54 Variable resistance type sensors

When an engine is idling the exhaust gas scavenging of the cylinders is poor. This has the effect of diluting the incoming mixture. The ECU must detect when the throttle is in the idling position, so that alteration of the air-fuel ratio can occur to ensure that the engine continues to run smoothly. At full engine load and full throttle, the mixture (air-fuel ratio) needs enriching, so the ECU also needs a signal to show that the throttle is fully open. These duties are performed by the throttle position switch. Figure 5.12 shows how the action of a throttle position sensor is based on the principle of the potential divider.

Vc = Constant voltage supply from computer VP = Voltage giving position of throttle

VP Varies with throttle angle

Fig. 5.12 The principle of the throttle position sensor

VP Varies with throttle angle

Fig. 5.12 The principle of the throttle position sensor

The sensor produces a voltage which is related to throttle position. The voltage signal is conducted to the ECU where it is used, in conjunction with other inputs, to determine the correct fuelling for a given condition.

There are two types of throttle position sensors in common use, and they are quite different in certain respects. Test procedures that will work on one type will not apply to the other type. Here again it is important to be able to recognize which type is being used in a particular application. Figures 5.13 and 5.14 show details of the throttle position switch.

Fig. 5.13 Lucas type throttle position switch
Fig. 5.14 Inside the throttle switch (Lucas)

The points to note about these two types of throttle position sensors is that they are primarily electrical. They do not require any great electrical or electronic knowledge in order to test them. However, the electrical signals which they produce, and which are used by the electronic control module (ECM), must be correct for given conditions. The throttle switch produces 'step' voltage changes at the idling and full throttle position since the ECM program requires precise identification of the throttle open and throttle closed positions. The potentiometertype throttle position sensor produces a steadily increasing voltage, from idling up to full throttle. It is, therefore, very important that any measurements taken during tests are accurate, and relate correctly to the angular position of the throttle butterfly, as well as the specific type of sensor under test. Figure 5.15 shows the throttle position sensor used on the Toyota 3S-GTE engine.

Contact points

Contact points

Fig. 5.15 Throttle sensor (Toyota)

Vcc is a constant voltage of 5 V supplied by the computer. Terminal E2 is earthed via the computer. The other two voltages, IDL and V^, relate to idling and throttle operating angle. Figure 5.16 shows how the voltages at the terminals IDL and Vta relate to the position of the throttle butterfly.

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