IAT/MAT SENSORS

Function

The terms IAT (intake air temperature) and MAT (manifold air temperature) are both used for the sensors whose main function is to advise the PCM of the temperature of the air entering the engine cylinders. Its resistance decreases as the surrounding air temperature increases, providing a signal to the PCM. The IAT sensor signal is used to adjust spark timing and to determine injector pulse width according to intake air density from the prepeogrammed PCM circuitry.

Air Temperatue Sensors

Several air temperature sensors are used in cars. They are described here.

Figure 1. Location IAT sensor 

1. An IAT (intake air temperature) sensor provides air temperature data for fuel strategy and spark advance modification in engine control systems. Three main designs are used.

·         A plastic NPTF (national pipe thread fine) can be threaded into plastic assemblies with brass inserts or pushed into rubber tubes using a hose clamp for retention.

·         A twist-lock that installs a keyholeshape opening on a flat surface of the air cleaner or intake ducting using an O-ring for sealing.

·         A brass theaded type using PTF-SAE short threads that can be threaded into metal intake manifolds, and plastic or rubber assemblies with theaded inserts. These sensors have an operating range between -40o to 150oC (302oF). Typical output ranges between 1K to 925K ohms, with a general output of 30K ohms. Each type of sensor operated on a NTC (negative temperature coefficient) concept.

2.      Ambient air temperature sensors are used to regulate the quantity of fresh are used to regulate the quantity of fresh air required to maintain the proper in-car temperature for heating or air conditioning equipment. The sensor consists of a disc thermistor whose resistance varies nonlinearly and is inversely proportional to the temperature. Its resistance and operating temperature is the same as that for the IAT sensor.

3.      A temperature manifold absolute pressure sensor is used in speed density systems (SDS) to measure the amount of air entering the engine. If consists of a absolute pressure sensor and an intake air temperature sensor (two sensors in one), and is designed to sense absolute pressure in the intake manifold. The sensor consists of a silicon diaphragm that deflects when pressure is applied. Signal amplification and conditioning circuitry are integrated in dual solid state circuits mounted to a hybrid thick-film ceramic circuit board. Designed to operate typically between 10 kPa tp 115 kPa (1.45 to 16.7 psi), its signal output will range between 0.5V to 4.5V.

IAT Location and Operation

            The actual location of the IAT will vary in different engines; however, it is typically located in the intake manifold piping/ducting or at the intake manifold plenum. Figure 2. shows the IAT location for a 1998 3.8L GMC V6.

            The intake MAT (manifold ait temperature) sensor is important because it updates the ECM as to the density of the air within the manifold (nonturbocharged and turbocharged engines). An example of a constructed as a thermistor (a combination of the words thermometer and resistor) and changes value based on temperature. Low intake manifold temperature produces a high resistance (usually about 100,000 ohms at -40°C/F; high temperature causes low resistance (approximately 70 ohms) at 130°C/266°F. The sensor receives a 5 volt input reference voltage from the engine ECM; its output voltage will vary based on the intake manifold air temperature. The ECM can calculate the air temperature from the sensor’s voltage return signal and adjust both the fuel and spark advance accordingly. Typically, this sensor has a fairly low resistance value. For example, on a 97 Chrysler Sebring, ohmmeter reading values will normally be between 9 and 11 kohms at a temperature of 25°C (77°F) and between 0.6 and 0.8 kohms at a temperature between 100°C (212°F).

Figure 2. Location of an IAT sensor and how its resistance value changes

MAT Sensor DTCs

Figure 3. Schematic of an IAT sensor circuit

            MAT (manifold air temperature) and IAT (intake air temperature) sensors measure the ambient or turbocharger-boost air temperature entering the engine. A 5 volt reference voltage from the PCM is applied to the sensor through a pull-up resistor. See a typical circuit Figure 3. Cold air will produce a high sensor resistance, resulting in the PCM monitoring a high signal voltage on the sensor signal circuit. Warm and hot air will cause the opposite effect-the sensor resistance will be much lower, and therefore the PCM sees a lower sensor voltage sigbal.

            The PCM monitoring circuit continuously monitors the MAT/IAT sensor for operating signals outside of the normal preprogrammed values. An intake air temperature in excess of 135°C (255°F) lasting for longer than 20 seconds at vehicle speeds in excess of 40 km/h (25 mph), for example, will set a DTC P0112 in PCM memory. In both cases, the MIL will illuminate during the second consecutive trip that the system has failed. In addition, the PCM will store the conditions present at this time in freeze frame and fail records data.

            On 1999 Ford Taurus/Sable vehicles equipped with electronically shifted transmissions, a DTC P0112 or DTC P0113 will create a symptom of incorrect EPC pressure, either high or low, which will result in harsh or soft shifts. Also, a DTC P0114, which is IAT related, indicates an out-of-range condition and indicates a higher- or lower-than-expected IAT temperature during KOEO (key-on-engine-off) and KOER (key-on-engine-running).

Taken From The Book : Automotive Electronics and Computer Systems (Robert N. Brady)