1997 ASTON MARTIN DB7 air conditioning

'^^p
Air Conditioning
In Car Controls
Fan Speed Control Switch (Mode Switch)
SC On Mode
The system is not operational, a residual
signal to the ECM ensures that the blower
flaps are closed to prevent air entering the

system.

1.
Low Blower Mode
The fan speed is lowandtheairconditioning
function switch is fully operational. Pin 13
on the ECM is earthed through the switch
and a 12V signal is returned to Pin 44.

2.
Medium Blower Mode
The fan runs at medium speed and the
function switch is fully operational. Pin 14
of the ECM is earthed through the switch
and a 12V signal is returned to Pin 44.
3. High Blower Mode
The fan speed is high and the function
switch is fully operational. Pin 15 at the
ECM is earthed through the switch and a
12V signal is returned to Pin 44.
DEF Defrost Blower Mode
The fan speed is
high,
the function switch
and temperature switch are not operational,
front screen vents are fully open and lower
flaps are fully closed. Pin 27 at the ECM is
earthed through the switch and a 12V signal
is returned to Pin 44.
Temperature Sensors
The system has three temperature sensors:
• the ambient sensor (Fig. 5)
the evaporator sensor (Fig. 6)
Figure 5.
Figure 6.
• and the in-car aspirated sensor.
Each sensor containsa semiconductor whose output
or sensing voltage varies with temperature changes.
All three are supplied via pin 43 of the ECM. The
sensing voltages are fed back into the module via
the following pins (Fig. 7):
• ambient temperature sensor - pin 34
• in-car aspirated sensor - pin 4
• evaporator temperature sensor - pin 5
At 0°C the sensing voltage should be 2.732 volts.
This voltage rises or falls by 0.01 volts for every 1°C
change in temperature; for instance, if the
temperature rises 5°C above zero, the voltage rises
by 0.05 volts to 2.782 volts. Sensors have built in
potentiometers which are preset at the factory and
must not be adjusted.

H

Figure 7.
Key to Fig. 7

1.
Sensor

2.
Pin 43 control module
3. Sensing voltages:
Pin 4 In-car aspirated temp sensor
Pin 5 Evap temp sensor
Pin 34 Ambient temp sensor

4.
Earth-ground
May 1996 8-23

Air Conditioning
In Car Controls ^2?
Evaporator Sensor
The evaporator sensor allov^^s the ECM to monitor
the temperature ofthe refrigerant in the evaporator
core continuously. When the temperature falls
below 0°C the ECM de-energises the compressor's
electromagnetic clutch and prevents refrigerant
from flowing through the system. The clutch is re­
engaged when the temperature rises.
Motorised In-Car Aspirated Sensor
The motorised aspirator (Fig. 8), which is fitted to
the passenger side facia underscuttle panel,
incorporates a motor driven fan (Fig. 8-1) that
draws air continuously over the in car temperature
sensor (Fig. 8-2).
Figure 8.
Key

1.

2.

3.

4.

5.
to Fig. 8
Motor
Sensor
Fan
Connector SCAO07
Connector SAC030
The motor (Fig. 8-1) is supplied, independently of
the air conditioning
system,
from the ign ition switch
(position 2). Its operating voltage range is 13.5 to
14.2 volts. Maximum current is 120 mA. The sensor

(Fig.
8-2) has a temperature operating range of -
30°C to +85°C. It is fed with 5 volts from the ECM
(pin 43), while the sensing voltage
is
supplied to pin
4 ofthe ECM. At 0°C the sensing voltage is 2.732V
± 0.002V. The rate of change of sensing voltage is
0.01 V± 0.002V per 1°C.
LC/0
0^5
U
Figure 9.
Key to Fig. 9

1.
Ignition switched supply to motor

2.
+5V supply to sensor from pin 43 of ECM
3. Sensor voltage output to pin 10 of ECM

