wiring ASTON MARTIN DB7 1997 Workshop Manual

mxm^^^?
Electrics
Airbag System
Airbag DM Fault Code 14
Primary crash sensor circuit short to ground
Airbag DM Fault Code 21
Safing sensor insecure mounting.
Normal Operation
The diagnostic module measuresthevoltage at pins

1
-2 and
1
-6. The normal voltage at these pins is 10
volts (±1 volt) with the ignition on, and battery
voltage with the ignition off.
If the voltage at the airbag diagnostic module
connector pins
1
-2 or
1
-6 drops below 5 volts, fault
code 14 will be generated and the airbag warning
lamp will be switched on. When generating a code
14 the diagnostic module also generates a signal to
blow its own internal thermal fuse. This action
disables the airbag deployment circuit. The airbag
diagnostic module fuse is non-repairable and the
module must be replaced after removing the short
circuit. If the voltage at pins 1-2 and 1-6 returns to

normal,
fault code 51 (blown thermal fuse) will be

logged.

Fault Analysis
WARNING: Read and adhere to all warnings and
safety procedures at the start of
this
section when
working on the airbag system.

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4)

2.
Disconnect the airbag diagnostic module and
remove the plastic wedge from plug
1
(slate).
Note:
The
plastic wedge contains a shorting bar which
would
short
together pins
1-3
(gnd),
1-11
and 1-2 when
the plug is disconnected.
3 Measure the continuity to ground at the following
pins on the diagnostic module harness connector:
1-2 RH crash sensor monitor
1-11 LH crash sensor feed

1
-6 LH crash sensor monitor
1-12 RH crash sensor feed
If no short circuit is detected, go to step 4.
If a short circuit
is
detected, disconnect the relevant
crash sensorandrepeatthecontinuity measurement
to isolate the circuit fault. Service the wiring or
replace the crash sensor as necessary. Refit the
plastic wedge to the diagnostic module connector.
Fit a new airbag diagnostic module and rearm the
airbags (6.4.21.4).
Normal Operation
The diagnostic module measures the resistance
between pins
1
-10 (safing sensor case ground) and
1-3 (monitor reference ground) at the diagnostic
module connector. If the resistance is greater than
2Q
a
fault code
21
will be generated and the airbag
warning lamp will be switched on.
Note:
a
good ground connection is vital.
The
wire from
pin
1-1
is riveted to the safing
sensor case
and the
case

must
be securely
grounded to the vehicle
body.

Fault Analysis
WARNING: Read and adhere to all warnings and
safety procedures at the start of this section when
working on the airbag system.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).
Disconnect the diagnostic connector and check for
continuity from pin 1-3 to chassis ground.
If continuity is good, go to step 3.
If the resistance reading is 2Q or more, service the
diagnostic module ground circuit to pin 1-3 as
necessary. Confirm a good ground path between
pins 1-10 and 1-3.Clearthecode21 and rearm the
airbags (6.4.21.4).
Check for continuity between pins
1
-10 and chassis
ground at the safing sensor.
If the resistance is above 2Q, check for continuity
from the safing sensor connector pin 5 to chassis

ground.
Also check the safing sensor mounting for

corrosion,
dirt or loose fixings. Service the wiring
and/or clean and secure the sensor mounting as
necessary, if a satisfactory ground is not achieved,
replace the safing sensor.
Make
a
final continuity test from diagnostic module
connector pin
1
-3 to
1
-10 to ensure a satisfactory
ground
path.
Reconnect the diagnostic module.
Clear the code
21.
Rearm the airbags (6.4.21.4).
September 1996 6-87

