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Diagnose Sensor Prob.

Air temperature sensors go by a variety of names. Ford has Air Charge Temperature (ACT), Vane Air Temperature (VAT) and Manifold Charging Temperature (MCT) sensors, General Motors has Manifold Air Temperature (MAT) sensors, Chrysler has Charge Temperature Sensors (CTS) and Bosch has Air Temperature Sensors (ATS).

Though all these sensors are basically the same in design and function as a coolant sensor, air temperature sensors are used in vehicles equipped with fuel injection for one of two purposes. On some systems, the sensor is used to monitor the temperature of the air entering the engine while the engine is cold. The sensor is mounted in the intake manifold with the tip exposed to air. It gives a much faster response to temperature changes than the coolant sensor. If the air is cold enough, the sensor triggers the cold start injector to spray additional fuel into the manifold to richen the fuel mixture. In this way it acts something like an electronic choke while the engine is cold. On some applications, the air temperature sensor is also used to delay the opening of the EGR valve until the engine warms up. In other applications the sensor is used to continuously monitor the incoming air temperature so the engine computer can alter the fuel mixture to compensate for changes in air density. Cold air is denser than warm air, so when the sensor reads cold injector duration is increased to maintain a balanced fuel mixture. If the sensor reads hot, then the duration of the injectors is shortened to prevent the mixture from becoming too rich.

All air temperature sensors work like coolant sensors. Their electrical resistance changes in direct response to temperature. When the computer applies a control voltage to the sensor, the amount of resistance encountered determines the return signal voltage. When the sensor is cold, resistance is high and the return signal to the computer is high. As the temperature rises, resistance drops and the voltage output goes down.

An air temperature sensor can sometimes be damaged by backfiring in the intake manifold. Carbon and oil contamination can also affect the accuracy and responsiveness of the sensor as can old age. Driveability symptoms for the type that are used to trigger the cold start injector include stalling or rough idle when cold. A faulty sensor may also allow the EGR valve on some applications to open before the engine is warm, resulting in cold hesitation or detonation.

On applications where air temperature is monitored continuously to help balance the fuel mixture, a faulty sensor can contribute to cold stumbling and warm surging. Sometimes what appears to be a fuel mixture balance problem due to a faulty air temperature sensor is in fact due to something else, like a restricted catalytic converter! A severe exhaust restriction will reduce intake vacuum and airflow causing the sensor to read hotter than normal. Once the engine is warm, the air temperature sensor will usually read a few degrees cooler than the coolant sensor.

Trouble codes that may indicate an air temperature sensor problem are:
* General Motors: Codes 23 & 25
* Ford: Codes 24, 54 & 64
* Chrysler: Code 23

An air temperature sensor can be checked several ways. On GM applications, the easiest way is to tap into the onboard electronics with a scan tool. Plugging into the ALDL connector will allow you to get a direct temperature reading (in degrees Celsius) from the sensor. You should see a slight drop in the temperature reading when the throttle is opened while the engine is running and air rushes into the manifold. You can also compare the air temperature and coolant sensor readings to see if they're close on a warm engine.

The sensor's resistance can also be checked with an ohmmeter. Remove the sensor and check the resistance when cold. Then blow hot air at the tip with a blow drier (never use a propane torch!) and watch for a drop in resistance. No change means a new sensor is needed.

All you can do with a defective sensor is replace it (unless the tip is coated with carbon, in which case cleaning the tip in solvent may restore its function). Be careful not to overtighten the sensor when it is installed, and use sealer on the threads to prevent vacuum leaks.

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