In practice, it is not quite as simple as this, because allowance must

1. Spiral spring 2. Compensating flap 3. Metering flap 4. Idling air passage

Vs = Output voltage

Vc = Constant voltage supply

The voltage Vs represents air flow Fig. 5.37 The potential divider applied to an air flow sensor

Vs = Output voltage

Vc = Constant voltage supply

The voltage Vs represents air flow Fig. 5.37 The potential divider applied to an air flow sensor be made for other contingencies. However, this should not detract from the value of knowing the basic principles because it is these which lead to the diagnostic checks that can be applied. Before we consider some basic checks that can be performed, we will have a more detailed look at the Lucas air flow meter of Fig. 5.35.

Figure 5.38 shows the principle of the Lucas 2AM sensor. The relative position of the measuring flap depends on the air flowing to the engine, and the return torque of the spring. This diagram gives a little more information because it shows a refinement, the idle air bypass, that is required for satisfactory operation of the device.

PumP Spiral

PumP Spiral

Air temperature Measuring Idle air Adjustment sensor flap bypass screw

Fig. 5.38 Schematic drawing of Lucas 2AM type air flow sensor

Air temperature Measuring Idle air Adjustment sensor flap bypass screw

Fig. 5.38 Schematic drawing of Lucas 2AM type air flow sensor

To compensate for production tolerances and different air requirements for similar engines at idling speed, it is desirable that the air-fuel ratio can be adjusted under idling conditions. The idle air bypass together with the adjustment screw permits the idle mixture to be adjusted to suit individual engine requirements. The air flow meter (sensor) incorporates a fuel pump switch. This switch is controlled by the initial (approximately 5°) movement of the measuring flap. This 'free play' is controlled by a very light spring, which is overcome by a very small airflow, i.e. idle speed. This method of switching the fuel pump ensures a minimum fire risk in the event of a collision, when fuel pipes could fracture.

To improve the stability of the measuring plate, when pressure variations in the inlet tract occur, a compensating flap is used. Both the measuring flap, and the compensating flap are part of the same casting and rotate about the same shaft center. Whatever force is felt by the measuring flap is also felt by the compensating flap and this reduces the effect of pressure fluctuations in the air inside the induction manifold. The equilibrium (balance) of the two flaps is only disturbed when the pressure on the engine side of the measuring flap is lower than atmospheric. This pressure is also felt on both sides of the compensating flap.

The air flow sensor also incorporates a temperature sensor which sends a signal to the ECU which uses it to calculate the mass of air. (Mass and volume of air are linked by temperature.) The approximate form of the signal output that can be expected from sensors of this type is shown in Fig. 5.39.

The following points should be noted.

1 The voltage is at its highest value when the throttle is wide open and the engine is accelerating.

2 The idling air flow produces a low voltage.

3 This dip is caused by the damping action that is caused by movement of the air

4 The voltage increases as the air flow increases with speed and throttle opening. 5.9.1 HOT WIRE MASS AIR FLOW SENSOR (MAF)

The 'hot wire' air flow meter, shown in Fig. 5.40, incorporates a small orifice inside the main body of the flow meter. The flow meter is positioned between the throttle body and the air cleaner. The main air supply for combustion therefore passes through this meter and a steady flow of the same air passes through the sensing orifice. In the sensing orifice are placed two wires, a compensating wire and a sensing wire. The compensating wire has a small electric current passing through it and the electronic circuit is able to determine the temperature of the incoming air by measuring the resistance of this wire.

The sensing wire is 'hot', about 100°C above the temperature of the compensating wire. This temperature is maintained by varying the current flowing through it. The air flowing to the engine has a cooling effect on the sensing wire so the current in the sensing wire is increased in order to maintain the temperature. As

Fig. 5.39 The approximate form of the voltage pattern

flap.

Fig. 5.40 A 'hot-wire' air flow sensor

air flow increases so the sensing current is increased, and as the air flow decreases so the sensing current decreases. The resultant current flow gives an accurate electrical 'analogue' of air flow which is used in the ECM as an element of the controlling data for the fuel supply.

Modern materials permit the resistive elements of the mass air flow sensor to be built into a metal foil element that is exposed to the intake air stream. Such sensors are frequently built into a housing which is inserted into the air intake system and the principle of operation is similar to that of the hot wire sensor described above.

Figure 5.41 shows the approximate form of the voltage signal that can be expected at the output of a mass flow type of air flow sensor. This pattern shows a voltage that varies as the engine is taken through a range of speeds by operating the throttle.

GENERAL SENSORS ^

HEN" CONTINUITY RANGE

™e ohm open close move

GENERAL SENSORS ^

HEN" CONTINUITY RANGE

™e ohm open close move

Fig. 5.41 Voltage pattern from a mass flow air sensor as recorded by Bosch PMS 100

oscilloscope

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