Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

318 Chapter Fifteen
coil. The switch closes when the pressure on
the low side of the refrigerant system rises to
a specified value, engaging the clutch. When
system pressure drops to a predetermined value,
the switch opens to shut off the compressor.
The switch operates to control evaporator core
pressure and prevent icing of the evaporator
cooling coils.
Air-conditioning systems may also use lowand
high-pressure switches as safety devices,
as follows:
• The low-pressure switch is closed during
normal compressor operation and opens
only when refrigerant is lost or ambient temperature
is below freezing.
• The high-pressure switch is normally
closed to permit compressor operation.
However, if system pressure becomes
excessive (generally 360–400 psi or
2,480–2,760 kPa), the switch acts as a relief
valve and opens to shut off the compressor.
Once pressure drops to a safe level, the
switch will close again and permit the compressor
to operate.
• A pressure relief valve on the compressor
high-pressure side may be used instead of a
high-pressure switch. Some systems have a
diode installed inside the compressor clutch
connector to suppress any voltage spikes
that might be produced by clutch circuit
interruption.
Other compressor clutch controls may include the
following:
• A power-steering pressure or cutout switch
to shut the compressor off whenever high
power-steering loads are encountered, as
during parking. The switch senses line pressure
and opens or closes the circuit to the
compressor clutch accordingly.
• A wide-open throttle (WOT) switch on the
throttle body or accelerator pedal to open
the circuit to the compressor clutch during
full acceleration.
• A pressure-sensing switch in the transmission
to override the WOT switch when the
transmission is in high gear.
Temperature Control
The basic electrical components of most airconditioning
systems have already been described.
As we have seen, they provide input to, and
protection for, the refrigeration system.
Figure 15-5. The electromagnetic clutch in this airconditioning
compressor prevents the compressor
from wasting energy.
Figure 15-6. Manual air conditioning system block
diagram. (GM Service and Parts Operations)