633 Pulse Width Modulated Injectors

Pulse modulated (power control) injectors have a high initial current to provide rapid opening of the injector valve. After the initial pulse, the ECM switches the earthing transistor on and off at high frequency. This permits the injector to remain open for the required time whilst preventing the circuit from overheating. Figure 6.7 shows the type of oscilloscope trace that should be obtained from this type of injector.

Fig. 6.7 Oscilloscope trace for a pulse width modulated injector

Referring to the Fig. 6.7, please note the following.

1 Current flow pulses on and off at a frequency that holds the injector valve open.

2 Voltage peak is caused by the voltage surge that occurs when the injector solenoid current is switched off and the magnetic field collapses. (In some systems this peak may be shortened because the system is provided with surge protection.)

3 Here the injector voltage is returned to battery voltage ready for the next injector action.

4 This represents the injector 'on-time'. (The sharp rise in voltage as the injector starts to pulse is caused by the sudden interruption of current as the high frequency pulsing commences.)

5 This is where the ECM switches on the driver transistor and starts the injection period.

6 This is the supply voltage, probably battery voltage.

These explanations of the methods of operation of petrol injectors, together with the oscilloscope traces, should give a good insight into the types of test that

Ignition on

Fig. 6.8 The general principle of a petrol injector test. (a) Testing injectors for leakage. (b) Testing the amount of fuel per injector

Ignition on

Fig. 6.8 The general principle of a petrol injector test. (a) Testing injectors for leakage. (b) Testing the amount of fuel per injector can be performed in order to assess the effectiveness of injectors and thus assist technicians in their diagnostic work. However, it is important to remember that the function of an injector is to provide the correct amount of fuel, in a combustible condition, as and when the engine requires it. This means that complete testing of injectors requires examination of other aspects of injector performance, such as the amount of fuel delivered and the spray pattern.

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