Pontiac FIREBIRD 1972 Service Manual page 215

1972 PONTIAC SERVICE MANUAL
,
ing, depending upon the amount of current flow from the
P'
amplifier. The transducer plunger is spring loaded so that
with no current flow through the windings, the plunger is
pulled out of the windings. When current flows through
the windings, the plunger is pulled into the winding, caus-
ing the transducer vacuum valve to have a slight leak to
outside air. This results in a lower vacuum level from the
regulated vacuum port of the transducer (See Fig. 1B-28).
If a decrease in current flow from the DC amplifier occurs,
the plunger moves out of the winding, opening the trans-
ducer vacuum valve and resulting in more vacuum from
the regulated vacuum port of the transducer.
The vacuum output from the transducer is fed to the
vacuum checking relay. When engine vacuum is applied
to the checking relay, the relay opens (Fig. 1B-29) allow-
ing the regulated vacuum output from the transducer to
be fed directly to the vacuum motor, positioning the mo-
tor. If engine vacuum to the checking relay is not present,
the relay closes (Fig. 1B-30), sealing the vacuum applied
to the vacuum motor. This locks the vacuum motor in
place until engine vacuum is again applied to the vacuum
relay.
The vacuum checking relay also has another section
which checks the input vacuum to the door diaphragms
whenever engine vacuum is not present. This holds the
doors in place until engine vacuum is again present.
The vacuum motor mechanism is positioned as the result
of the vacuum from the transducer. The vacuum motor
mechanism is directly connected to electrical wiper con-
tacts that control the various blower speeds.
The mechanism is also connected to a rotary vacuum
valve in the programmer and to an output shaft.
The rotary vacuum valve channels vacuum to various
vacuum diaphragms and to the rotary vacuum valve on
FROM TRANSDUCER
/
TO VACUUM MOTOR
,
ENGINE VACUUM INPUT
CHECKED VACUUM OUTPUT
Fig. 16-29 Checking Relay
-
Open
I
FROM TRANSDUCER
/
/
r
TO VACUUM MOTOR
ENGINE VACUUM INPUT
CHECKED VACUUM OUTPUT
Fig. 16-30 Checking Relay
-
Closed
the control head. The output shaft controls the position of
the air-mix door (temperature door) controlling the tem-
perature of the air which is distributed into the passenger
compartment.
As the vacuum motor in the programmer moves, a gear
meshed with the mechanism rotates the amplifier feed-
back potentiometer. This potentiometer indicates when
the vacuum motor has reached the proper position and
cancels out the change in resistance of the sensor string or
the temperature dial. This signals the amplifier to stop the
movement of the vacuum motor.
Whenever an appreciable amount of variation in resist-
ance occurs in the sensor string or the temperature dial,
the vacuum motor moves to supply either warmer or
cooler conditioned air to counteract the resistance change.
AMPLIFIER THEORY OF OPERATION
The purpose of the three-stage amplifier used in the Auto-
matic Temperature Control System is to amplify the DC
signals that are created due to resistance changes in either
the temperature dial, the in-car sensor or the ambient
sensor. The feedback potentiometer (Fig. 1B-3 I), temper-
ature dial, in-car sensor, ambient sensor and R12 form a
voltage divider circuit that results in a voltage at the base
of Q1. Also at Q1 in the emitter circuit, R13, R15 and D l
form a voltage divider resulting in a fixed voltage on the
emitter of Q1. During stable operation of the ATC system,
the voltage difference between the base and the emitter
remains 'constant. This causes Q1 to conduct a constant
amount of current (electron flow).
This current flow in Q1 actually flows from ground,
through the E-B diode of Q3, through the E-B diode of Q2
and through Q1. This current flow forward biases 4 2 and
43, the Darlington amplifier, and causes it to conduct.