Air Flow Meter/Sensor: Description and Operation

Mass Airflow Sensor and Manifold Absolute Pressure Sensor

The MAF sensor contains a hot wire resistance element that forms part of a Wheatstone bridge. Air flowing around the hot-wire cools it, so altering the value of its resistance. The consequent change in the voltage dropped across the resistance is compared with the voltage dropped by the other resistance arms of the Wheatstone bridge to determine the airflow. The MAF sensor is continually monitored by OBD routines. A DTC is recorded if the input signal from the sensor to the ECM is outside pre-defined thresholds at the high or low end of the scale.

High/Low Input Failure and Ground Monitor
These are continuous monitors. The voltage from the sensor is compared to a failure threshold defined in the software. If the voltage is below the low threshold, then a timer starts to increment. Once this timer exceeds another threshold, then a failure flag is set and a DTC is stored. If the voltage is over the high threshold defined in the software, then a timer starts to increment. Once this timer exceeds a threshold, then a failure flag is set and a DTC is stored. For MAF sensor ground open monitoring, the voltage on the ground pin of the MAF sensor is monitored in the same way as described above.

Range/Performance Failure
The monitor operates continuously whilst the entry conditions are met. Every 0.128 seconds the airflow sensor monitor compares the actual airflow with an estimated airflow, which is calculated by a model. Similarly, every 0.032 seconds the MAP sensor monitor compares the actual MAP with an estimated pressure, which is calculated by a model. The models to calculate the estimated airflow and pressure have look-up tables that use engine speed, throttle angle and atmospheric pressure to derive base values and compensation values by which the estimated airflow and pressure are calculated.

Whether the MAF sensor and the MAP sensor are behaving normally is determined if the difference between the actual and estimate values are below a calibrated threshold for more than 5 seconds. Whether the MAF sensor and the MAP sensor are behaving abnormally, as failed components, is determined if the difference between the actual and estimated values is greater than a calibrated threshold for fifteen seconds continuously. The monitors have the ability to make a normal judgments followed by failed judgments or vice versa as the monitors run continuously whilst the entry conditions are met.