Blower Motors; Potentiometers; Lower Feedback Potentiometer - Aston Martin DB7 Workshop Manual

Air Conditioning
Lower Feedback Potentiomenter
"3^^?
Blower Motors
Potentiometers
The air conditioning system has two blower motors
(Fig. 1 -7) that operate together to maintain the flow
of air into the car at the desired level. The motors are
supplied from an ignition switched supply through
transistorised control circuits fitted in the motor
outlets. The circuits steplessly vary the speed of the
motors at 1 and 2 selections of the mode switch and
operate the motors to high speed when 3 is selected.
The electronic components are assembled on a
heat sink and include a power transistor (Fig. 1 -9)
and feedback diode (Fig. 1-8). The switches are
supplied and controlled by the ECM.
When the mode switch is set to 3, the high speed
relay (Fig. 1-10) is energised from pin 16 of the
ECM, opening a path to earth-ground, and allowing
full battery voltage to be applied to the motor. At 1
and 2, the motor is supplied with a continuously
variable voltage by the power transistor and the
earth-ground return is made via the ECM. The
feedback diode enables the ECM to sense the
voltage at the negative terminal of the blower
motor.
Lower Feedback Potentiometer
The lower feedback potentiometer determines the
position of the lower blend flap in the air
conditioning unit and feeds this information to the
ECM. The ECM is thereby able to command the
lower flap servo motor to move the flap to a new
position and maintain the temperature of the air to
the feet and rear outlets at the desired level.
V 1
• 2
V 3
Figure 1.
Key to Fig. 1
1. Output signal from Pin 16 ECM
2. Power feed
3. Blower feedback
Left hand Pin 22, Right hand 33
4. Blower output Left Pin 3 1 , Right 32
5. Control switch earth-ground
6. ECM earth-ground Pin 45
7. Blower motor
8. Feedback diode
9. Power transistor
10. High speed relay
Figure 1.
1.
2.
3.
+5 volts from ECM Pin 43
Feedback signal to ECM Pin 29
Earth-Ground
The potentiometer is supplied with +5V from pin
43 of the ECM and returns its feedback signal via
pin 29. The feedback signal is 10OmV (COLD AIR)
to 1.2 V (HOT A! R). The potentiometer also provides
a single feedback signal of 2.9V when the blend
flap is in DEFROST. In this position, the feet and
rear outlets are closed and all air is directed to the
screen.
8-26 May 1996