Air Temp Sensor / On car

Air Temperature Sensor

The air temerature sensor (ATS) is also know as thermistor air (THa) and intake air temperature(IAT) sensor.This sensor is located in the ait cleaner box or sumwhere in the intake system. The air temp sensor helps the ecu calculate air density. Hot air is less dense than cold air. The ecu uses this information to do calculations for air fuel ratio mixes.

The IAT works exactly like the engine coolant temperature sensor. It is also a negative temperate coefficinet.When temperature increases resistances decreases and when temperature decreases resistace increases.

To do the experiment we did it like how we checked the ECT. As the IAT was made of plastic we didnt wana melt it by heating it up with a heater gun. And we also wouldnt have been able to know the temperature we heated it with. So we put it in water and heted it  over the stove. Temperatures were checked with a thermometer.At 20 degrees celsius the resistance was 2.9k. As we heated it up the resistance droped down to 800 ohms at around 50 degrees Celsius. This was within manufactures specifications and we could tell that the IAT was in good fine as the resistance gradually decreased when the temperature increased.  

Temperature (degrees celsius)	Resistance(ohms)
16	2.9k
20	2.5k
25	2.1k
30	1.7k
35	1.4k
40	1.1k
45	1k
50	800

On Car Intake Air Temperature

I found the IAT on the intake air clean box. I back probed the input wire  of the IAT and connected my multimeter to read DC voltage. Red probe to the input and the black probe to bappery negative post. Then i turned the igniton on.

I got a reading of 3.4v which is  more than the ECT voltage.

This shows that this is colder than the ECT sensor.

The IAT receives a 5v reference to the IAT thermistor. This is a NTC type thermistor which mean as temperature increases resistance will decrease. There is an internal resistor inside the ecu. When the air is hot, resistance decreases at the IAT so the voltage drop at the internal resistor is higher therefore less voltage goes to the IAT Input.The signal that the ecu looks at is voltage after the internal resistor as shown in the above wiring diagram. So when theres hot air signal sees a low voltage. When the air gets cold, resistance is high at the IAT and more voltage is needed to go through the thermistor so the signal sees a high voltage.

The IAT works with the MAP sensor to calculate the amount of air entering the engine. Cold air is more dense the hot air. So when the IAT senses cold air the ecu will need to open the injector longer to get the air fuel mixture right. If there is hot air entering the engine and the signal at the IAT is telling the ecu that theres cold air,the injectors will squirt more fuel making the air fuel mixture rich which will make the engine suffer from poor fuel economy and bad emmisions. If theres cold air getting in but the IAT is telling the ecu that theres hot air the mixture will be lean therefore the car will loose power.

Things that can cause an incorrect IAT signal to the ECU:

 - poor grounding at the ECU (connection corroded)

 - poor ECU grounding

 - Theres not a good supply voltage from the ECU

 - Open Circuit on the earth side or power side in the circuit

 - Bad thermistor