Air Flow Meter/Sensor: Description and Operation

Mass Air Flow Sensor:

The mass air flow sensor, located in the intake tract downstream of the air cleaner, measures the volume of air flow into the engine intake. The sensor has a cast alloy body with an integral electronic module. Some of the air flows through a bypass channel containing two small wire coils: a sensing coil and a compensating coil. The sensing coil is electrically heated by the electronic module; the compensating coil remains unheated.

Intake air volume is determined by measuring the electrical current required to keep the temperature differential of the two coils constant. This measurement occurs in the circuitry of the sensor. The sensor then provides a voltage signal representing air flow to the ECM. The theoretical full range of the signal is 0-5 volts. Idle voltage is normally 1.10-1.40 volts and torque converter stall (full throttle; 1850-2150 rpm) voltage is normally 3.00-3.30.

An adjustable potentiometer is incorporated into the mass air flow sensor to provide for idle fuel trim adjustment during initial engine setup. This potentiometer operates independently of the air flow elements and provides a constant voltage signal to the ECM. The idle trim potentiometer has an adjustment range of approximately 0-1000 ohms, measured between pins 1 and 6 of the mass air flow sensor connector. The ECM applies a 5 volt reference to pin 6 and a ground to pin 1. With the mass air flow sensor connected, the reference measured at pin 6 must be less than 4 volts or DTC 11 will be flagged. Typically, the voltage is between 2 and 3 volts.