Electrics
Airbag System [D:B3-2?
Airbag DM Fault Code 22
Safing sensor output circuit shorted to battery
voltage.
Normal Operation
The diagnostic module measures the voltage at pin
2-5 (airbag module feed) and 2-5 (safing sensor
output monitor) at thediagnosticmoduleconnector.
The voltage at these pins is dependant on charging
system voltage as shown in the table below. If the
voltage at either pin exceeds 5 volts,
a
fault code 22
will be generated.
in 2-5
2.3V
2.4V
2.5V
2.7V
2.8V
3.0V
3.1V
3.2V
3.4 V
3.5V
3.7V
3.8V
4.0V
4.1V
4.3V
Pin 2-6
2.3V
2.4V
2.5V
2.7V
2.8V
3.0V
3.1V
3.2 V
3.4 V
3.5V
3.7V
3.8V
4.0V
4.1V
4.3V
Charge Volts
9.0V
9.5V
10.0V
10.5V
11.0V
11.5V
12.0V
12.5V
13.0V
13.5V
14.0V
14.5V
15.0V
15.5V
16.0V
Possible Causes
• A short between pins 2-5 or 2-6 and another

wire.
(The wiring to the safing sensor carries
voltages above 5 volts).
• A short between the cable reel cassette circuit
and other 12 volt circu its at the steering column

head.

• A short across the normal ly open contacts of the
safmg sensor.
• Vehiclechargingsystem voltage too
high.
If the
generator output is greater than 17V, fault code
22 may be logged.
Fault Analysis
WARNING: Read and adhere to all warnings and
safety procedures at the start of this section when
working on the airbag system.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).
Check the voltage at the battery with the engine
running at approximately 1500 rpm.
If the voltage is 14.8 ±0.5 volts, the charging
voltage is within specification, go to step 3.
If the voltage is significantly above 14.8 volts (the
voltage regulator set point) service the charging

system.
Clear the code 22. Rearm the airbags
(6.4.21.4)
Check in the area of the diagnostic module
connector for harness damage which could cause
a short circu it to 12 volts. (Note: The airbag system
harness runs are sheathed in black plastic protection
and harness damage is unlikely.)
If no harness damage is evident, go to step 4.
If harness damage is identified, service the wiring as
necessary. Clear the code 22. Rearm the airbags
(6.4.21.4).
Switch on the ignition. Monitor the voltage at pins
2-5 and 2-6 whilst rotatingthe steering from lock to
lock in both directions.
If the voltage at both pins remains below 5.0 volts,
go to step 5.
If the voltage rises to above 5.0 volts, service the
cable reel cassette or wiring as necessary. Clear the
code 22. Rearm the airbags (6.4.21.4).
With the ignition on, check the voltage at pins 2-5
and 2-6 whilst manipulating the bulkhead and
safing sensor harnesses.
If the voltage rises above 5.0 volts, service the
wiring fault in the harness area being manipulated
at the time the voltage rise occurred.
If no voltage rise occurs, fit a new safing sensor as
the most likely cause of the code 22. Clear the code
22 and rearm the airbags (6.4.21.4).
6-88 September 1996

>^ '—-y ^ Electrics
-^ ^ ^ Airbag System
Airbag DM Fault Code 23
Safing sensor input- battery feed/return open circuit.
Normal Operation
The diagnostic module measures the voltage on
diagnostic module pin
1
-9 at all times. The voltage
can be as high as 25 volts when the backup power
supply is fully charged.
Pin
1
-9 is connected to pin 2-11 inside the safing
sensor and the voltage on both these pins shou
Id
be
the same. If the voltage on pin 2-11 fails below the
voltage on pin
1
-9, a code 23 is generated.
Possible Causes
• Open circuit in the wiring to diagnostic module
pin 1-9.
• Open circuit in the wiring to diagnostic module

pin2-n.

Fault Analysis
WARNING: Read and adhere to all
warnings
and
safety procedures at the start of
this
section when
working on the airbag
system.

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).

2.
Disconnect the diagnostic connector and thesafing
sensor connector. Check for continuity from DM

pi n 1
-9 to safing sensor harness connector
pi n
7 and
from DMpin 1-11 to
safing
sensor harness connector
pin 6.
If the continuity of both leads is good, go to step 3.
If a continuity fault is detected, repair the wiring as
necessary. Reconnect ail components. Clear the
code 23. Rearm the airbags (6.4.21.4).
3. Check the continuity between pins 6 and 7 of the
safing sensor.
If the continuity is good, replace the diagnostic
module. Rearm the airbags (6.4.21.4).
If a continuity fault is detected, replace the safing
sensor. Reset the code 23. Rearm the airbags
(6.4.21.4).
September 1996 6-89

