T
S
The IAT sensor is technically known as a thermistor, which is a two-wire, temperature sensitive variable resistor. There are two types of thermistors including, those with a positive temperature coefficient (PTC-type) and those with a negative temperature coefficient (NTC-type). With a PTC thermistor, resistance increases as air temperature rises.
In contrast, the resistance of an NTC thermistor drops as the temperature of the air becomes hotter. PTC thermistors were used on some of the first computer controlled engines, such as the fuel-injected V8 in the
1975 Cadillac Seville. By the early 1980s, virtually all manufacturers began using NTC thermistors for temperature detection.
Depending on the engine, the IAT sensor will be mounted on the intake manifold, the air cleaner, or the intake air duct.
INTAKE AIR TEMPERATURE (IAT)
IAT OPERATION
The PCM sends a 5-volt signal to the IAT sensor through a current limiting resistor.
When air temperature is low, sensor resistance is high. Under this condition, IAT signal voltage remains close to 5 volts, since there is little voltage drop across the resistor. As air temperature rises, the resistance of the
IAT sensor steadily drops. This results in a corresponding reduction in signal voltage.
The accompanying chart shows the relationship between intake air temperature, sensor resistance, and signal voltage. The PCM uses the IAT signal primarily for adjusting ignition timing.
CPU
The IAT sensor is a thermistor used to detect air temperature. As the temperature of the incoming air increases, sensor resistance decreases. This causes a voltage drop across an internal resistor, which the PCM processes as the IAT signal.
IAT OPERATION
140
122
104
86
212
194
176
158
68
50
-40
Sensor Resistance
(ohms)
667
973
1459
2238
177
241
332
467
3520
5670
100,700
Signal Voltage
2.00
2.47
2.97
3.46
.75
.97
1.25
1.59
3.89
4.25
4.95
As the temperature of the incoming air increases, sensor resistance drops. This results in a corresponding reduction in IAT signal voltage.
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