Optimize Systems with Air Temperature Sensors

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Purpose

Air temperature sensors are used on fuel injected vehicles to perform one of two functions:

• On certain vehicles, the air temperature sensor monitors the temperature of the air entering the engine during warm-up (while the engine is cold). The sensor is mounted in the intake manifold in a position such that the tip of the sensor is exposed to incoming air.
  
  If the temperature of the incoming air is sufficiently cold, the vehicle’s computer signals the cold start injector to inject additional fuel into the manifold, resulting in a “rich” air/fuel mixture.
  
  In such applications the sensor serves as an electronic choke while the engine is cold. The air temperature sensor responds more quickly to changes in operating temperature than the coolant sensor, and provides a more reliable and efficient means of regulating the air/fuel ratio during engine warm-up.
  
  The signal from the sensor may also be used by the vehicle’s computer to delay opening of the EGR valve until the engine reaches normal operating temperature.
  
• In other cases, the air temperature sensor continuously monitors incoming air temperature. In these applications, the vehicle’s computer uses temperature information from the sensor to adjust the air/fuel mixture to compensate for changes in air density (cold air is denser than warm air).
  
  When the temperature of the incoming air is sufficiently cold, the vehicle’s computer increases the injector duty cycle (the injector stays “on” longer) to maintain a balanced fuel mixture. If the incoming air temperature becomes sufficiently hot, the injector duty cycle is decreased to “lean out” the air/fuel mixture.

Depending on the vehicle make, model, and sensor application, air temperature sensors are identified by a variety of names, including:

Bosch

• Air Temperature Sensor (ATS)
  

Chrysler

• Charge Temperature Sensors (CTS)
  

Ford

• Air Charge Temperature (ACT) Sensor
  
• Vane Air Temperature (VAT) Sensor
  
• Manifold Charging Temperature (MCT) Sensor
  

General Motors

• Manifold Air Temperature (MAT) Sensor
  

Location

Air temperature sensors are typically threaded into the intake manifold or throttle body. 

On some vehicles, the air temperature sensor may be incorporated in the Mass Airflow (MAF) or Vane Airflow (VAF) sensors.

Operation

The air temperature sensor is essentially a temperature-sensitive variable resistor called a “thermistor.” The electrical resistance of the sensor changes in direct proportion to changes in air temperature. During normal vehicle operation, the vehicle’s computer applies a control voltage to the sensor, and monitors the return signal voltage. The amount of resistance applied by the sensor determines the value of the return signal voltage.

Most air temperature sensors are “negative coefficient” type sensors; the sensor’s resistance decreases as air temperature increases. When the temperature of the incoming air is cold the sensor’s resistance is high, and the return signal to the vehicle’s computer is high. As the temperature of the incoming air increases, sensor resistance becomes lower, and the voltage output decreases.

Drivability Symptoms

Air temperature sensors can be damaged by backfiring in the intake manifold. Additionally, carbon deposits and oil contamination, as well as normal wear and “aging” can affect the sensitivity, accuracy and responsiveness of the sensor.

A malfunctioning or defective air temperature sensor will generally result in an improper air/fuel mixture with the engine at cold and/or warm temperature. The following symptoms may indicate a damaged or defective air temperature sensor, or trouble in a related system:

Engine Cold

• Stalling, stumbling or rough idle
  
• Hesitation
  
• Audible pinging or knocking while accelerating or driving under loadEngine Warm
  
• Surging or racing
  

In some cases, symptoms generally associated with a fuel mixture balance problem due to a faulty air temperature sensor may be caused by a failure or malfunction in a related system or component. Be sure to confirm a faulty air temperature sensor as the cause before replacing the sensor.

Testing

Air temperature sensor operation is easily checked with an ohmmeter using the following procedure. BE SURE the sensor is cold before testing.

• Unplug the wiring harness from the air temperature sensor, and remove the sensor from the engine.
• Connect an ohmmeter to the sensor’s signal pins. Note the sensor’s resistance.
• Heat the sensor tip with a blow dryer or other low temperature heat source. DO NOT use a propane torch or other high temperature heat source. DO NOT overheat the sensor.
• Observe the ohmmeter and verify that the sensor’s resistance decreases as heat is applied.
• Remove the heat source and disconnect the ohmmeter from the sensor. Allow the sensor to cool.
• Apply thread sealer to the sensor threads to prevent vacuum leaks, then reinstall the sensor in the engine. DO NOT over-tighten the sensor when installing.
• Reconnect the wiring harness to the sensor’s connector.

Troubleshooting

If sensor resistance does not decrease when heat is applied, the sensor is defective.

Repair/Replacement

Excessive carbon deposits and oil contamination can affect sensor operation. If the sensor tip appears to be contaminated, clean carbon and oil deposits from the sensor using a mild solvent, then retest. BE SURE to rinse residue solvent from the sensor BEFORE applying heat.

The air temperature sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

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