Electrics
Airbag System =2?
Airbag DM Fault Code 24
Safing sensor output - battery feed/return open
circuit.
Normal Operation
The diagnostic module measures the voltage on
diagnostic module pins 2-5 and 2-6 the voltage
varies with the vehicle charge level
as
shown in the
table below:
in 2-5
2.3V
2.4 V
2.5V
2.7V
2.8V
3.0V
3.1V
3.2V
3.4V
3.5V
3.7V
3.8V
4.0V
4.1V
4.3V
Pin 2-6
2.3V
2.4V
2.5V
2.7V
2.8V
3.0V
3.1V
3.2V
3.4V
3.5V
3.7V
3.8V
4.0V
4.1V
4.3V
Cliarge Volts
9.0V
9.5V
10.0V
10.5V
11.0V
11.5V
12.0V
12.5V
13.0V
13.5V
14.0V
14.5V
15.0V
15.5V
16.0V
The diagnostic module also measures the voltage at
pin 1-7 (battery input). Using the battery input
voltage, the system can predict the expected voltage
level on pins 2-5 and 2-6. If the voltage on pins 2-
5 or 2-6 is higher or lower than expected, a fault
code 24 will be declared.
Possible Causes
• Open circuit or high resistance in the wiring
harness or safing sensor.
• Intermittent battery voltage on pin 1-7
• The resistance of the pin 2-5 to 2-6 circuit to

ground.
This circuit should be open to ground at
all times.
Fault Analysis
WARNING: Read and adhere to all warnings and
safety procedures at the start of
this
section when
working on the airbag system.

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).

2.
Probe the battery input voltage terminal
1
-7. Start
the engine and monitor the charge voltage level
using a digital multimeter. Run the engine at idle
speed and look for any instability in the voltage

level.
Run the engine at a constant medium speed
and then at a constant high speed and repeat the
observation for voltage instability.
If the voltage levels are stable, go to step 3.
If instability of the voltage level is observed,
Investigate and rectify the problem in the charging
circuit.
3. Switch off the ignition. Disconnect the diagnostic
module and check for OQ continuity from pins 2-5
to 2-6.
If no resistance is detected, go to step 4.
If any resistance is detected between pins 2-5 and

2-6,
service the wiring or safing sensor to achieve
Ofi continuity.

4.
Measure the resistance to ground from pins 2-5 and

2-6.

If no short circuit is detected, replace the diagnostic
monitor. Rearm the airbags (6.4.21.4).
If
a
short to grou
nd
is detected, service the wiring or
safing sensor as necessary. Rearm the airbags
(6.4.21.4).
6-90 September 1996

o; D^?
Electrics
Airbag System
Airbag DM Fault Code 32
Drivers airbagcircuit high resistance or open circuit.
Normal Operation
The diagnostic module measures the resistance
across pins 2-4 and 2-5 every time the ignition is
switched on. The resistance should be 1.6 to
1.8£2.

This value is made up of the airbag
1
.OQ and the
cable reel cassette windings 0.3-0.4Q per winding.
If the total resistance in the drivers airbag circuit
exceeds 4.0^, fault code 32 will be declared and
the airbag lamp will be illuminated.
Note: The
resistance
of
the airbag simulator is
2.5Q,

therefore expect total resistance readings
of 2.8 - 2.9Q
with
the simulator
fitted.

Caution: The drivers airbag and the cable reel
cassette connectors contains shorting bars which
short circuit pins 1 and 3 of the connectors when
disconnected.
DO NOT REMOVE THE AIRBAG SHORTING
BAR

