ENGINE MANAGEMENT SYSTEM – TD6 DESCRIPTION AND OPERATION If the IAT sensor fails the ECM uses a default intake air temperature of -CF. In the event of an IAT sensor failure, any of the following symptoms maybe observed: l Over fuelling, resulting black smoke emitting from the exhaust l Idle speed control inoperative. Boost Pressure (BP) Sensor The BP sensor is located on the rear of the inlet manifold, adjacent to the fuel rail pressure sensor. The sensor provides a voltage signal to the ECM relative to the intake manifold pressure. The BP sensor has a three pin connector which is connected to the ECM and provides a V reference supply from the ECM, a signal input to the ECM and aground for the sensor. The BP sensor uses piezo ceramic crystals. These are sensitive to pressure changes and oscillate at different rates according to air pressure. The oscillation changes the resistance of the piezo crystal. The resistance through the crystal produces an output voltage from the sensor of between 0 and V, proportional to the pressure in the intake manifold. A signal of Vindicates low pressure and Vindicates high pressure. This signal is processed by the ECM which determines the manifold pressure by comparing the signal to stored values. The ECM then uses the signal for the following functions: l Maintain manifold boost pressure l Reduce exhaust smoke emissions when driving at high altitude l Control of the EGR system l Control of the vacuum control module. If the BP sensor fails, the ECM uses a default pressure of 0.9 bar (13 lbf/in 2 ). In the event of a BP sensor failure, the following symptoms maybe observed: l Altitude compensation inoperative (black smoke emitted from the exhaust) l Active boost control inoperative.