Chapter 7 Computer Control Fundamentals 133
Air Conditioning Compressor Relays and Cruise
Control
The ECM receives an on-off signal from the air condi-
tioner or cruise control from the air conditioner compressor
clutch relay or cruise control module. Some driver-operated,
on-off switches deliver a signal to the ECM to indicate that a
device or system is functioning. An example is the air con-
ditioner compressor relay, which energizes the compressor
clutch coil. The relay and clutch are, in some cases, wired
as an input connection to the ECM. When the relay ener-
gizes the compressor clutch, a low-voltage signal informs
the ECM that the air conditioning compressor is on.
Air Conditioning High- and Low-Pressure
Sensors
The primary function of a high-pressure switch in
an air conditioning system is to monitor system pressure
and shut off the compressor if pressure becomes too high,
Figure 7-27. The high-pressure switch allows the ECM to
monitor and compensate for high pressure. Many systems
also have a low-pressure sensor. This sensor notifies the
Figure 7-25. The shift sensor on a manual transmission/transaxle allows the ECM to monitor which gear the transmission is in. The
ECM uses this information to adjust engine output.
Electrical wires
Operating linkage
Slotted adjustment holes
Electrical switch
Mounting bracket
Transmission linkage
Figure 7-26. The power steering pressure switch tells the ECM
when the steering wheel is locked. The ECM can then increase
engine speed to compensate for the extra load.
High-pressure
hose
Pressure switch
Figure 7-27. Air conditioning pressure switches are monitored
by the ECM for system operation, overpressure, and low refrig-
erant level.
Refrigerant-
applied pressure
Dual-pressure
switch
Movable
contact
Pressure switch terminals