UNDER ANY CIRCUMSTANCES.
The harness shorting bars may be temporarily
removed during the following resistance checks.
Possible Causes
• Excessive resistance in the cable reel cassette
connections.
• Open circuitor high resistance in the cable reel
cassette windings.
• Open circuit or high resistance in the wiring
harness
• Open circuit or high resistance in the drivers
airbag module.
• Defective diagnostic module.
WARNING: Do not attempt a direct resistance
check on the airbag module. Accidental
deployment can occur due to the induced voltage
from the measuring equipment.
3.
Fault Analysis
Disarm the airbag system and fit airbag simulators
(6.4.21.4).
Disconnect the diagnostic module and remove the
plastic wedge from connector 2 (black). Measure
the resistance between pins 2-4 and 2-5. The
resistance should be 2.8 - 2.9Q.
If the resistance is 2.8 - 2.9Q, go to step 3.
if the resistance is not in the range 2.8 - 2.9D, isolate
the continuity problem noting the following:
a) Pin 2-4 is shorted to pin 2-5 by a shorting link
. when plug 2 of the diagnostic connector is

removed.
Removal of the plastic wedge from
the plug will remove the short circuit.
b) Both halves of the cable reel conneaor have
shorting links which will short pin 1 to pin 3
when the connection is removed. Removal of
the plastic wedge horn either half will remove
the short circuit in that part of the wiring.
c) A shorting link is fitted in the airbag module
connector. Do not attempt to remove this
shorting link or the plastic wedge. Do not
attempt to measure the resistance of the airbag
module.
d) Carefully replace all plastic wedges on
completion of the circuit checks.
The fault is therefore in either the airbag module or
in the airbag diagnostic module.
Substitute a known good diagnostic module. If
code 32 recurs with the airbag simulator fitted,
there is a fault in the wiring.
If the fau
It
code 32 does not recur, rearm the airbags
(6.4.21.4) and retest.
If fau
It
code 32 recu
rs
with the ai rbag modu
le
fitted,
the airbag module is faulty. Replace the drivers
airbag module (4.5.01.3).
September 1996 6-91

Electrics
Airbag System ^^2?
Airbag DM Fault Code 33
Passengers airbag circuit high resistance or open
circuit.
Normal Operation
The diagnostic module measures the resistance
across pins 2-2 and 2-3 every time the ignition is
switched on. The resistance should be 0.9 to 1.1Q.
If the total resistance in the passengers airbag
circuit exceeds 4.0Q, fault code 33 will be declared
and the airbag lamp will be illuminated.
Note:
The
nominal
resistance
of the airbag simulator
is

2.5£2,
therefore
expect
a
total circuitresistancereadingof
2.4 - 2.6Q with the simulator
fitted.

Caution: The passenger airbag connector contains
shorting bars which short circuit pins 1 and 3 of
both sides of the connector when disconnected.
DO NOT REMOVE THE AIRBAG SHORTING
BAR

UNDER ANY CIRCUMSTANCES.
The harness shorting bar may be temporarily
removed during the following resistance checks.
Possible Causes
• Open circuit or high resistance in the wiring
harness
• Open circuit or high resistance in the passenger
airbag module.
• Defective diagnostic module
WARNING: Do not attempt a direct resistance
check on the airbag module. Accidental
deployment can occur due to the induced voltage
from the measuring equipment.
Fault Analysis

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).

2.
Disconnectthediagnostic module and remove the
plastic wedge from connector 2 (black). Measure
the resistance between pins 2-2 and 2-3. The
resistance should be 2.4 - 2.6Q.
if the resistance is 2.4 - 2.6Q, go to step 3.
Ifthe resistance is not in the range 2.4-2.6S2, isolate
the continuity problem noting the following:
a) In the diagnostic module connector, pin 2-2 is
shorted to pin 2-3 by a shorting link when plug
2 of the diagnostic connector is removed.
Removal of the plastic wedge from the plug wi
11

remove this short circuit.
b) At the airbag module harness connector, pin 1
is shorted to pin 3 when the airbag is
disconnected. Removal of the plastic wedge
from the plug will remove this short circuit.
c) The shorting link fitted in the airbag module
connector will short pin 1 to pin 3 when the
airbag is disconnected. Do not attempt to
remove
this
shorting link or the plastic wedge.
Do not attempt
to
measure the resistance of the
airbag module.
c) Carefully replace all plastic wedges on
completion of the circuit checks.
3. The fault is therefore in either the airbag module or
in the airbag diagnostic module.
Substitute a known good diagnostic module. If
code 33 recurs with the airbag simulator fitted,
there is still a fault in the wiring or the simulator.
Ifthe fault code 33 does not recur, rearm the airbags
(6.4.21.4) and retest.
If fault code 33 recurs with the airbag module fitted,
the airbag module is faulty. Replace the passengers
airbag module (7.8.02.6).
6-92 September 1996

