CRD (Bosch) fuel rail pressure sensor - voltage

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

How to perform the test

View connection guidance notes.

1. Using manufacturer's data, identify the output signal circuit for the fuel rail pressure sensor.
2. Ensure the engine is stopped.
3. Connect PicoScope channel A to the signal circuit.
4. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
5. Start the scope 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 s.
8. Depress the accelerator pedal to its maximum load position for 2 to 3 s.
9. Release the accelerator pedal and let the engine return to idle for around 5 to 10 s.
10. Turn off the engine and ignition.
11. With your live waveform on screen stop the scope.
12. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

• With the ignition on and the engine off, the sensor output is at 0.5 V, corresponding to a rail pressure of 0 bar.
• As the engine cranks the sensor output rises rapidly to about 1.3 V, indicating the idle operating rail pressure of around 280 to 320 bar.
• With the accelerator pedal fully depressed, the engine accelerates creating a rise in rail pressure.
• After reaching a peak around 3.5 V, the rail pressure is reduced whilst the high engine speed is maintained and the sensor voltage drops to 2.5 V: with the vehicle stationary, the engine is unloaded and a high rail pressure is not required to maintain its speed.
• When the accelerator pedal is released, the engine returns to idle and the sensor output returns to 1.3 V, as originally observed.
• 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.

Further guidance

The fuel rail pressure sensor provides feedback to the ECM, which regulates the rail pressure by varying the volume of fuel delivered by the high-pressure pump and/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.

GT138-EN