Common Rail Pressure Sensor

General description 
The common rail pressure sensor is located on the fuel rail. Its function is to monitor the fuel pressure in the common rail. 
Sensor is used by the ECM as a part of the calculation for the % duty cycle applied to the Fuel Pressure Control Valve and Fuel Quantity Control Valve. 
It’s a three-wire sensor: +5V powered from the ECM, output signal and ground. Output range is 0.5 ÷ 4.5V for Bosch, Delphi and Siemens rail pressure sensors and 1.0 ÷ 4.2V - for the Denso sensors.

Appearance 
Fig. 1 shows a typical common rail pressure sensor.

Fig. 1

Principle of operation of the common rail pressure sensor

Pressure measurement results from the bending of a steel diaphragm on which are located polysilicon strain-gauge elements. These are connected in the form of a Wheatstone bridge. This permits high signal utilization and good temperature compensation. The measurement signal is amplified in an evaluation IC and corrected with respect to
offset and sensitivity. At this point, temperature compensation again takes place so that the calibrated unit comprising measuring cell and ASIC only has a very low temperature-dependence level. Part of the evaluation IC is applied for a diagnostic function which can detect the following potential defects:
 – Fracture of a bonding wire to the measuring cell.
 – Fracture anywhere on any of the signal lines.
 – Fracture of the bridge supply and ground.

Order to verify functionality of the common rail pressure sensor

• Testing the output signal

• Connect the ground lead of the oscilloscope to chassis ground.
• Start the engine and left it idling
• Connect the active probe to the sensor’s output wire (usually in the middle).
• Watch the oscilloscope screen. Pressure must be in the range between 25 to 35MPa during engine idling.
• Press the throttle sharply and then release it, pressure must rise up to 100MPa and then reduce to around 30MPa.

In fig.2 you’ll be able to monitor pressure (y axis) change as a function of time (x axis).

Fig.2

• Possible failures in the common rail pressure sensor:
    - Chaotic output signal

• Chaotic output signal is, when the voltage signal changes randomly, drops to zero and disappears.
  This usually happens when an inefficient common rail pressure sensor is present. In this case the sensor must be replaced.

    - Missing of signal voltage

• Check whether the supply voltage is applied (+5.0V).
• Check for grounding problems.
• If the supply voltage and the grounding are correct, check the signal wire between the common rail pressure sensor and the onboard controller.
• If the supply voltage and/or grounding are not correct, check the condition of the wires between the sensor and the ECU.
• If all wires of the sensor are correct, check all connections for the reference voltage and ground of the onboard controller. If they are correct under suspicion falls the ECU.

     - The common rail pressure sensor power supply or signal is equal to the car battery voltage.

• Check for a short circuit to the positive terminal of the car battery.