Bosch CRDi injector under acceleration (using a low-current clamp)

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 the current clamp into Channel A on the PicoScope.
  • Select the 20 A range if required and switch the current clamp on.
  • Press the zero button before connecting the clamp to the circuit.
  • The current is controlled and limited by the vehicle's Electronic Control Module (ECM). Place the current clamp on the fuel injector's supply wire. It may be necessary to connect to each wire in turn and view the waveform in order to identify the correct one. If access is difficult, it may be necessary to pull back some of the loom's outer shielding to allow sufficient room for connection.

Figure 1 shows the current clamp connected to one of the injector wires.

Waveform notes

The first example waveform shows the two distinctive points of injection, the first pulse being the 'pre injection' phase, and the second the 'main' injection phase.

As the throttle is opened and the engine is accelerated, the second example shows that the 'main' injection pulse expands in a similar way to a petrol injector.

In the third example the throttle is released and the 'main' injection pulse disappears until the engine returns to just above idle.

Under certain engine conditions a third phase may be seen. This is called the 'post injection' phase and is mainly concerned with controlling the exhaust emissions.

Technical information

The amount of fuel that is injected into the engine is finely calculated by the vehicle's Electronic Control Module (ECM) from information received from the engine's various sensors. Also, the length of time that the injector is held open is determined by the fuel pressure.

At a lower engine speed, the pump delivers a lower pressure and therefore a longer duration is required. As the engine and pump speeds increase, the injector duration decreases, but due to the higher pressure a greater quantity of diesel is delivered to the engine.

The point of injection determines the injection timing. This is determined by many factors including: engine speed, engine load and engine temperature. The injector is initially supplied with 80 V to lift the injector's pintle, and then with 50 V to hold the pintle open. These higher voltages are supplied from a capacitor that collects the induced voltage from the previous injection in the cycle. Unlike conventional diesel injection systems which only utilise a single injection period, the HDI system can have up to three!

Pre-injection is used to inject a small amount of fuel into the engine. The fuel immediately combusts and is used as the ignition source for the 'main' injection period. This type of two-stage injection reduces the characteristic diesel 'knock'.

Main injection is the conventional period of injection whose duration is determined by the vehicle's ECM.

Post injection is used under certain engine conditions to reduce the amount of pollutants emitted from the vehicle's exhaust system.


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.

Suitable accessories

  • 30 A (low amps) current clamp


  • 20 A / 60 A DC (low amps) current clamp


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Guided test: Bosch Injectors (Current Under Acceleration)