Relays And Switches - Pontiac Fiero 1988 Service Manual

1B-4 A!R CONDITIONING
Complete wiring diagrams and diagnosis for the
A/C Electrical System are in Section 8A. Section 8A
also contains additional diagnostic information
regarding air flows and vacuum logic.
Vacuum Lines
Vacuum lines are molded to a connector which
is attached to a vacuum control switch on the control
head assembly.
In case of leakage or hose collapse, it will not be
necessary to replace the entire harness assembly.
Replacement can be made by cutting the hose and
inserting a plastic connector. If an entire hose must be
replaced, cut all hoses off at the connector and then
attach hoses directly to the control head vacuum
switch. (NOTE: The Fiero uses an electric motor to
control mode selection. Therefore, it will not have a
vacuum harness.)
Vacuum Tank
During heavy acceleration, the vacuum supply
from the carburetor drops. A check valve in the vac-
uum tank maintains vacuum so that, under load condi-
tions, vacuum will be available for continuous use.
R E L A Y S AND SWITCHES
High-Pressure Compressor Cut-Off Switch
The high-side, high-pressure cut-off switch in
the rear head of the compressor is a protective device
intended to prevent excessive compressor head pres-
sures and reduce the chance of refrigerant escape
through a safety relief valve. Normally closed, this
switch will open the circuit at a high-side pressure of
approximately 2700 kPa (430 psi ± 20 psi) and
reclose the circuit at approximately 1379 kPa (200 psi
± 50 psi).
Low-Pressure Cut-Off Switch
Compressor protection is provided on some cars
by a low-pressure cut-off switch which will open in
the event of a low-charge condition. This switch can
be located in the liquid line or in the rear head of the
compressor. This switch will also keep the compres-
sor from running during cold weather.
Pressure Cycling Switch
The refrigeration cycle (on and off operation of
the compressor) is controlled by a switch which
senses the low-side pressure as an indicator of evapo-
rator temperature. The pressure cycling switch is the
freeze protection device in the system and senses
refrigerant pressure on the suction side of the system.
This switch is located on a standard Schrader-type
valve low-side fitting. This switch also provides com-
pressor cut-off during cold weather.
Additional compressor protection results from
the operating characteristics of the low-side pressure
cycling system. If a massive discharge occurs or the
orifice tube becomes plugged, low-side pressures
could be insufficient to close the contacts of the pres-
sure switch. In the event of a low charge, insufficient
cooling accompanied by rapid compressor clutch
cycling will be noticed at high air temperatures.
If replacement of the pressure cycling switch is
necessary, it is important to note that this may be done
without removing the refrigerant charge. A Schrader-
type valve is located in the pressure switch fitting.
During replacement of the pressure switch, a new
oiled O-ring must be installed and the switch assem-
bled to the specified torque of 6-13 N*m (5-10 lb. ft.).
Power Steering Gut-Off, o r Anticipate
Switch
Engine idle quality on some cars is maintained
by cutting off the compressor (switch normally
closed) when high power steering loads are imposed.
On other cars the switch (normally open) provided a
signal to the ECM to allow engine control systems to
compensate for high-power steering loads.
W i d e - O p e n T h r o t t l e
(WOT)
Compressor
Cut-Out Switch
A switch located on the throttle controls of some
carburetor equipped cars opens the circuit to the com-
pressor clutch during full throttle acceleration. The
switch activates a relay that controls the compressor
clutch.
During full throttle acceleration on cars
equipped with TBI or EFI, the TPS sends a signal to
the ECM, thereby controlling the compressor clutch.
Air Conditioning Time Delay Relay
This relay on some cars controls the current to
the entire air conditioning system and provides a short
delay of air conditioning operation upon start-up.
Constant Run Relay
Engine idle quality on some cars is maintained
by a "constant run" system (constant run relay) that
eliminates compressor cycling during engine idle for a
predetermined time after the vehicle has come to rest
from road speed. If the idle period continues for an
extended time, the A/C system may return to a con-
ventional C.C.O.T. mode for a short time to prevent
system freeze-up. The A/C control relay and constant
run relays are both controlled by the Electronic Con-
trol Module (ECM) which determines operating con-
ditions by evaluating input from the distributor
(engine speed), vehicle speed sensor, air sensor and
A/C compressor "on" signal.