Mass Air Flow And Temperature Sensor
The air filter housing is located in the front left corner of the engine compartment. The air outlet connection of the air filter housing incorporates a Mass Air Flow and Temperature (MAFT) sensor. The MAFT sensor is secured with 2 screws and measures the air entering the clean air duct to the turbocharger.
The MAFT sensor receives a 12V power supply from the Powertrain Control Module (PCM) relay in the Engine Junction Box (EJB). Further 3 connections provide signal feedback to the PCM for intake air volume and temperature.
The MAF sensor works on the hot film principle. The 2 sensing elements are contained within a film. The first sensing element is maintained at ambient (air intake) temperature, for example 25°C (77°F). The second sensing 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 2 elements. The PCM uses the differential 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 value. This default air mass value 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 temperature of the air flow is measured with a Negative Temperature Coefficient (NTC) sensor in the MAFT sensor. The NTC sensor works on the principle of decreasing resistance in the sensor as the temperature of the intake air increases. The change in voltage is proportional to the temperature change of the intake air. Using the voltage output from the MAFT 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 MAFT 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 MAFT sensor fails the PCM implements the default strategy based on engine speed.
In the event of a MAFT 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 temperature sensor of the MAFT sensor fails the PCM uses a default intake air temperature of -5°C (23°F).
In the event of a temperature sensor failure any of the following symptoms may be observed:
- Over fueling, resulting black smoke emitting from the exhaust
- Idle speed control inoperative.