Advanced automotive fault diagnosis Figure 9.61

a roller in a set position until an impact above a
predetermined limit provides enough force to over-
come the spring and the roller moves, triggering a
micro switch. The switch is normally open with a
resistor in parallel to allow the system to be moni-
tored. Two switches similar to this may be used to
ensure that the bag is deployed only in the case of
sufficient frontal impact. Note the air bag is not
deployed in the event of a roll over. The other main
type of crash sensor can be described as an
accelerometer. This will sense deceleration, which
is negative acceleration.
The final component to be considered is the
ECU or diagnostic control unit. When a mechan-
ical type crash sensor is used, in theory no elec-
tronic unit would be required. A simple circuit
could be used to deploy the air bag when the sen-
sor switch operated. However, it is the system
monitoring or diagnostic part of the ECU which
is most important. If a failure is detected in any
part of the circuit then the warning light will be
operated. Up to five or more faults can be stored
in the ECU memory, which can be accessed by
blink code or serial fault readers. Conventional
testing of the system with a multimeter and jump
wires is not to be recommended as it might cause
the air bag to deploy!
A block diagram of an air bag circuit is shown
in Figure 9.64. Note the ‘safing’ circuit, which 
is a crash sensor that prevents deployment in the
event of a faulty main sensor. A digital based 
system using electronic sensors has about 10 ms
at a vehicle speed of 50 km/h to decide if the 
supplementary restraint systems (SRS) should 
be activated. In this time about 10 000 computing
operations are necessary. Data for the develop-
ment of these algorithms is based on computer
simulations but digital systems can also remem-
ber the events during a crash allowing real data to
be collected.

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