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Computer Sensor Operation, Diagnosis, and Service 599<br />
TompEralure is decrgasing,<br />
caosing lhg r€sistancs lo incraase<br />
Temperature sensors arg normally<br />
Negative T€mp6rature Coef{icient<br />
thermislors fNTC)<br />
Tamporaturo sansor roadings ar€ typically<br />
mado ovor a long poriod ol lims<br />
Figure <strong>26</strong>-5 A typical ECT sensor being tested using a digital storage oscilloscope. The<br />
illustration shows the voltage of the sensor after the engine was stopped. As the<br />
resistance of the sensor increased, the voltage decreased. (Courtesy of Fluke<br />
Corporation)<br />
ffi INTAKE AIR<br />
TEMPERATURE SENSOR<br />
The intake air temperature (IAT) sensor is a negative<br />
temperature coefficient (NTC) thermistor that<br />
decreases in resistance as the temperature of the<br />
sensor increases. The IAT sensor can be located in<br />
one of the followins locations:<br />
{<br />
tt<br />
I<br />
I<br />
In the air cleaner housing<br />
In the air duct between the air filler and the<br />
throttle body as shown in Figure <strong>26</strong>-6<br />
Built into the mass air flow (MAF) or air flow<br />
sensor'<br />
Scren ed into the intake manifold where it senses<br />
the temperature of the air entering the cylinders<br />
$##Yffi: An IAT installed in the intake mAnifold is the<br />
most likely to suffer damage due to an engile backfire.<br />
which can often destroy the sensor.<br />
The purpose and function of the intake air temperature<br />
sensor is to provide the engine computer<br />
(PCM) the temperature of the air entering the engine,<br />
r Cold air-is more dense and contains more<br />
oxygen and therefore requires a richer mixture<br />
to achieve the proper air-fuel mixture. Air at 32'<br />
F (0' C) is I4Vo denser than air at 100" F (38" C).<br />
r Hot air-is less dense and contains less oxygen<br />
and therefore requires a leaner mixture to<br />
achieve the proper air-fuel mixture.<br />
The IAT sensor is a low-authority sensor and is<br />
used b1- the computer to modify the amount of fuel<br />
Figure <strong>26</strong>-6 The IAT sensor on this General Motors<br />
3800 V-6 engine is in the air passage duct between the air<br />
cleaner housing and the throttle plate.<br />
and ignition timing as determined by the engine<br />
coolant temperature sensor.<br />
Engine temperature is most accurately determined<br />
by looking at the engine coolant temperature<br />
(ECT) sensor. In certain conditions, the IAT has an<br />
effect on performance and driveability. One such
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