4.
Sensor earth-ground to pin 4 of ECM
5. Motor earth-ground
A. Motor
B. Sensor
Ambient Temperature Sensor
An ambient temperature sensor (Fig. 10-1) is fitted
in the plenum air intake to provide the ECM with
information on the temperature ofthe air entering
the air conditioning unit and so offset the in-car
temperature at extremes of ambient. The voltage
signal output from the sensor is proportional to the
temperature of the surrounding air. The sensor
temperature range is -30°C to 85°C. At 0°C the
output ofthe sensor is 2.732V ± 0.005V. The rate
of change is + 0.01 V ± 0.002V per
1
°C.
Figure 10.
Key to Fig. 10

1.
Ambient temperature sensor

2.
+5 volts from ECM Pin 43
3. Sensing signal to ECM Pin 34

4.
Earth-ground
8-24 May 1996

•=2?

Air Conditioning
Water Temperature Switch / Blower Motors
Water Temperature Switch
The water temperature switch (Fig. 1) is fitted to the
lower side of the heater matrix inlet pipe.
Figure 7.
It is connected to pin 21 of the ECM. The switch
contacts are open to prevent the blower motors
operating until the temperature of the water in the
heater matrix reaches 30°C. The water temperature
switch is overridden when cold air is demanded,
defrost mode is selected and fan speed 3
is
selected.
Figure 2.
Key to Fig. 2

1.
Earth-ground

2.
Output to Pin 21 ECM
May 1996 8-25

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
V3
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 31, 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

^?
Air Conditioning
Upper Feedback Potentiometer / Servo Motors
Upper Feedback Potentiometer
The upper feedback potentiometer determines the
position of the upper blend flap in the air
conditioning unit and feeds this information to the
ECM enabling it to command the upper flap servo
motor to move the flap to a new position and
maintain the desired temperature of the air at the

dashboard,
centre, screen and side demist vents.
Servo Motors
Lower Servo Motor
The lower blend flap assembly has two inlets and a
single outlet which are wholly or partially blocked
by the flap to control the temperature of air entering
the lower half of the vehicle.
V 1
-•2
V3 -•2

Figure
2.

1.

2.

3.
+5 volts from ECM Pin 43
Feedback signal to ECM Pin 30
Earth-Ground
The potentiometer is supplied with +5V from pin
43 of the ECM and returns its feedback signal via
pin 30. The feedback signal is
1
OOmV (COLD AIR)
to 1.9V (HOT AIR).
Figure L

1.
Energising voltage Lower Servo ECM Pin 37
(Upper ECM 40)

2.
Energising voltage Lower Servo ECM Pin 41
(Upper ECM 42)
A servo motor
(Fig.
1) drives the lower blend flap to
the desired position via a 1500:1 reduction gear
box. The motor is bidirectional and energised from
pins 37 and
41
of the
ECM.
The energising voltages
have the following values: LOW ± O.OV to 2.0V;
HIGH± 7.0V to 9.5V.
Upper Servo Motor
The upper servo motor (Fig. 1) drives the upper
blend flap to the desired position through a 1500:1
reduction gearbox. Like the lower servo motor it is
bi-directional and energised by the ECM (pins 40
and 42). The energising voltages are:
LOW + O.OV to 2.0V
HIGH + 7.0V to 9.5V.
May 1996 8-27

Air Conditioning
Vacuum System •^^
Vacuum System The flaps in the cabin air distribution vents and the
water valve in the pipeline from the engine coolant
system to the heater matrix are all operated by
vacuum actuators. The vacuum forthese
is
supplied
by four solenoids mounted in pairs behind the front
footwell outlets. Each solenoid and its associated
pipe work is identified by a colour:
Defrost
Auto Re-circulation
Water valve
Centre vent
Green
Blue
Red
Black.
The vacuum supply pipes to the re-circulation and
centre vent actuators are fitted with restrictors in
order to slow down the operation of the flaps and
avoid hunting. The re-circulation flaps can take up
to 30 seconds to move to a new position.
Vacuum is piped to the solenoids from the engine
manifold through a reservoir. The solenoids are
energised by signals from the ECM in response to
demand ,sensing and feedback signals.
-T^^^T 2
3
figure 7.
Key

1.

2.

