Circuit Description

Intake air temperature sensor is built into the mass airflow meter and senses the intake air temperature. A thermistor built in the sensor changes the resistance value according to the intake air temperature. The lower the intake air temperature becomes, the greater the thermistor resistance value, and the higher the intake air temperature becomes, the lower the thermistor resistance value. See Fig 1 and Fig 2 . The intake air temperature sensor is connected to the ECM. The 5 volt power source voltage in the ECM is applied to the intake air temperature sensor from terminal THA via resistor "R". Resistor "R" and the intake air temperature sensor are connected in series. When the resistance value of the intake air temperature changes in accordance with changes in the intake air temperature, the potential at terminal THA also changes. See INTAKE AIR TEMPERATURE SENSOR RESISTANCE  table. Based on this signal, the ECM increases the fuel injection volume to improve driveability during cold engine operation. Possible causes are:

• IAT sensor circuit is open or shorted.
• Defective IAT sensor (built into MAF sensor).
• Defective ECM.

Fig 1: Identifying Intake Air Temperature Resistance Values

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Fig 2: Identifying Intake Air Temperature Sensor Circuits

Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002