Air Flow Meter/Sensor: Description and Operation

Sensor Schematic:

Sensor Cut-Away:

The mass air flow meter uses a platinum hot wire. the hot wire air flow meter consists of a platinum hot wire, thermistor and a control circuit installed in a plastic housing. the hot wire air flow meter works on the principle that the hot wire and thermistor located in the intake air bypass of the housing detect any changes in the intake air temp.
The hot wire is maintained at the set temp. by controlling the current flow through the hot wire. This current flow is ten measured as the output voltage of the air flow meter.
The circuit is constructed so that the platinum hot wire and thermistor provide a bridge circuit, with the power transistor controlled so that the potential of A and B remains equal to maintain the set temp.
The intake air temp. sensor is built into the air flow meter and senses the intake air temp.
A thermistor built in the sensor changes the resistance value according to the intake air temp.

Figure 1:

The lower the intake air temp., the greater the thermistor resistance value, and the higher the intake air temp., the lower the thermistor resistance value (See fig. 1).
The intake air temp. sensor is connected to the ECM (See below). The 5 V power source voltage in the ECM is applied to the intake air temp. sensor from the terminal THA via resistor R. That is, the resistor R and the intake air temp. sensor are connected in series. When the resistance value of the intake air temp. sensor changes in accordance with changes in the intake air temp., the potential at terminal THA also changes. Based on this signal, the ECM increases the fuel injection volume to improve driveability during cold engine operation. If the ECM detects the DTC "P0110", it operates the fail safe function in which the intake air temp. is assumed to be 20°C (68°F).