Automotive Electrical and Electronic Systems Classroom Manual Fifth Edition Update by John F. Kershaw

348 Chapter Fifteen
Figure 15-50. SRS components. (DaimlerChrysler
Corporation)
Diagnostic Monitor Assembly
The diagnostic monitor contains a microcomputer
that monitors the electrical system components
and connections. The monitor performs a
self-check of the microcomputer internal circuits
and energizes the system readiness indicator
during prove-out and whenever a fault
occurs. System electrical faults can be detected
and translated into coded indicator displays. If a
certain fault occurs, the microcomputer disables
the system by opening a thermal fuse built into
the monitor. If a system fault exists and the indicator
is malfunctioning, an audible tone signals
the need for service. If certain faults occur, the
system is disarmed by a firing circuit disarm
device incorporated within the monitor or diagnostic
module.
Trouble codes can be retrieved through the use
of a scan tool or flash codes, and on some models
through the digital panel cluster (if equipped). As
CAUTION: When servicing the air bag system, the service manual will instruct you when and
how to disarm the system. Failure to follow these procedures can result in injury and additional
costly repairs to the vehicle.
to conduct a system self-check to let the driver
know that it is functioning properly, to detect
an impact, and to send a signal that inflates the
air bag. The air bag module is located in the
steering wheel for the driver and in the dash
panel for passengers; it contains the air bag and
the parts that cause it to inflate. The knee
diverter cushions the driver’s knee from impact
and helps prevent the driver from sliding under
the air bag during a collision. It is located
underneath the steering column and behind the
steering column trim.
Electrical System Components
The electrical system generally has the following
parts:
with all diagnostics, consult the appropriate service
manual for the correct procedures.
An air-bag-system backup power supply is
included in the diagnostic monitor to provide air
bag deployment power if the battery or battery
cables are damaged in an accident before the
crash sensors close. The power supply depletes its
stored energy approximately one minute after the
positive battery cable is disconnected.
Sensors
The sensors detect impact (Figure 15-51) and
signal the air bag to inflate. At least two sensors
must be activated for the air bag to inflate. There
are usually five sensors: two at the radiator support,
one at the right-hand fender apron, one at
the left-hand fender apron, and one at the cowl in
the passenger compartment. However, a few systems
use only two sensors—one in front of the
radiator and another in the passenger compartment.
There is an interlock between the sensors,
so two or more must work together to trigger the
system. Keep in mind that air bag systems are
designed to deploy in case of frontal collisions
CAUTION: The backup power supply energy must be depleted before any air bag component
service is performed. To deplete the backup power supply energy, disconnect the positive battery
cable and wait one minute.