Airbag Dm Fault Code 52; Airbag Dm Fault Code 53 - Aston Martin DB7 Workshop Manual

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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 w i t h i n 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 w i 1 1 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 w i l l
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