How does the Inlet Air Temperature (IAT) Sensor function?

General

IAT sensors sense the air temperature on the inlet side of the engine. The voltage signal, produced by the sensor, is passed to the ECU. The ECU evaluates the signal and influences the fuel injection timing and ignition timing.

Function

Depending on the inlet temperature the resistor of the IAT changes. If the temperature rises up the sensor is reduces the voltage of the sensor. The ECU assesses the voltage value because it is in direct relation to the inlet air temperature. That means low temperature high voltage, high temperature low voltage.

Effects of failure

A faulty IAT can be noticed through:

  • engine warning light illumination
  • hard starting
  • less engine power
  • higher fuel consumption
  • fault code present

Causes of failure

  • mechanical damage
  • wire short-circuit/open circuit
  • internal short-circuit
  • sensor tip soiled

Diagnostics

Internal Resistance Check

For fault recognition consider the following system tests:

  • read out fault codes
  • electrical leads checked for correct fitting and contact

Use a Multimeter to check.

1. Find out the internal resistance. It is dependent on the temperature, cold engine high ohm, hot engine low ohm. According to the manufacturer:

Resistance of the Sensor Plug to ECU

25°C
2.0 – 5.0 Kohm
80°C
300 – 700ohm

Note the set point specification.

2. Wiring harness to the ECU plug, check for continuity and short-circuit to earth.

  • Measurement with a ohmmeter between sensor plug and removed ECU plug, measured value: < 1 Ohm (circuit diagram needed for pin definition)
  • Voltage Check

    Respective pin from the sensor plug checked for short-circuit to earth, ECU plug removed, measured value: >30 MOhm

3. Measurement with a voltmeter on the sensor plug, checking the supply voltage. ECU plug fitted, ignition on, measured value approximately 5 V. If the voltage is not enough check the operation voltage and the ground of the ECU(circuit diagram needed).

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1 Comment

Filed under Electronics, Inlet Air Temperature Sensor

One response to “How does the Inlet Air Temperature (IAT) Sensor function?

  1. Mike rothschild

    Thanks, my Ford e-150 became hard to start when warm, always started when it was cold. Changed the air temp sensor and have not had the problem since.

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