Fuel Rail Pressure Sensor The fuel rail pressure sensor is located on the end of the fuel rail, at the rear of the engine compartment. The sensor uses a diaphragm in contact with the pressurised fuel. The diaphragm is attached to a piezo crystal which changes resistance as the diaphragm reacts to changes in fuel pressure. The deflection of the diaphragm equates to approximately 1 mm per 500 bar (7252 lbf/in 2 ). The ECM supplies the fuel pressure sensor with a V reference voltage to abridge circuit. The returned voltage, depending on pressure, is in the range of 0 – 70 mV. This is passed through an evaluation switch within the sensor and outputs an analogue signal to the ECM which can be between 0.5 and V. The ECM compares the received signal voltage against stored data and converts the signal into a fuel pressure value. The ECM uses the pressure value to control the fuel rail pressure control valve located on the high pressure pump, to control the fuel pressure supplied to the fuel rail and the injectors. The ECM also uses the fuel rail pressure signal in conjunction with the LP fuel sensor signal to calculate the 'setpoint' pressure used during engine operation. The ECM will monitor the fuel rail pressure only if the engine speed is greater than 550 rev/min, the fuel rail pressure control valve is operational and there is no stored faults for the fuel rail pressure sensor. The ECM monitors the following fuel rail pressure related variables: l fuel rail pressure within maximum and minimum limits l controlled variable at fuel rail pressure control valve within minimum and maximum limits l plausibility of rail pressure and controlled pressure variable. If the ECM registers an 'out of range' deviation for any of the above a fault is stored in the ECM memory. This result in immediate engine shutdown. Possible causes of the 'out of range' deviation areas follows: l fuel tank empty l fuel system problem upstream of high pressure fuel pump l internal or external leakage in the high pressure system l high pressure fuel pump output inadequate (pump failure) l injector leak-off rate excessive l control quantity for injectors too high l fuel rail pressure control valve failure. The wires to the pressure sensor are monitored by the ECM for short and open circuit. The ECM also monitors the V supply. If a failure occurs a fault is recorded in the ECM memory. The ECM uses a default rail pressure value which results in the injection quantity being limited. In the event of a failure of the fuel rail pressure sensor, the following symptoms maybe observed: l Engine will not start l Severe loss of power l Engine stalls.
ENGINE MANAGEMENT SYSTEM – TD6 DESCRIPTION AND OPERATION 18-1-27