DIESEL
A6
CIRCUIT OPERATION
2
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.