Discovery II (LT) V8-4.0L (2002)
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Absolute Pressure Sensor: Description and Operation
Mass Air Flow (MAF)/Inlet Air Temperature (IAT) Sensor (C0149)
The MAF/IAT sensors are combined into a single unit and located between the air filter housing and the inlet manifold. The ECM receives input
signals from the MAF/IAT sensor to calculate the mass of air flowing into the engine inlet manifold.
Input/Output
The MAF sensor has both electrical input and output. Input to the MAF sensor comes from two different sources. Battery voltage is supplied to the
MAF sensor via fuse 2 of the engine compartment fuse box. The MAF sensor also utilises a 5 volt reference input via pin 7 of connector C0636 of the
ECM. The MAF sensor output voltage is measured via pin 23 of connector C0636 of the ECM.
The IAT sensor has only electrical output. Output from the IAT sensor is measured at pin 34 of connector C0636 of the ECM, this is a variable
voltage/resistance measured by the sensor to provide air temperature information to the ECM.
The MAF/IAT sensor share the same sensor earth. Sensor earth is via pin 9 of connector C0636 of the ECM.
The MAF/IAT sensor and its connector has silver plated terminals for its low current signals to protect against corrosion. DO NOT apply 12 V to the
5 V supply, as this will destroy the internal circuitry. The MAF/IAT sensor should not be dropped or roughly handled and should be kept free from
contamination.
Mass Air Flow (MAF) Sensor
The MAF sensor utilises a "hot film" element contained in the air intake duct to monitor the mass of the airflow being drawn into the engine. The
MAF sensor contains two sensing elements, one element is controlled at ambient temperature (e.g. 25 °C (77 °F)), while the other is heated to 200 °C
(360 °F) above the ambient temperature (e.g. 225 °C (437 °F)).
When the intake air passes the heated element, it cools it down, so lowering the resistance of the hot film element. In order to maintain the same
temperature, the circuit to the heated element has to supply more current. The change in current causes a corresponding change in potential difference
to be detected in the monitoring circuit. This change is supplied to the ECM as a voltage between 0 and 5 V, where it is processed by the ECM's
internal mapping to interpret the data as a measure of the mass of air flow.
The measured air mass flow is used by the ECM to determine the fuel quantity to be injected in order to maintain the stoiciometric air:fuel mixture for
optimum engine performance and low emissions.
Normal operating parameters of the MAF sensor are as follows:
