Air Charge Temperature Sensor


These thermometer-like devices convert temperature into electric signals to be interpreted by on-board electronic systems.



Used in on-board electronic systems to measure the following:

  • Air charge temperature: In electronic injection and electronic ignition systems, they measure the temperature of the ingested air.
  • Indoor and outdoor room temperature: In electronic climate systems, they measure the air temperature.
  • Battery temperature: In integrated alternator control systems, they measure the temperature of the battery.


Operation Principle

The main components of temperature sensors for automotive systems are thermistors – resistors of the NTC type, which stands for negative temperature coefficient. These sensors contain a capsule where the NTC element is assembled (Fig. 1). 


As shown below (Fig. 2), the main feature of this thermistor is an accentuated variation in its electric resistance in relation to the temperature it is subjected to.

temperature increase -> resistance decrease
temperature decrease -> resistance increase


The assembly of the sensor depends on the intended application. If used to measure the temperature of the engine, the NTC element is located inside a protection capsule, isolated from the coolant. As for sensors aimed at measuring air temperature (air cooling, indoor or outdoor air), the NTC element is exposed to the air flow.
Important: The air charge temperature sensor (ACT) can be associated to the manifold absolute pressure (MAP) sensor, forming a combined sensor that in some cases is identified by the MAT acronym. 



Air charge  temperature sensor: In the intake manifold (multi-point systems) or on the  cap of the throttle body  (mono-point systems).



The air charge temperature sensor is used to:

  • adjust the point of ignition.
  • calculate the mass of ingested air in speed density systems.


When It Does Not Work

  • Air temperature sensor: Detonation; irregular idle speed, overheating.



Important actions when changing the temperature sensor:

  • Avoid excessive tightening.
  • Bleed air out of the cooling system (remove air bubbles).



Three types of fault apply to these sensors:

  1. The sensor sends the wrong information, but within its range.
  2. The sensor sends the wrong information outside its range (the sensor is short or open circuited).
  3. The information is wrong (short or open circuit) for certain temperatures (intermittent fault).


In all cases, the diagnosis can be made with a OBD2 Scanner equipment or a voltmeter.
For case 1: Visualize operation parameters and make comparisons with actual engine temperature or cooling air temperature.
For case 2: Use the mode for reading stored faults.
For case 3: With the sensor connected and using a voltmeter, check for possible non discontinuities (tension peaks) in the sensor’s signal measurement, while the engine heats from room temperature to its regular working temperature. The sensor analysis (short or open circuit) requires an ohmmeter. To check the calibration, in addition to the ohmmeter, the characteristic curve and the calibration table supplied by the manufacturer are necessary.



  • Make sure the electronic temperature sensor is appropriate for each vehicle model.
  • Never perform maintenance while the cooling system is still hot, due to high risk of burns.
  • At any evidence of overheating, park the vehicle in a safe place and turn off the engine immediately, otherwise you may damage the engine.
  • Check the coolant level weekly, with a cold engine.
  • Always use the specified coolant for your vehicle at the proper rate.
  • Perform the temperature sensor preventive maintenance every 30,000 km.



All MTE-THOMSON products are guaranteed for 01 (one) year against faulty materials or manufacturing defects. The guarantee is limited to the replacement of the faulty part; it cannot be extended to cover defects caused by misuse, neglect, accident, or wear and tear. We cannot accept liability for consequential loss or damage which is claimed to have resulted from the use of one of our products. More information at:


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