Pontiac FIREBIRD 1972 Service Manual page 217

1B-30 1972 PONTIAC SERVICE MANUAL
This results in current flow through the transducer. This
constant current through the transducer produces a con-
stant vacuum output from the transducer.
If the resistance of a sensor increases as the result of a
temperature decrease or if the temperature setting on the
temperature dial is increased, a larger voltage will be pre-
sent at the base of Q1. This reduces the current flow
through Q 1, which in turn reduces the conduction of the
Darlington amplifier through the transducer. Low trans-
ducer current produces high vacuum at the transducer
vacuum output. The vacuum motor moves in the direction
of "more heat". The feedback potentiometer is mechani-
cally connected to the vacuum motor mechanism. As the
vacuum motor moves to the "increased heat" position, the
feedback "pot" reduces in resistance. This movement
yields a cancelling effect to the increased resistance which
caused the movement. When the feedback "pot" com-
pletely offsets the increase, the voltage at the base of Q1
is the same as earlier described in the stable operation and
the movement stops.
If the temperature at a sensor increases or the temperature
dial is moved to a lower temperature setting, the voltage
at the base of Q1 will decrease. More current will now
conduct through Q1, 4 2 and 43. The increased current
flow through Q1 causes 4 2 and 4 3 to conduct more cur-
rent through the transducer. The vacuum at the trans-
ducer output will now decrease, causing the vacuum
motor to move in the direction of more cooling. As the
vacuum motor moves, the feedback "pot" increases in
resistance, offsetting the original resistance decrease. The
amplifier has now stabilized again and the movement
stops.
The .1 Mfd. capacitor at the base of Q1 filters out any
high-frequency AC signals from entering the DC am-
plifier. AC signals result in erratic operation of the ATC
system. The 2 Mfd. capacitor controls the reaction time of
the amplifier and helps to stabilize its operation. This
eliminates any oscillations in the system operation. R25 is
a feedback resistor that reduces the gain of the DC am-
plifier and results in more stable operation of the system.
Diode D l is physically located under the Darlington am-
plifier. When the Darlington heats up, it tries to conduct
more current. D l also heats up and reduces in resistance.
This lowers the emitter voltage of Q1 and reduces its
conduction. This results in lower conduction of the Dar-
lington amplifier, thereby bringing it back to its original
conduction level.
Diode D2 is a spike suppression diode. This prevents any
voltage spikes from the input supply line from entering the
amplifier.
ELECTRICAL SYSTEM OPERATION
OFF Position
When the control head selector lever is in the OFF posi-
tion*(~&. 1B-32), the system is locked in "LO blower"
operation when the ignition switch is in the RUN position
and the engine thermal switch or the in-car temperature
switch are closed. The engine thermal switch closes whe-
never the engine water temperature is above 120°F. The
in-car temperature switch closes whenever the in-car tem-
perature is above 75" F.
With ignition switch in the RUN position, power is sup-
plied to the programmer allowing it to operate. The in-car
sensor, the ambient sensor and the temperature dial set-
ting on the control head cause the programmer to move
the air-mix door. The proper air mixture is then dis-
tributed from the heater outlets maintaining the in-car
temperature at the comfort level selected on the tempera-
ture dial (the compressor is not running in the OFF posi-
tion and comfort level is limited to outside air when dial
is set at the minimum setting).
VENT Position
With the selector lever in the VENT position (Fig. 1B-33),
the LO Relay will be closed and the system will come on
immediately with the fan locked on "Lo blower" speed.
The vacuum motor in the programmer is in the A/C
position since the in-car sensor and the ambient sensor are
shorted out by the control head switch. The ambient
switch is closed when the ambient temperature is above
32" F. However, the compressor will not run since no
voltage is applied to it through the control head switch.
The Auto Relay and Hi Relay are always open in the
VENT position.
LO Position
With the selector lever in the LO position (Fig. 1B-34), the
system will come on when the in-car temperature is above
75" F., which closed the in-car temperature switch or
when the engine water temperature reaches 120" F. The
Lo Relay is then closed, allowing the blower to be locked
on "Lo blower" speed. The Auto Relay and Hi Relay
remain open. When the ambient temperature is above 32"
F., the ambient switch is closed, supplying voltage to ener-
gize the compressor clutch via the control head switch.
AUTO Position
With the selector lever in the AUTO position (Fig. 1B-35),
the system will come on when the in-car temperature is
above 75" F., which closes the in-car temperature switch
or when the engine water temperature reaches 120" F. The
Lo Relay and the Auto Relay are both closed and the
position of the blower wiper contacts in the programmer
determines the speed at which the blower operates. When
the ambient temperature is above 32" F., the compressor
clutch is energized via the ambient switch and the control
head switch. The Hi Relay always remains open. If the
system is calling for maximum cooling, the system will
operate in the recirculation mode; however, there is no
change in the electrical function when this mode is
achieved.