Bosch CDI3 fuel pressure sensor

The purpose of this test is to check the fuel system on a common rail diesel engine by using the fuel rail pressure sensor.

Connection guidance

Connection for diagnostic work will vary dependent on the application.

Technicians should whenever 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 the 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 to the control module.
  • When testing an actuator, it is desirable to gain access to the actuator.

How to perform the test

  1. Using manufacturer data, identify the output signal circuit for the fuel rail pressure sensor.
  2. Ensure the engine is stopped.
  3. Connect PicoScope channel A.
  4. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  5. Select GO or press the space bar to see live data.
  6. Turn on the ignition and wait for the glow plug light to extinguish (if applicable).
  7. Crank and start the engine and let it idle for 5 to 10 seconds.
  8. Depress the accelerator pedal to its maximum load position for 5 to 10 seconds.
  9. Release the accelerator pedal and let the engine return to idle for around 5 to 10 seconds.
  10. Turn off the engine and ignition.
  11. With your live waveform on screen select STOP or press the space bar to stop your capture.
  12. Use the WAVEFORM BUFFER and ZOOM tools to examine your waveform.

Waveform notes

The measurement starts with the ignition on, the engine off and the sensor output at 0.5 V, corresponding to a pressure of 0 bar.

As the engine cranks and starts, the sensor output rises to about 1.3 V, corresponding to a rail pressure of about 280 bar, a common value at idle in this early generation of the system.

With the pedal fully depressed, the Engine Control Module (ECM) increases the injection quantity, which accelerates both the engine and a rise in rail pressure. After reaching a peak pressure at about 3.5 V, the ECM acts to regulate the rail pressure whilst maintaining maximum engine speed: with the vehicle stationary, the engine is unloaded and a high rail pressure is not required to maintain its speed, as indicated by the sensor output, which settles at about 2.5 V.

When the accelerator pedal is released, the engine returns to idle and the sensor output returns to 1.3 V, as was observed previously.

After the engine is stopped and the ignition is switched off, the signal drops slowly back to 0.5 V over about 10 s, after which the ECM powers down and the sensor output falls to 0 V.

Waveform Library

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

Example 2-pin pressure sensor

Example 3-pin pressure sensor

Further guidance

The fuel rail pressure sensor provides feedback to the ECM, which regulates the rail pressure by either varying the volume of fuel delivered by the high-pressure pump or controlling the flow of excess fuel from the rail to the fuel return system. The ECM uses its internal calibration tables maps to calculate the desired fuel rail pressure and injection duration according to the accelerator pedal position indicating driver demand, engine speed, load, and temperature etc.

The rail pressure sensor is calibrated to output a signal between 0.5 V and 4.5 V with changing rail pressure across the range 0 bar to 1600 bar. This provides two opportunities for the ECM to undertake a signal plausibility check: any voltage above or below these values, for example, either at 0 V or 5 V, will indicate that the sensor has failed.

A high-pressure system leak can be indicated if the sensor output rapidly drops to 0.5 V after the engine has been stopped, as might occur with a leaky injector, rail pressure regulator, inlet metering valve or pump.

Please note, the example waveform was captured with an unloaded engine; with the engine fully loaded over a prolonged period, the sensor output voltage will be above 2.5 V and, in the extreme case, up to 4.5 V.

Refer to vehicle technical data for specific test conditions and results.

Diagnostic trouble codes

P0087 – Fuel rail pressure sensor or rail pressure too low

P0088 – Fuel rail pressure sensor or rail pressure too high

P0190 – Fuel rail pressure sensor circuit open/short/failure

P0191 – Fuel rail pressure sensor circuit performance/range

P0192 – Fuel rail pressure sensor circuit low input

P0193 – Fuel rail pressure sensor circuit high input

P0194 – Fuel rail pressure sensor circuit intermittent

GT138-4

Disclaimer
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

  • Premium 6-way breakout lead set

    £269.00

  • Back-pinning Probe Set

    £40.00

  • Large Dolphin/Gator Clips

    £10.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £47.00

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2 comments | Add comment

Mike Valentine
January 26 2017

Hi X - These figures were obtained from the workshop manual for the vehicle under test. Thanks.

X
December 26 2016

Where did you get the values like 280Bar at 1.3V and 1600Bar at 4.5V. Is there any chart ?

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Guided test: Bosch CDI3 fuel pressure sensor