VAG PD piezo unit injector

The purpose of this test is to check a PD injector current draw under different operating conditions non intrusively.

How to perform the test

The wiring loom for the injectors can be accessed at the round multi-plug at the rear of the cylinder head. It may be necessary to remove the multi-plug by releasing the locking pin and then carefully stripping back the loom insulation to expose adequate wire to make the connection. Refit the multi-plug and repair the loom insulation once the tests have been concluded.

Channel A

Plug the current clamp into Channel A of the scope. Set the clamp to the 20 amp setting and auto-zero it. Place the clamp around one of the wires to the injector to be tested, referring to the manufacturer's wiring diagram if necessary.

The connection is illustrated in Figure 1.

Note: This waveform has been set up in Auto trigger mode, with a rising-edge trigger. If the waveform does not start with a positive pulse, the current clamp has been connected the wrong way round. Remove it and replace facing the opposite direction.

Waveform notes

These example waveforms show the injector current under different operating conditions.

Example 1: The first injector phase is pre-injection and the second is the main injector phase.

Example 2: As the engine speed is raised, the control module increases the duration of the main injector phase.

Example 3: On overrun, only the pre-injection phase remains.

Technical information

The piezo-mechanical unit injectors are located in the cylinder head and the wiring runs through a circular multi-plug at the rear of the head to the individual injectors. The injectors are unlike common-rail injectors in that the high fuel pressure is generated by an additional rocker-arm running from the camshaft and acting directly onto the injectors, so there is no high-pressure pump. These injectors can operate across a wide range of pressures from 130 to 2,200 bar.

Piezo unit Injector fitted

Figure 2 - Piezo Unit Injector fitted to a cylinder head

This system is referred to by the Volkswagen Audi group as Pumpe-Düse (PD).

There may be up to five injection phases:

  • Pilot phases (0 to 2): to combat 'diesel knock' by gently starting combustion
  • Main injection phase (1): for the power and torque characteristics
  • Secondary injection (0 to 2): to increase combustion temperature during diesel particulate regeneration.
Piezo injector element in 2 states

Figure 3

Piezo injectors have the advantage over conventional electromagnetic injectors of a very quick reaction time - some four times faster.

Piezo injectors contain a 'stack' of about 300 wafer-thin ceramic plates. When the switching voltage is applied, the plates expand and act upon the injector needle valve, which in turn opens the nozzle needle and injects the fuel into the cylinder.

Figure 3 shows a piezo element in its two different states. Illustration 1 shows the stack with no supply voltage and illustration 2 shows the increase in height of the stack when the supply voltage is switched on.

Note 1: Once the stack is in the injector open position, it does not need to have a continuous supply voltage and remains in the open state until the close command. The 'open' and 'close' commands are the positive and negative current spikes in the waveform. For this reason, never disconnect the injector's multi-plug with the engine running. If the injector is in the open state it could hydraulic-lock the engine with diesel fuel. The engine will cut out if the control module detects any faults with the injectors or driver circuit.

Note 2: This unit injector is from a 2.0 litre PD VAG engine with piezo injectors. Other engine capacities have the electromechanical unit injectors described in a different topic.


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: VAG Piezo PD (current)