Unit-injection (VAG PD) piezo injector - voltage and current

The purpose of this test is to examine the electrical operation of a VAG PD injector, current, voltage and earth circuits.

How to perform the test

View connection guidance notes.

Do not electrically disconnect a piezoelectric injector with the engine running.  Ensure secure diagnostic electrical connections are made.

1. Use manufacturer data to identify the function of the injector circuits.
2. Connect the Low amp clamp to PicoScope Channel A.
3. Select the 20 A scale and zero the clamp.
4. Connect the clamp to the injector supply circuit.
5. Connect PicoScope Channel B to the same supply circuit.
6. Connect PicoScope Channel C to the injector switched earth circuit.
7. Start the engine and allow it to idle.
8. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
9. Start the scope to see live data.
10. With your waveforms on screen stop the scope.
11. Stop the engine and turn the ignition to off.
12. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

The orientation of the current clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

Channel A

This shows the injector current. At the point of injection the current rises from zero to around 15 amps where there is a brief interruption before continuing to rise to just over 17.5 amps. The injection current remains high for a short period before reducing to about 11 amps and then enters a multi-pulsing phase.

Channel B

This shows the injector live circuit. It is important to note that the injectors are live switched; at the start point of injection the normal battery supply voltage is shown at around 14 V. The supply voltage is modulated to create the injection phases throughout the injector duration of around 2 ms.

Channel C

This shows the injector earth circuit. At the end of the injection period the voltage in the injector's electromagnetic windings collapses and this induced voltage can be seen rising to around 50 volts.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, injector current.

You can refine your search by selecting, make VW.

Further guidance

The electromechanical unit injectors are located inside 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 on the injectors to compress the fuel, so there is no high-pressure pump. This action generates between 1,800 and 2,050 bar of pressure.

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

The electrical control also differs from common-rail as the supply voltage is at normal battery voltage. The switching of the injections is conducted by the positive supply and not by controlling the earth path.

As with all modern injectors there is a pilot and main injection phase to control noise and emissions and maintain a smooth combustion. The rise of the injector current from zero to the maximum is the pilot injector period. The period of the multi-pulsing current is the main injection phase.

GT385