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Intake Air Temperature Sensors (IAT)
November 2012
IAT function
The Intake Air Temperature sensor (IAT) has been utilised
as an Engine Control Unit (ECU) input signal from the very
first EFI systems to the currently used systems, as a
requirement for calculating the Air Mass volume for the
incoming air charge. This is, to assist in determining the
correct engine fuel requirement to suit the operating air
temperature.
What a lot of hot air
It is important to maintain correct combustion mixtures
during the extremely hot winds of summer as well as the
freezing night winds of winter. The density of the air varies
with the air temperature and with this, the amount of
required fuel to ensure the correct mixture.
More now than in the past, the IAT plays a larger role in the
mixture control due to the greater importance placed on
controlling emission levels. That is, earlier vehicles with
faulty IAT sensor causing a malfunction light to illuminate,
with a fault code logged and little driveability problems to a
Limp Home Mode condition on some of the later vehicles to
ensure that the IAT circuit is rectified quickly.
Location, Location, Location
Before we go any further, let us identify the IAT sensor unit
as it may be integrated into various other Engine
Management components.
1. Stand alone IAT sensor
A two wire NTC (Negative Temp Coefficient) thermistor
device that produces a resistance change in accordance to
the exposed air temperature that it monitors. This may be
located in the intake air tube between the air filter box and
the throttle body.
Also may be threaded directly in the inlet manifold and is
sometimes called a MAT sensor (Manifold Temp Sensor)
2. Integrated in the MAP sensor (T-MAP sensor)
MAP sensors that are mounted directly on the inlet
manifold may or may not integrate the IAT sensor. The
MAP combination with the IAT may be identified by an
extra 4th pin at the connector. That is a standard MAP
sensor requires only 3 pins to function.
The 4th pin if fitted identifies the T-MAP sensor which
utilises the common MAP sensor earth to complete the IAT
circuit.
3. Integrated in the MAF (Mass Air Flow) sensor or
AMM (Air Mass Meter)
Multiple pin configurations may or may not indicate an
integrated IAT sensor. The minimum circuit requirement for
an Air Mass Sensor to function is 3 and units with this
number of terminal configurations, do not incorporate an
IAT sensor.
A greater number than 3 terminals on the AMM generally
indicates the presence of an IAT sensor but there are
exceptions and care must be taken when renewing these
units to ensure the correct one is fitted.
4. Other typical areas
In the Air Flow Meter housing (AFM) on earlier vehicles
located at the inlet of the air flow chamber and also on the
Karman Vortex AMM housing, located at the inlet of the air
flow chamber
Testing the IAT sensor
Generally, this unit would not be on the “to check” list
unless a fault code was logged. A typical scanner will
indicate the running intake air temperature at time of
testing. It is important to note that a higher than normal IAT
voltage would result in a richer mixture and would affect
both emissions and fuel consumption.
Alternative Test methods - on standalone units
Alternative Test methods -on AMM units
Alternative Test methods – on T-MAP sensors
Note: Heating the IAT sensor with a hot air gun may assist
in quicker diagnosis.
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