in:m3'^?
Electrics
Airbag System
Airbag DM Fault Code 34
Drivers airbag circuit low resistance or short circuit.
Normal Operation
The diagnostic module measures the resistance
across pins 2-5 and 2-4 every time the ignition is
switched on. The resistance should be 1.6 to 1.8Q.
This value is made up of the airbag
1
.OQ and the
cable reel cassette windings 0.3-0.4£2 per winding.
If the total resistance in the passengers airbag
circuit falls below 0.7Q, fault code 34 will be
declared and the airbag lamp will be illuminated.
Note: The nominal
resistance
of the airbag simulator is
2.5Q, therefore expect
a
total circuit resistance readingof
3.1 -
3.3£2
with the simulator
fitted.

Caution: The drivers airbag connector contains
shorting bars which short circuit pins 1 and 3 of
both sides of the connector when disconnected.
DO NOT REMOVE THE AIRBAC CONNECTOR
SHORTING BAR UNDER ANY CIRCUMSTANCES.
The airbag harness shorting bar may be temporarily
disabled during the following resistance checks.
Possible Causes
• Poorly mated cable reel cassette connector. If
the connector is poorly mated, the shorting bars
may not be fully retracted.
• Faulty shorting bar may short circuit 2-4 and 2-
5 together.
• Short circuit in the cable reel cassette windings.
• A short across the airbag terminals in the airbag
module.
Fault Analysis
Disarm the airbag system and fit airbag simulators
(6.4.21.4).
Disconnect the diagnostic module and remove the
plastic wedge from connector 2 (black). Measure
the resistance between pins 2-5 and 2-4. The
resistance should be 3.1 - 3.3Q.
If the resistance is 3.1 to 3.3Q, go to step 4.
If the resistance is below 3.1 - 3.3Q, go to step 3.
Disconnect the cable reel cassette connector at the
steering column and remove the plastic wedge.
Measure the resistance from pins 2-4 and 2-5 at the
diagnostic module connector.
If the resistance is infinity, check the cable reel
cassette for short circuits. Replace the column
switchgear assembly if necessary. Refit the plastic
wedges. Reconnect the diagnostic module. Clear
the code 34. Rearm the airbags (6.4.21.4).
If the resistance is less than infinity, replace the
main airbag harness. Refit the plastic wedges.
Reconnect the diagnostic module. Clear the code

34.
Rearm the airbags (6.4.21.4).
Since the circuit resistance is in specification, the
fault must lie in the diagnostic module or in the
airbag module.
Substitute a known good diagnostic module. If
code 34 recurs with the airbag simulator fitted,
there is still a fault in the wiring or the simulator.
If the fault code 34 does not recur, rearm the airbags
(6.4.21.4) and retest.
If fault code 34 recurs with the airbag modulefitted,
the airbag module is faulty. Replace the drivers
airbag module (4.5.01.3).
September 1996 6-93

Electrics
Airbag System ^^?
Airbag DM Fault Code 35
Passengers airbag circuit low resistance or short
circuit.
Normal Operation
The diagnostic module measures the resistance
across pins 2-2 and 2-3 every time the ignition is
switched on. The resistance should be 0.9 to
1.1Q..

