Mass Air Flow/Intake Air Temperature Sensor
The combined Mass Air Flow (MAF)/Intake Air Temperature (IAT) sensor is located on the top cover of the air filter housing and is secured with two screws. The sensor is downstream of the air filter and measures the air entering the clean air duct to the turbocharger.
The MAF/IAT sensor receives a fused 12V power supply from the Powertrain Control Module (PCM) relay in the Engine Junction Box (EJB). Three further connections provide signal feedback to the PCM for intake air volume and temperature.
The MAF sensor works on the hot film principle. Two sensing elements are contained within a film. One element is maintained at ambient (air intake) temperature, for example 25°C (77°F). The other element is heated to 200°C (392°F) above the ambient temperature, for example 225°C (437°F). Intake air entering the engine passes through the MAF sensor and has a cooling effect on the film.
The PCM monitors the current required to maintain the 200°C (392°F) differential between the two elements in the MAF. The differential is used to provide a precise, non-linear, frequency based signal which equates to the volume of air being drawn into the engine.
The PCM checks the calculated air mass against the engine speed. If the calculated air mass is not plausible, the PCM uses a default air mass figure which is derived from the average engine speed compared to a stored characteristic map. The air mass value will be corrected using values for boost pressure, atmospheric pressure and air temperature.
The IAT sensor within the MAF/IAT sensor incorporates a Negative Temperature Coefficient (NTC) resistor in a voltage divider circuit. The NTC resistor works on the principle of decreasing resistance in the sensor as the temperature of the intake air increases. As the NTC resistor allows more current to pass to ground, the voltage sensed by the PCM decreases. The change in voltage is proportional to the temperature change of the intake air. Using the voltage output from the IAT sensor, the PCM can correct the fueling map for intake air temperature. The correction is an important requirement because hot air contains less oxygen than cold air for any given volume.
The MAF/IAT sensor output is a digital signal proportional to the mass of the incoming air. The PCM uses this data, in conjunction with signals from other sensors and information from stored fueling maps, to determine the precise fuel quantity to be injected into the cylinders. The signal is also used as a feedback signal for the Exhaust Gas Recirculation (EGR) system.
If the MAF sensor fails the PCM implements the default strategy based on engine speed. In the event of a MAF sensor signal failure, any of the following symptoms may be observed:
- Difficult starting
- Engine stalls after starting
- Delayed engine response
- Emission control inoperative
- Idle speed control inoperative
- Reduced engine performance.
If the IAT sensor fails the PCM uses a default intake air temperature of -5°C (23°F). In the event of an IAT sensor failure, any of the following symptoms may be observed:
- Over fueling, resulting black smoke emitting from the exhaust.
- Idle speed control inoperative.