Injectors - multi-point (voltage)

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

Plug a BNC test lead into Channel A of the PicoScope. Place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe or Multimeter Probe onto the test lead with the coloured moulding (positive). Attach the black clip onto the battery negative terminal and probe the switched earth side of the injector with the Back-pinning Probe or Multimeter Probe. Alternatively the TA012 two pin test lead adapter can be used, as illustrated in Figure 1.

With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings. Snap the accelerator quickly from idle to full throttle and observe the waveform expanding under acceleration and (on some models) shutting off on over-run. The waveform is stabilised by using the falling voltage to trigger the oscilloscope.

Example waveform

Waveform notes

The injector is an electromechanical device which is fed by a 12 volt supply. The voltage will only be present when the engine is cranking or running, due to the voltage supply being controlled by a tachometric relay.
The length of time the injector is held open will depend on the input signals seen by the Electronic Control Module (ECM) from its various engine sensors.

The held open time or 'injector duration' will vary to compensate for cold engine starting and warm-up periods. The duration time will also expand under acceleration. The injector will have a constant voltage supply while the engine is running and the earth path will be switched via the ECM, the result can be seen in the example waveform. When the earth is removed a voltage is induced into the injector and a spike over 80 volts is recorded.

The height of the spike will vary from vehicle to vehicle. If you see approximately 35 volts, it is because a zener diode is used in the ECM to clamp the voltage. Make sure the top of the spike is squared off, indicating the zener dumped the remainder of the spike. If it is not squared, that indicates the spike is not strong enough to make the zener fully dump, meaning there is a problem with a weak injector winding. If a zener diode is not used in the computer, the spike from a good injector will be 60 or more volts.

Multi-point injection may be either sequential or simultaneous. A simultaneous system will fire all 4 injectors at the same time with each cylinder receiving 2 injection pulses per cycle (720° crankshaft rotation). A sequential system will receiving just 1 injection pulse per cycle, this is timed to coincide with the opening of the inlet valve.

As a very rough guide the injector durations for an engine at normal operating temperature, at idle speed are:

• 2.5 ms - Simultaneous
• 3.5 ms - Sequential

Technical information

The multi-point injector is an electromechanical device which is fed by a 12 volt supply from either the fuel injection relay or from the Electronic Control Module (ECM).

The voltage in both cases will only be present when the engine is cranking or running, due to both voltage supplies being controlled by a tachometric relay.

The injector is supplied with fuel from a common fuel rail. The length of time that the injector is held open for will depend on the input signals seen by the engine management ECM from its various engine sensors. These input signals will include:

• The resistance of the coolant temperature.
• The output voltage from the airflow meter (when fitted).
• The resistance of the air temperature sensor.
• The signal from the Manifold Absolute Pressure (MAP) sensor (when fitted).
• The position of the throttle switch / potentiometer.

The held open time or injector duration will vary to compensate for cold engine starting and warm-up periods, i.e. a large duration that decreases the injection time as the engine warms to operating temperature.
The duration time will also expand under acceleration and contract under light load conditions.

Depending on the system encountered the injectors can fire either once or twice per cycle. The injectors are wired in parallel with simultaneous injection and will all fire together at the same time (see Figure 2). Sequential injection, as with simultaneous, has a common supply to each injector but unlike simultaneous has a separate earth path for each injector (see Figure 3). This individual firing allows the system, when used in conjunction with a phase sensor, to deliver the fuel when the inlet valve is open and the incoming air helps to atomise the fuel.
It is also common for injectors to be fired in 'banks' on 'V' configured engines (see Figure 4). The fuel will be delivered to each bank alternately. In the case of a Jaguar V12 the injectors are fired in 4 groups of 3 injectors.

Because of the frequency of the firing of the injectors, it is expected that a sequential injector will have twice the duration, or opening, than that of a simultaneous pulse. This will however be determined by the injector flow rate.

The injector consists of a solenoid operated valve which is held in the closed position by a spring until the earth circuit is completed by the ECM. When the electromagnetic field lifts the pintle off its seat, fuel is delivered to the engine. The total lift on the pintle is approximately 0.15 mm (6 thou) and has a reaction time around 1 millisecond.

The injector consists of a solenoid operated valve which is held in the closed position by a spring until the earth circuit is completed by the ECM. When the electromagnetic field lifts the pintle off its seat, fuel is delivered to the engine. The total lift on the pintle is approximately 0.15 mm (6 thou) and has a reaction time around 1 millisecond.

Figure 6 shows an electronic fuel injector.

AT382-1

Suitable accessories