If the total resistance in the passengers airbag
circuit falls below 0.7S2, fault code 35 will be
declared and the airbag lamp will be illuminated.
f^ote:
The
nominal
resistance
of the airbag simulator
is

2.5£2,
therefore expect
a
total circuit
resistance
reading of
2.4 - 2.6Q with the simulator fitted.
Caution: The passengers airbag connector contains
shorting bars which short circuit pins 1 and 3 of
both sides of the connector when disconnected.
DO NOT REMOVE OR DISABLE THE AIRBAG
CONNECTOR SHORTING BAR UNDER ANY
CIRCUMSTANCES.
The airbag harness shorting bar may be temporarily
disabled during the following resistance checks.
Possible Causes
• Poorly mated airbag harness connector. If the
connector is poorly mated, the shorting bars
may not be fully retracted.
• Faulty shorting bar may short circuit 2-2 and 2-
3 together.
• A short across the airbag terminals in the
passengers airbag module.
Fault Analysis

1.
Disarm the airbag system and check the security of
the passengers airbag module connector before
disconnecting.
If the connector is secure, complete the disarming
procedure, fit the airbag simulators (6.4.21.4). Go
to step 2
If the connector is insecure, service as necessary,
clear the code 35, rearm the airbags. Check that
code 35 does not recur.

2.
Disconnectthediagnostic module and removethe
plastic wedge from connector 2 (black). Measure
the resistance between pins 2-2 and 2-3. The
resistance should be 2.4 - 2.6Q.
If the resistance is 2.4 - 2.6Q, go to step 4.
If the resistance is below 2.4 - 2.6fl, go to step 3.
3. Disconnect the airbag simulator at the passengers
airbag module and remove the plastic wedge from
the harness connector. Measure the resistance
between pins 2-2 and 2-3 at the diagnostic module
connector.
If the resistance is infinity, the airbag harness is
good at this time. Swap the airbag simulators at the
drivers and passengers airbag connectors. Repeat
the resistance check. If the resistance is less than
infinity, there is a fault in the passengers airbag
harness connector. Service the connector or replace
the harness as necessary. Refit the plastic wedges.
Reconnect the diagnostic module. Clear the code

35.
Rearm the airbags.
If the resistance is less than infinity, service the
bulkhead harness as necessary. Refit the plastic
wedges. Reconnect the diagnostic module. Clear
the code 35. Rearm the airbags (6.4.21.4).

4.
Since the circuit resistance is in specification, the
fault must lie in the diagnostic module or in the
airbag module.
Substitute a known good diagnostic module. If
code 35 recurs with the airbag simulator fitted,
there is still a fault in the wiring or the simulator.
If the fault code 35 does not recur, rearm the airbags
(6.4.21.4) and retest.
If fault code 35 recurs with the airbag module fitted,
the airbag module is faulty. Replace the passengers
airbag module (7.8.02.6).
6-94 September 1996

Electrics
Airbag System ^?
Airbag DM Fault Code 51
Airbag diagnostic module - thermal fuse blown.
WARNING: The diagnostic module will blow the
internal fuse whenever a short circuit to ground
occurs in the deployment circuits. Itdoesnotblow
because of
excessive
current
flow.
The thermal
fuse is not a serviceable item. No attempt should
be made to replace or
bypass
the fuse.
WARNING: Do not install a new diagnostic
module until the short has been located and
corrected. A short which goes away before
corrective action has been taken must be regarded
asan intermittentfault. Installinga newdiagnostic
module when there is an intermittent fault will
result in repeated diagnostic module failures and
repeated repairs.
Fault Analysis
Disarm the airbag system and fit airbag simulators
(6.4.21.4).
Inspecttheconnectorsandterminalsfor any foreign
metal objects or damaged terminals which could
be the cause of the short circuit.
Check each of the circuits in the possible causes list
and isolate the intermittent short circuit. Rectify the
fault.
See the warnings above about replacement of
diagnostic modules when intermittent shorts are
present.
Replace the diagnostic module.
Reconnect all components and rearm the airbags
(6.4.21.4).
Normal Operation
In order to prevent airbag deployment in the event
of a short circuit to ground, the diagnostic module
blows its internal fuse as soon as a short to ground
is detected. This action removes all battery and
back-up power from the deployment circuits. The
diagnostic module will flag the higher priority
codes 13 or 14 while the short to ground exists. If
the short is intermittent and temporarily corrects
itself, the d iagnostic modu le wi
11
flag code
51.
If the
short to ground returns, the diagnostic module will
again flag the higher priority code 13 or 14 instead
of code 51.
Code 51 after Airbag Deployment
The internal wiring may be shorted to the metal
airbag housings after the airbags are deployed. The
diagnostic module will see this as a short to ground
in the deployment wiring, it will flagacode 14and
blow the internal fuse. As the deployed airbags

