Air Flow Meter/Sensor: Description and Operation

Air Flow Meter:

PURPOSE AND LOCATION
The air flow meter consisting of air flow sensor and body is installed between air cleaner and AFM outlet hose. It detects the amount of air drawn into the engine and sends that information to ECM as a voltage signal. ECM uses the signal as one of the signals to control fuel injectors.

Hot Wire Operation Theory:

OPERATION
When a hot wire energized with a constant current (resistance wire heated by a current to a high temperature) is placed in the air flow, its heat is absorbed by the air flow and temperature rise of the hot wire reduces (heat is conducted to air, which moves and absorbs heat). The less air flows (is drawn), the less heat is dissipated (absorbed) from the hot wire and the more air, the more heat dissipation (absorption) from the hot wire. Therefore, if the temperature of the hot wire is maintained constant by regulating the amount of electric current flowing to the hot wire, the amount of air flow (intake air) can be measured by the amount of flowing current, for the amount of current corresponds to that of the intake air.

Air Flow Meter Circuit:

The air flow sensor uses the above described principle and has a circuit as shown. The hot wire temperature is detected by the circuit in which the hot wire and reference resistor are connected. Also, as the hot wire temperature is affected by the air temperature, the intake air temperature is detected by the circuit in which the cold wire and reference resistor are connected so as to compensate the changes in hot wire temperature. And by putting these two circuits in the bridge structure, the current flowing to the hot wire is regulated to corresponds to the amount of the intake air and thus the hot wire temperature is kept constant.

Then the current flowing to the hot wire, as a voltage signal, is transmitted to ECM. As it corresponds to the amount of the intake air as described above, the voltage rises higher as the intake air increases.