3.

4.

5.
6.
7.
8.
9.

10.

11.

12.

13.

14.

to Fig. 1
Vacuum reservoir
Defrost solenoid
Defrost-demist actuator
Restrictors
Centre vent solenoid
Centre vent actuator
Defrost vacuum pipe
Centre vent vacuum pipe
Recirc. solenoid
Recirc actuator
Recirc. vacuum pipe
Water valve solenoid
Water valve actuator
Water valve vacuum pipe
Figure 2.
5
7
10
Key to Fig. 2

1.
Defrost (Green) solenoid

2.
+12V Defrost Input from ECM pin 12
3. Defrost output to ECM Pin 11

4.
Recirc (Blue) solenoid
5. +12V Recirc. input from ECM Pin 3
6. Water valve (Red) solenoid
7. +12V Water valve input from ECM Pin
1
7
8. Centre vent (Black) solenoid.
9. +12V Centre vent input from ECM Pin 18

10.
ECM earth-ground
8-28 May 1996

^?
Air Conditioning
Pressure-Temperature Graphs
Pressure-Temperature Graphs
To obtain Bar multiply the Ibf/in
^
by 0.068
To obtain kgf/cm^ multiply
Ibf/in^
by 0.07
High Side (Ibf/in^) - Ambient (°C)
350
i 250
200
150
•inn
15 18 21 24 27 30 32 35 38 40 43 46 49
C
Low Side (Ibf/in^) - Ambient (°C)
80
70
60
CM C
~ 50
n
40
30
20
10
May 1996 8-29

Air Conditioning
System Fault Diagnosis ^=2?
System Fault Diagnosis
Probable causes of faults can be found by comparing actual system pressures, registered on the manifold gauge set
or recovery-recharge-recycie station, and the pressure to temperature relationship graphs found on the previous

page.
The chart below shows the interpretation that may be made by this difference. The 'Normal' condition is that
which is relevant to the prevailing ambient and evaporator temperatures.
Note: If erratic or
unusual gauge movements
are experienced, check the equipment
against a
known manifold
gauge
set.

Low Side
Normal
Gauge
Normal to low
Low
Low
Low
Low
High
High
High Side Gauge
Normal
Normal
Low
Low
Low
Normal to high
Low
High
High
Fault Finding
High
Symptom
Discharge air initially cool
then warms up
As above
Discharge air slightly cool
Discharge air warm
Discharge air slightly cool or
frost build up at expansion
valve
Discharge air slightly cool
Compressor noisy
Discharge air warm and high
side pipes hot
Discharge air warm
Sweating or frost at evaporator
Diagnosis
Moisture in system
As above
HFC 134A charge low
HFC 134A charge very low
Expansion valve stuck closed
Restriction in High side of system
Defective reed valve
HFC 134A charge high or
condenser malfunction
Expansion valve stuck open
Caution:
The microprocessor is extremely sensitive
and should only
be tested
using
a
digital multimeter with no
less
than a 3.5
digit display and a
resistance
of no
less than
2 M
ohms.
The use
of any other form of multimeter will
damage the microprocessor

irreparably.
Note: Always allow time for the
servo
motors and blower motors to come to a
rest
before starting a
check.

The car should be in a workshop and the ambient
temperature
should
be
stable
e.g.
24''C
(75°F)
for at
least
30 minutes before
commencing the automatic
check.

At 24°C the
sensor
voltage is
2.972
V
± 2 mV.
The
rate of
change
is lOmVper
1
°C.

Unless
stated
otherwise,
all
checks
are carried out at the ECM
test plugs

Mode Switch: Off Ignition Switch: Aux 2
Signal Pin No.
Battery supply 1
Recirc input 9
Earth-ground 2
Earth-ground 6
Earth-ground 10
Earth-ground 38
Earth-ground 45
From mode switch 44
To mode switch 12
Voltage
n to 14v
0to2V
0 to 40mV
0 to 40mV
0 to 40mV
0 to 40mV
0 to 40mV
0 to 12V
10 to 13.3V
8-30 May 1996