126 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
temperature sensors work like coolant temperature sensors,
which are discussed later. In some cases, the coolant and
air-temperature sensors are identical.
Intake-Air Temperature Sensors
When the air entering the engine is warm, less fuel is
needed to operate the engine. The intake-air temperature
sensor (IAT) monitors the temperature of the incoming air,
Figure 7-10. An IAT’s resistance changes in response to air
temperature. On some vehicles, the sensor is part of the
mass airfl ow sensor. Intake-air temperature sensors are also
known as manifold air temperature (MAT) sensors.
Barometric Pressure Sensors
Some materials change resistance as the pressure
placed on the material changes. As a result, the strength of
a voltage signal passing through the material varies as the
pressures placed on the material changes. Some of these
materials are piezoelectric crystals.
Air pressure is different at sea level than it is in the
mountains. A vehicle adjusted to operate normally at sea
level will perform differently at higher elevations with-
out readjustments. Barometric pressure sensors measure
atmospheric pressure. The sensors send a voltage signal
to the ECM. In turn, the ECM compares the input to that
from the manifold vacuum sensor and adjusts the air-fuel
ratio, spark timing, and other outputs to maintain good
performance.
Engine Sensors
Engine sensors measure conditions in or on the engine.
Some of the engine conditions measured include tempera-
ture, speed, manifold vacuum, and airfl ow.
Manifold Absolute Pressure Sensors
Almost all engines have a manifold absolute pressure
(MAP) sensor. Manifold absolute pressure refers to the dif-
ference between the pressure in the intake manifold and
the outside air when the engine is running.
MAP sensors use a pressure-sensitive material and
operate in the same way as a barometric pressure sen-
sor. The ECM uses the MAP sensor input in combination
with other sensor inputs to determine the engine load.
The ECM then determines the best ignition timing and
air-fuel ratio for the situation. The manifold vacuum and
barometric pressure sensors are usually installed in the
same housing. The schematic for a MAP sensor is shown
in Figure 7-11.
Figure 7-9. A transducer is simply a wire coil with a metal
plunger. Movement of the metal plunger causes fl uctuations in
the coil’s magnetic fi eld. This results in a voltage change that
can be read by the ECM.
Transducer
housing
Return
spring
Coil
Plunger
Ground
From ECM
+
Figure 7-10. The intake air temperature (IAT) sensor allows the
ECM to compensate for air temperature when determining the
correct air-fuel ratio.
IAT Sensor
ECM
To TPS
IAT signal
Reference
Voltage
Sensor ground
Figure 7-11. Manifold absolute pressure (MAP) sensors are
used to determine the difference in air pressure between the
manifold and outside air.
To coolant
sensor
To TPS
ECM
MAP sensor
Sensor ground
MAP sensor signal
Reference
voltage
Manifold
absolute
pressure
(vacuum)