632 Conventional Switching To Earth

Heavy duty transistor switches that are controlled by the ECM (as shown in Fig. 6.5) are used to complete the circuit to earth in order to operate the injectors.

In this case the oscilloscope pattern will be similar to that shown in Fig. 6.6. In order to limit current flow in the injector circuit the earth path may include a series resistor. In some applications the injector solenoid winding is designed to have a higher resistance.

By reference to the trace in Fig. 6.6 it is possible to see the following.

• The high voltage at (1) is caused by the surge that occurs when the current is switched off and the magnetic field of the injector solenoid collapses.

• This is the point at which the ECM switches off the transistor and current ceases to flow. It is the end of the injection pulse.

Fig. 6.5 Earth switching of petrol injectors
Fig. 6.6 Oscilloscope trace of voltage for one pulse of the injector

This represents the period of time for which the injector is delivering fuel and the actual time (in ms) can be determined by reading off the time base scale of the oscilloscope.

This represents the voltage supplied to the injector (normally system voltage is 12 V).

• This is the point at which the ECM switches on the driver transistor, to earth the solenoid winding, and it is the point at which injection commences.

Do It Yourself Car Diagnosis

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