Plug the current clamp into Channel A of the scope. Set the clamp to the 20 amp setting if required and auto-zero it. Place the clamp around one of the injector's multi-plug wires. It may be necessary to carefully remove some of the loom tape, or untwist the wires, to allow adequate space for the clamp's jaws.
The connection is illustrated in Figure 1.
Note: If the waveform does not start with a positive pulse, the current clamp has been connected the wrong way around. Remove it and replace facing the opposite direction.
These example waveforms show the injector voltage and current under different operating conditions. An auto/rising trigger has been used to stabilize the waveform.
Example 1: There are two pre-injection phases before the main injection phase at engine idle speed.
Example 2: At higher engine speed or load, the main injection phase expands. One of the pre-injection phases may also disappear.
Example 3: On overrun, the main injection phase cuts off and a multi-pulsing injection phase is created to allow excess fuel to be discharged into the leak-off circuit.
Figure 2 shows a Delphi injector.
The Delphi injectors are instantly recognisable by their thin pencil-like design and clip-on multi-plug wiring connections. They can be found on vehicles from manufacturers such as Ford, Renault and Nissan.
The operating strategy of the injector is controlled by the engine's Electronic Control Module (ECM) or Injector Driver Module (IDM), and may vary between vehicle manufacturers even when the same Delphi injectors are used. For example, the above waveform has been captured from a Renault vehicle and has two pre-injection phases at idle speed, while the same injector test on a Ford vehicle has only one pre-injection phase.
As with all common rail systems there may be various injection phases:
As soon as the injector's solenoid is energised, the needle valve inside the injector lifts and fuel injection begins, creating a slight pressure drop in the common-rail.
The initial current, illustrated in Figure 3, is called the call current. It is higher than the hold current to overcome the injector's needle valve inertia.
At the end of the injection phase the injector solenoid is de-energised and fuel injection stops.
If there is a sudden change of demand in fuel, such as when pressure on the accelerator is completely removed and there is little or no load on the engine, the ECM / IDM allows the fuel pressure to be discharged through the injector and into the leak-off circuit.
During this phase, the injector multi-pulses as illustrated in example waveform 3. If the time base was to be increased, it would show many of these pulses before returning to normal idle speed. The three illustrated current peaks show that there is a call current but no hold current. This prevents fuel being injected into the cylinder, by allowing the fuel to pass through the injector into the leak-off circuit.
If any Delphi injectors are identified as faulty and are replaced, the new injectors need to be re-coded into the ECM / IDM using a scan tool, as they are individually calibrated.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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