Unit-injection - solenoid injector (VAG PD): current and voltage

The purpose of this test is to monitor the power supply, earth circuit and current flow to determine the correct operation of a PD injector.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should wherever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator.

To avoid damage to your scope, you may need to use an attenuator for this test.

These instructions do not refer to an attenuator as it is based on our PicoScope 4425 automotive scope.

If you are using a previous PicoScope Automotive model, you will need either a 10:1 or 20:1 attenuator and will need to adjust the Probe settings for the relevant channel.

These settings can be found under Channel Options, then:
• Probe > 10:1 Attenuator
• Probe > 20:1 Attenuator

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 connections. Refit the multi-plug and repair the loom insulation once the tests have been concluded.

  1. Use manufacturer data to identify the function of the injector circuits.
  2. Connect the Low amp current clamp into Channel A on your PicoScope. Switch on the current clamp and select the 20 A scale. Zero the current clamp and connect to the injector supply circuit.
  3. Connect channel B to the same supply circuit.
  4. Connect channel C to the injector switched earth circuit.
  5. Start the engine and allow it to idle.
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Select go or press the space bar to see live data.
  8. With your live waveforms on screen select stop or press the space bar to stop your capture.
  9. Stop the engine and turn the ignition to off.
  10. Use the Waveform Buffer and Zoom tools to examine your waveform.

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.

Example PD-Unit injector

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.


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

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


  • Premium 6-way breakout lead set


  • Back-pinning Probe Set


  • Flexible Back-pinning Probe


  • Large Dolphin/Gator Clips


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Guided test: Unit-injection - solenoid injector (VAG PD): current and voltage