Diesel – circuit operation

DIESEL – CIRCUIT OPERATION

CIRCUIT OPERATION

Sensors Engine Speed Sensor (X255)

The engine speed signal is crucial to the system, as the information from the sensor is used in virtually all of the strategies within the ‘DDE’ (Digital Diesel Electronics) and its Engine Control Module (ECM) (Z132). Through this sensor, the ECM (Z132) knows if the engine is turning, how fast it is turning and approximately where the engine is in its cycle. The sensor is of the ‘Hall effect’ type, which sends out a pulse to the ECM (Z132) every time a ‘tooth’ is sensed on the flywheel (the flywheel has six ‘teeth’). If the sensor fails, then the warning lamp is activated and the ECM (Z132) enters a ‘limp home’ mode where the ECM (Z132) looks at the signal from the needle lift sensor in one of the injectors. The needle lift sensor gives one pulse per injection, i.e. one pulse per twelve engine speed sensor pulses. The response of the ECM (Z132) to changes in engine parameters will, therefore, be considerably slower, and a higher idle speed is initiated to try to compensate at low engine speeds.

Needle Lift Sensor (X318)

The engine has six diesel fuel injectors, one of which has a sensing element at the tip which informs the ECM (Z132) exactly when the injector fires (the ‘beginning of injection’ signal). The ECM (Z132) uses this to correct the injection timing and also to back up the engine speed signal in case that sensor fails. If this sensor fails, the warning lamp will switch on and the vehicle will enter ‘limp home’ mode with reduced engine power/performance and lack of throttle response (as there is a lack of feedback on the injection timing).

Engine Coolant Temperature Sensor (X126)

This sensor is a ‘thermistor’ (a temperature dependent resistor) where the voltage output varies in proportion to coolant temperature. The ECM (Z132) uses this information in many strategies, i.e. to correct the injected fuel quantity and timing (especially during cold starts), length of glow plug timing, etc. The sensor is located in the top of the engine block. In case of a failure, the warning lamp is not activated and the ECM (Z132) selects a substitute value of 50
C for glow plug and ignition timing and uses the fuel temperature to correct the fuel quantity, glow plug timing will not be correct, possibly resulting in long crank times in cold weather

as well as slight fuelling effects. These symptoms may not be noticeable.

Air Temperature Sensor (X311)

This sensor is another thermistor, located in the inlet manifold, just after the intercooler, and measures the actual temperature of the turbo booster air entering the engine. To calculate the maximum amount of injected fuel allowed, the ECM (Z132) needs to know the volume of air flowing into the engine. Normally a bulky, heavy and expensive air flow meter is necessary, but the ECM (Z132) can calculate the air flow from the air temperature and air pressure, so eliminating the need for an air flow meter. If the air temperature sensor fails, a substitute value of 30
C is used, so by producing an incorrect air flow reading and a reduction in engine power.