1997 ASTON MARTIN DB7 Workshop Manual

Air Conditioning
Electronic Control Module r^^?
Description
The ECM (Fig 1) controls the air conditioning
system so that selected temperaturesare maintained.
It cannot be repaired but input and output signals
can be measured, using the PDU, for diagnostic
purposes.
Input-Output Voltage Signals
The following voltage signal levels are input and
output from the ECM:
Digital input levels:
Low
High
Analogue input levels:
General
Blend Flap Potentiometer
Digital output levels
Relays On
Off
Vacuum manifold Pull-in
Drop-out
Analogue output levels
Blend Flap Servo Motors
Blower Motors
0.16 to 1.1V
3.70 to 4.50V
0 to 0.2885 V
0 to 5.0V
8.0V
1.0 to 5.0V
5.6V (typical)
3.5V (typical)
0.5 to 6.5V
0.5 to 2.5V
figure 1.
The ECM
is
turned on when the ignition
is
turned on
to auxiliary position 1. its operating range is 13.5-
14.2 volts.
The ECM power unit supplies +5 volts for the
temperature sensors, feedback potentiometers and
high speed relays.
An integrated circuit protects the ECM against
reversed polarity and voltage surges.
Caution: Take care when connecting test equipment. If
test pins are allowed to be shorted together, to
ground or to positive supplies the ECM will be
damaged.
8-20 May 1996

^n:m^^^

Air Conditioning
In Car Controls
In Car Controls
Temperature Demand Switch
In-cartemperatu
res are selected by
the temperature
demand switch (Fig. 1).
Face Level Differential Controller
The face level differential control alters the
temperature of the air distributed through the face
level vents. The switch is coupled to a 10 K ohm
potentiometersupplied with+5 volts
from
pin 43 of
the control module
(Fig.
3). Temperature, from the
face
vents,
is
decreased by moving the switch
anti­

clockwise and increased by moving it clockwise.
6.6 K
lOKQ
Linear
+ 5v
7 Temperature
Differential Signal
10CND
Figure 1.
The switch is coupled to
a
590 ohm potentiometer

(Fig.
2) supplied with +5 volts from pin 43 of the

ECM.
The output voltage
is
from zero to
2.885
volts
which represents a range of temperatures from 16
to 38°C. Rotation of the switch is restricted
mechanically to 180° of travel.

Figure
3.
590 Q Linear
1 GND
.2 Temperature Demand Signal
•3 +5v
410 Q

Figure
2.
May 1996 8-21

Air Conditioning
In Car Controls '^^
Air Conditioning Function Switch
The air conditioning function switch (Fig. 4) has
five positions: Economy (ECO), Air conditioning

(AC),
Manual (MAN), Re-circulation and Demist
(DEM). These settings have thefol lowing functions:
ECO (Economy) Inthissettingtheairconditioning
compressor is switched off. No cooling is
available but incoming air may be heated to
the selected temperature. This mode puts a
minimum load on the engine. Pin 23 at the
ECM is earthed through the switch to select
economy mode.
AC This setting allows the heating and cooling
functions to be performed automatically,
the required in-cartemperature
is
maintained
and the air is dehumidified. Pin 26 at the
ECM is earthed through the switch to select
this mode.
MAN (Manual) In the Manual mode the fan speed
and temperature control can be set at any
level and heating or cooling will be
performed despite the internal temperature
of the car. Pins 26 and 19 at the ECM are
earthed through the switch to select this
mode
V (Re-circulation) With this setting selected
the external air inlets are closed and
conditions are maintained by circulating
the air within the car. Heating and cooling
functions are controlled automatically to
give the required in-cartemperature. Pins 9
and 26 at the ECM are earthed through the
switch to select the manual Re-circulation.
DEM (Demist) Air is directed directly onto the
screen in this mode. Air speed and
temperature depend on the selected fan
screen and in-cartemperature. Heating and
cooling functions are still controlled
automatically to give the required in-car
temperature. Pin 8 and 26 at the ECM are
earthed through the switch to give the
required in-car temperature.

B
liD^
-• 1
-• 2
-• 3
-• 4
-• 5
,'^
1 -• 6
-• 7
-• 8
Figure 4.
Key to Fig. 4

1.
AC signal Pin 26 ECM

2.
MAN signal Pin 19 ECM
3. REC signal Pin 9 ECM

4.
DEM signal Pin 8 ECM
5. Earth-ground Pin 1 ECM
6. Earth-ground Pin 1 ECM
7. AC signal Pin 26 ECM
8. ECO signal Pin 23 ECM
A ECO
B AC
C MAN
D Re-circulation
E DEM
8-22 May 1996

'^^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