Diesel – circuit operation

DIESEL – CIRCUIT OPERATION

Boost Pressure Sensor

The boost pressure sensor signal is used in conjunction with the air temperature signal to calculate volume air flow into the engine. The sensor is located on the rear bulkhead, with the pressure tap just after the charge air cooler. If the sensor fails, a substitute value of 490 hPa is used by the ECM (Z132), producing a reduction in power due to a fuel quantity limiting to 21mg/stroke.

Throttle Position Sensor (X171)

The DDE system is a ‘Drive by Wire’ system i.e. the throttle pedal does not directly control a throttle disc or the amount of fuel injected into the engine, but accelerator movements or ‘drivers request’ are sensed and the information is passed to the ECM (Z132). The ECM (Z132) calculates the maximum allowable fuel quantity from the air flow into the engine, engine speed, temperature, etc. It also includes information from strategies such as smoke limitation, active surge damping, automatic gear change, fuel reduction, etc. to calculate the final figure. When driving, if the ‘drivers request’ signal is smaller than the maximum allowable quantity, then the requested quantity is injected. However, if the requested quantity is greater than the maximum allowable, then the latter quantity is injected rather than the driver’s demand. Therefore the Throttle Position Sensor (X171) is very important to the system. It is located within the cab, close to the pedal assembly itself. The unit consists of a potentiometer and has three outputs: 1. Throttle Position - pin 37 ECM (Z132) The sensor outputs the pedal position to the ECM (Z132), which uses the information as described above. 2. Idle Position Switch - pin 25 ECM (Z132). The sensor has a separate idle position switch which informs the ECM (Z132) of the pedal status in the form of a simple on/off signal. This information is used by the ECM (Z132) to implement
idle speed control’ and other strategies i.e.
overrun fuel shut-off’. 3. Kick Down Switch. This switch is currently not used.

Fuel Temperature Sensor

A thermistor is also located inside the injection pump. The fuel temperature sensor signal is used to adjust the quantity of injected fuel, especially during temperature extremes. The signal is also used to back up the Engine Coolant Temperature Sensor (X126). If this sensor fails, the ECM (Z132) uses a

substitute value of 60  C and only slight effects on fuelling may possibly be noted.

Fuel Quantity Feedback Sensor

Located within the injection pump, this sensor sends the ECM (Z132) information regarding the actual quantity of fuel injected. Failure of the sensor or corrupted signals will illuminate the warning lamp and cause the engine to stall or not start. A second check, a plausibility check against the needle lift sensor, also takes place.