Repeat Test

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Fig. 7.18 Voltage pattern for the full movement of the accelerator pedal

Fig. 7.18 Voltage pattern for the full movement of the accelerator pedal

In Fig. 7.19(a) the arrow at (5) indicates a rise in sensor voltage which shows that air flow into the manifold is increasing, as would be expected with an increase in throttle opening and engine speed. At section (2) the maximum air flow is entering the engine and this is the type of flow that happens when the engine is running at steady load high speed. At (3) the voltage is falling, showing a decrease in air flow into the manifold. At (4) the sensor voltage is at its minimum and this happens when the throttle valve is closed. In this position a switch, incorporated into the air flow sensor, normally causes the ECM to switch to idle control. Figure 7.19(b) shows the type of pattern that may be obtained from a defective potentiometer type of air flow meter. The spikes indicate that parts of the potentiometer track are defective and these would cause a problem whenever the sensor wiper is located in these positions.

1 Spikes in downward direction indicate a short to ground or an intermittent open in the resistive carbon strips.

2 Peak voltage indicates maximum air flow entering intake manifold.

3 A voltage decrease indicates less air is flowing into the intake manifold.

4 Minimum voltage indicates a closed throttle plate.

5 A voltage increase identifies an increase of airflow into the intake manifold.

1 Spikes in downward direction indicate a short to ground or an intermittent open in the resistive carbon strips.

2 Peak voltage indicates maximum air flow entering intake manifold.

3 A voltage decrease indicates less air is flowing into the intake manifold.

4 Minimum voltage indicates a closed throttle plate.

5 A voltage increase identifies an increase of airflow into the intake manifold.

Fig. 7.19 Potentiometer-type air flow sensor voltage signal Mass air flow sensors

As described in Chapter 5, these sensors normally rely on changes in resistance in the sensing element that are brought about by changes in air flow. The signal output may be of analogue or digital form depending on the type of sensor. One obviously needs to check this sort of detail before attempting any tests. Figure 7.20 shows the oscilloscope connected to the signal wire of a sensor that gives an analogue signal.

When the scope is securely connected and safely positioned, the engine is started and allowed to idle. The engine speed is slowly increased whilst observing the scope screen. If the sensor casing is very lightly tapped with a small screwdriver it should show up any poor connections by breaks, or blips in the scope pattern. Figure 7.21 shows a signal pattern for an analogue MAF in good condition.

Fig. 7.20 Testing an analogue air flow sensor
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