Common Rail fuel pressure control

Snap-on discuss how to diagnose problems with pressurised fuel systems

Published:  24 February, 2015

Fuel pressure is generated by a continuously operating high pressure pump. The engine ECU controls the fuel pressure using a closed loop strategy. The closed loop comprises the engine ECU, the rail pressure sensor and the rail pressure control valve.

The ECU receives information from the rail pressure sensor as a variable voltage. It then uses this information to determine the fuel pressure required. The ECU then sends a command to the fuel pressure regulator via a duty cycle to achieve the desired pressure.

The fuel pressure sensor is a three-wire device with battery voltage on one wire, earth on another and the third wire is the signal wire. On the signal wire you will find half a volt KOEO (key on engine off). The voltage at idle will be approximately 1.2 volts rising to approximately 3 volts snap throttle.

The fuel pressure control solenoid is a two wire device with battery voltage on one wire. The ECU then pulls the other wire down to earth to complete the circuit and increase the rail pressure. The time this signal spends at earth is displayed on a scan tool as a percentage. On most systems a low duty cycle equals low pressure and high duty cycle equals high fuel pressure. To achieve the correct pressure at idle a command of about 18% is normal for most systems.

This duty cycle signal can be checked with a scope set to measure voltage, set the voltage to 20 volts and the time scale to 20ms. Back probe the fuel pressure regulator and it should look like the image below. If you own a Snap on scope you can turn this signal into a live percentage reading as follows.

Press the back button twice to come back to the scope main menu, from here pick graphing multimeter. In the next menu you can pick the type of signal you are wanting to measure, one of the options is duty cycle click this option. The graphing multimeter will then convert the signal into a live percentage reading.

Lab scope set to voltage

Fuel pressure control solenoid duty cycle (yellow trace)

Fuel pressure sensor voltage (green trace)

Graphing multimeter

Fuel pressure control solenoid duty cycle (yellow trace)

Fuel pressure sensor voltage (green trace)

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