cool,
the short circuit may rectify itself. The
diagnostic module will then flag a code 51. This
intermittentfault must be investigated and corrected.
Possible Causes
A short circuit to ground in any of the following:
DM pins 2-2, 2-5 or 2-6 to the safing sensor
DM pins 2-2, 2-5 or 2-3 to the passenger airbag
DM pins 2-2, 2-4 or 2-5 to the drivers airbag
DM pins
1
-2 or
1 -11
to RH crash sensor
DM pins
1
-6 or
1
-12 to LH crash sensor
6-98 September 1996

in:[M3^?
Electrics
Airbag System
Airbag DM Fault Code 52
Back-up power supply - voltage boost fault.
Normal Operation
A back-up power supply consisting of a capacitor
and voltage booster is contained within the
diagnostic module. The voltage booster charges
the capacitor to 24 ±
1
volt when the ignition is on.
The back-up power supply is connected to
diagnostic module pin 1-9 via a diode. Since the
resistance of pin 1-9 to ground is infinite, the
capacitor has no discharge path and the back-up
supply
is
maintained ata high
voltage.
Thediagnostic
module monitors this voltage and will flag a code
52 if a minimum of 23V is not achieved within 45
seconds of switching on the ignition.
Possible Causes
Low back-up supply voltage may be caused by:
• Partial or total short circuit of pin 1-9 to ground.
• Boost circuit failure within the diagnostic
module.
Fault Analysis

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).

2.
Disconnect the diagnostic module. Check for a
partial or permanent short circuit to ground from
diagnostic module harness connector pin 1-9.
If no short circuit is detected, go to step 3.
If a short circuit is detected, service the wiring as
necessary. Reconnectthediagnostic module. Clear
the code 52. Rearm the airbags.
3. Using a high resistance digital multimeter, set up to
measure the voltage from diagnostic module pin
1
-
9 to ground. Switch on the ignition.
If the voltage is in the range 24 ± 1 volt, the back­
up power supply is good. Suspect an intermittent
short circuit to ground and recheck the output
circuit from pin 1-9.
Ifthe voltage is below the range 24 ±
1
volt, replace
the diagnostic module.

4.
Rearm the airbags (6.4.21.4).
Airbag DM Fault Code 53
internal diagnostic module failure.
Normal Operation
The diagnostic module constantly measures the
voltage on pins
1
-2,
1
-6,
1 -11
and
1
-12. It should
be 10
±1
volt. The voltage is regulated by a resistor
within the diagnostic module and the resistance to
ground ofpins
1-2,6,11
and 12 should be infinite.
The resistance across the normally open contacts of
the crash sensors should also be infinite. If the
resistance of the pins to ground or across the crash
sensors becomes measurable, the voltage wi
11
drop.
At between 5V and lOVthe diagnostic module will
recognise an internal voltage regulation failure and
flag a code 53 fault and the airbag warning light will
be illuminated.
Notes: A dead short to ground in the
crash
sensor circuits
will drop the voltage below 5V and the diagnostic
module will flag
a
code
14
fault.

The
diagnostic module
performs several self-tests as
the
ignition is switched on. If it fails any
test,
fault code 53
will be flagged.
Possible Causes
• A decrease in resistance from infinity between
the primary crash sensor circuits and ground.
• An internal diagnostic module self-test failure.
Fault Analysis

1.
Disarm the airbag system and fit airbag simulators
(6.4.21.4).

2.
Check each of pins 1-2, 1-6, 1-11 and 1-12 for
continuity to ground.
If resistance of all circuits is infinity, go to step 3.
If any resistance other than infinity is detected,
service the circuit wiring as necessary. Reconnect
all connectors and switch on and clear all fault
codes.
Switch off the ignition and then switch on again.
Check that the airbag warning lamp comes on at
'ignition on' and extinguishes after approximately
six seconds on satisfactory completion of the airbag
system self-check.
3. Rearm the airbags (6.4.21.4).
September 1996 6-99