132 Auto Engine Performance and Driveability
Reed switches are placed near a magnet attached
to the moving part to be monitored. The magnet moving
across the reed switch causes the two reeds to move apart
and then spring back together. This starts and stops the cur-
rent flowing through the switch. The variation in current
can be read by the ECM. Reed switches are usually installed
on the speedometer cable where it enters the speedometer
head. These switches are used for cruise control as well as
to provide inputs to the ECM.
Brake and Transmission Switches
Brake light and neutral safety switches are on-off
switches, which are the simplest type of inputs to the ECM.
The switch can be either on to allow current flow or off to
prevent current flow. These switches tell the ECM whether
the brake is being applied or a particular transmission/trans-
axle gear is selected.
Most automatic transmission/transaxle sensors are
pressure switches, such as the one shown in Figure 7-24.
Most manual transmission sensors are mechanical switches
operated by the shift linkage, Figure 7-25. Additional
switches are sometimes installed on the transfer case of
four-wheel drive units.
Accessory Equipment Sensors
Accessory equipment sensors are used to monitor
such conditions as pressure in the power steering system
and whether the air conditioner or cruise control is on or
off. This allows the ECM to adjust idle speed, spark timing,
and other functions to compensate for the additional load
placed on the engine.
Voltage Regulator
The alternator voltage regulator acts as a sensor. It sends
an output voltage signal at a very low amperage directly to
the ECM. In some cases, the ECM reads the voltage directly
from the alternator output terminal to the battery. The ECM
can then adjust idle speed to compensate for low- or high-
voltage conditions.
Power Steering Pressure Switch
Some computer control systems have a pressure switch
installed on the high-pressure hose in the power steering sys-
tem, Figure 7-26. The switch is used to determine whether
the wheels are being turned to lock. This allows the ECM to
compensate by raising engine idle and deactivating the air
conditioning compressor. This switch operates in the same
manner as the pressure switches installed in the automatic
transmission.
Figure 7-22. The vehicle speed sensor (VSS) allows the ECM
to monitor vehicle speed. Speed sensors are also located on
each wheel or in the rear differential of many vehicles.
Transmission/transaxle
output speed signal
Powertrain
Control Module
Vehicle Speed
Sensor (VSS) Assembly
Figure 7-23. Reed switches are magnetically operated. They
are used to measure the rotating speed of a part. (Nissan)
Field plate
OPEN CLOSED
Magnetic lines
of force
Magnetic lines
of force
Reed
switch
Field plate
Figure 7-24. Pressure switches such as this one are installed on
the case or valve body of automatic transmissions/transaxles to
sense internal fluid pressure. When hydraulic pressure applies
a band or clutch, the fluid pressure also activates the pressure
switch. This signals the ECM that a shift has taken place.