Injectors - Multi-point (Current) Using a low 0 to 60 amp current clamp
How to connect the oscilloscope
Example waveform and notes
How to connect the oscilloscope when testing:- the current waveform on a multi-point injector
Connect the 60A current clamp to channel A of the ADC-212 using the TA000 test lead.
The current clamp should be placed around the fuel injectors supply wire. If the wire is not easily accessible, unplug the connector from the injector and use the TA011 two pin break out lead. The 60A clamp can be placed around either the blue or the yellow exposed cable section of the adapter, as illustrated in figure 36.1. As there is no commonality to which terminal caries the supply, it may be necessary to observe both waveforms and select the correct one.
Example multi-point injector (current) waveform
Multi-point injector (current) waveform notes
It can be clearly seen from the example waveform that the waveform is clearly 'split' into two easily defied areas. The first part of the waveform is responsible for the electromagnetic force lifting the pintle, in this example the time taken is approximately 0.6 ms. At this point the current can be seen to fall before rising again as the pintle is held open. With this in mind it can be seen that the amount of time that the injector is held open is not necessarily the same as the time measured. It is not however possible to calculate the time taken for the injectors spring to fully close the injector and cut off the fuel flow. This test is ideal for identifying an injector with an unacceptably slow solenoid reaction time. Such an injector would not deliver the required amount of fuel and the cylinder in question would run lean.
Technical information - multi-point electronic injectors
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 36.2). 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 36.3). 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 36.4). 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.
Figure 36.5 show a cross section of an electronic fuel injection. Figure 36.6 shows an electronic fuel injector.