The IAT sensor or air-temperature
sensor. Yes it is in almost every car. The IAT sensor is a cousin of
the ECT (coolant sensor). It is a secondary input to the ECM. It
however, could cause loss of fuel efficiency as well as the CEL
(check engine light) ON.
Theory of Operation
The intake air temperature sensor (IAT), also known
as the air charge sensor( if screwed to the intake manifold runner),
like the ECT is also a thermistor device. It measures the air
temperature of the incoming air, and also like the ECT, it has a
negative temperature coefficient. This means that as temperature
increases its internal resistance decreases and viseversa. The IAT
is a 2-wire sensor. One of the wires is an ECM supplied sensor
ground and the other a reference voltage (usually 5.00 volts) that
is also supplied by the ECM through a dropping resistor inside the
ECM. As the air temperature changes, the IAT internal resistance
also changes. This action also changes the voltage drop across the
IAT sensor’s thermistor.
The ECM uses the IAT signal to adjust the air-fuel mixture
in accordance with air density. Air density changes with temperature. This
means that a running engine on a cold day sucks in more air than on a hot
day. Although the IAT measures air temperature, it is really telling the ECM
how dense the air is. The ECM also uses this signal to modify spark advance,
acceleration enrichment and determines when to enable EGR operation.
Conditions that Affect Operation
The air temperature sensor is usually placed in the air
duct, but in older vehicles it was also screwed on the intake manifold. The
later was also called “Air Charge Sensors” and would get severely
contaminated with carbon from fuel and oil buildup. Anything that prevents
the IAT sensor from accurately reading the temperature of the passing air
would send the wrong signal to the ECM. A rich mixture is usually the result
of a contaminated or coated IAT, since the ECM would tend to react as if the
temperature is colder than what it really is. The IAT sensor can be removed
and cleaned with carburetor cleaner, so long as it is not damaged. When this
sensor is placed in the air cleaner, as in most new vehicles, it tends to
last much longer. This placement shields the IAT sensor from the severe heat
and carbon buildup inside the engine. Some of the import makes place it
inside the VAF or MAF sensor, which provides a great deal of protection.
NOTE: The ECM, as with the
ECT sensor, may also substitute the IAT signal value in case of a fault.
When reading the scanner’s serial data, be aware of any signal substitution.
An IAT scan reading that does not change or stays around 80 to 120 º F could
be a substituted value.
NOTE: On late 70’s injected
Cadillac Sevilles, the ECM will have a hard time controlling fuel enrichment
upon acceleration if the IAT sensor is defective. These systems will exhibit
severe hesitation, since they depend greatly on this signal for fuel
enrichment.
Component Testing
The diagnostics testing routine for the IAT is similar to the ECT. Just
remember that the IAT senses air temperature instead of coolant temperature.
