Discovery 2. Manual - part 467
ENGINE MANAGEMENT SYSTEM - TD5
DESCRIPTION AND OPERATION 18-1-11
Ambient Air Temperature and Pressure (AAP) sensor
The AAP sensor is located in the top of the air filter housing. It provides voltage signals relative to both ambient air
pressure and temperature to the ECM. The AAP sensor produces a voltage between 0 and 5 volts proportional to the
pressure level of the air in the air filter housing. A reading of 0 volts indicates low pressure and a reading of 5 volts
indicates high pressure. The ECM uses the signal from the AAP sensor for the following functions:
l
To maintain manifold boost pressure.
l
To reduce exhaust smoke emissions while driving at high altitude.
l
Control of the EGR system.
The sensor also supplies a voltage between 0v and 5v proportional to ambient temperature. The ECM uses this signal
for the following functions:
l
Exhaust gas over temperature protection.
l
Turbocharger overspeed protection.
Input/Output
Connector C0158 pin 8 of the ECM supplies the AAP sensor with a 5 volt power supply. The pressure output from the
AAP sensor is measured at pin 10 of the ECM connector C0158, the temperature output from the AAP sensor is
measured at pin 31. The earth path is via pin 30 of ECM connector C0158.
The AAP sensor can fail the following ways or supply incorrect signal:
l
Sensor open circuit.
l
Short circuit to vehicle supply.
l
Short circuit to vehicle earth.
l
Contaminated sensor element.
l
Damaged sensor element.
l
Resistance in wiring harness.
In the event of an AAP sensor signal failure any of the following symptoms may be observed:
l
Altitude compensation inoperative (engine will produce black smoke).
l
Active boost control inoperative.
l
Turbocharger boost pressure limited to 1 bar (14.5 lbf.in
2
).
l
EGR altitude compensation inoperative.
The MIL will not illuminate in an AAP sensor failure, and the ECM will use a fixed default value from its memory.