This document provides an overview of a training module on supplemental restraint system (SRS) theory and operation. The module objectives are to identify SRS components and describe sensor input and component operation. The module importance is outlined as improving technician skills, productivity, repairs, and customer satisfaction. An overview of SRS and components is provided, along with diagrams of typical SRS circuits and operations.

1. Individualized
A U T O M O B I L E TE C H N I C A L TR A I N I N G
Skills Training
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 2001 American Honda Motor Co., Inc. – All Rights Reserved.
04/15/02
RSC01
K N O W L E D G E M O D U L E
SRS Theory and Operation
OBJECTIVE
On completion of this module, you will evaluate
your knowledge of “SRS Theory and Operation”.
You will be able to:
• Locate and identify components in the Sup-
plemental Restraint System (SRS).
• Describe sensor input and component opera-
tion in the SRS system.
The module will be completed when you have
answered all the test questions and received a
score of 90% or higher.
WHY THIS MODULE IS IMPORTANT
Increasing your knowledge of “SRS Theory of
Operation” will enable you to:
• Improve your skills as a technician.
• Increase productivity and profitability.
• Provide Fixed First Visit repairs.
• Reduce come-back repairs.
• Increase customer satisfaction.
MODULE OVERVIEW
In this module you will study the information on
“Supplemental Restraint Systems” and complete
the Self-Evaluation.
Use the Worksheet in this module as a resource
to complete the Self-Evaluation.
Fill in the appropriate answers in the Self-Evalua-
tion. After completion, go to the Online Test Site,
enter your answers, and submit them for scoring.
The skills studied, practiced and learned in this
module apply to vehicles with dual stage/dual
threshold, side airbag SRS.

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PREREQUISITE
None.
DECISION POINT
If you feel that you have a sufficient understand-
ing of “SRS Theory of Operation” as outlined in
the module’s objectives, you can bypass this
module.
To do this, and receive credit for this module you
will need to successfully complete the Self-Evalu-
ation. Otherwise, proceed with the module
WHAT YOU WILL NEED
1. SRS Theory of Operation Self-Evaluation.
2. Access to a computer with Internet or Interac-
tive Network (iN) to complete the on-line self-
evaluation.
Overview
To help automobile occupants survive the tre-
mendous forces of a crash, vehicle designers
have developed numerous safety features.
Built-in safety features include:
• A strong metal framework that forms a “safety
cage” around the occupants.
• Front and rear “crush zones,” which help
absorb crash energy in a frontal or rear-end
collision.
• A collapsible steering column, which can
reduce driver injuries.
• Energy-absorbing materials throughout the
passenger compartment.
WORKSHEET

3. SRS Theory and Operation
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Seat belts have proven to be the single most
effective device in reducing crash injuries and
fatalities. When properly worn, seat belts help
keep an individual from striking interior parts of
the car. They also keep an individual connected
to the car so they can take advantage of the car’s
built-in energy absorption, as well as preventing
an individual from being ejected during a colli-
sion.
Front airbags were first installed on vehicles in
the late 1980’s. Since then, they have evolved
into a very sophisticated occupant protection sys-
tem. Airbags today serve to prevent or reduce
the impact of vehicle occupants against interior
components in frontal or side impact collisions.
As part of a vehicle’s occupant protection sys-
tem, airbags are designed to work with, not
replace, seat belts.
Front airbags, which are part of a car’s Supple-
mental Restraint System (SRS), help protect the
driver and the front passenger against head and
chest injuries in frontal vehicle impacts. Side air-
bags help reduce the severity of injuries to the
driver’s or front seat passenger’s upper torso in
some side impact collisions.
Side airbags are designed to inflate only when
they can be helpful, and to not inflate when they
cannot help. For example, a side airbag is not
likely to inflate during a sideswipe, but one or
both side airbags could inflate during a rollover.
Front airbags provide supplemental protection
only in frontal crashes. Seat belts should always
be used to provide maximum protection in roll-
overs and all crashes.

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How You Can Be Hurt in a Frontal Crash
In a 30-mph head-on crash into a barrier, a car
stops in about 1/10th of a second — less than the
blink of an eye. What happens to an occupant
during this time?
If a car collides head-on with a solid barrier at 30
mph, it comes to a stop almost instantly. Without
a seat belt or an airbag, the occupant will con-
tinue forward at 30 mph until they strike the car’s
interior, and potentially suffer very serious inju-
ries.
However, if the occupant is wearing a seat belt,
the belt will restrain them and reduce the chance
of injury. A front airbag can further reduce the risk
of injury to their head and chest.
If you are not
stopped by a
seat belt or
an airbag,
you will strike
the interior of
the car with a
force equiva-
lent to falling
out of a third-
story window
and landing
face-first on
pavement.
However, if
you are wear-
ing a seat belt
and your front airbag deploys, the belt will
restrain you and reduce the chance of injury, and
the airbag will provide additional protection for
your head and chest.
TECH NOT E
Any airbag that has deployed must be replaced
along with the SRS control unit, seat belt ten-
sioner(s), buckle tensioners and other related
parts. (Some models do not require control unit
replacement.)

5. SRS Theory and Operation
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04/15/02
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SRS Component Location
1
2
3
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
1. Left front impact sensor
2. Right front impact sensor
3. SRS indicator light
4. ‘Side Airbag Off’ indicator light
5. Front passenger’s airbag
6. OPDS sensor/seat back foam pad
7. Front passenger’s side airbag
8. Right side seat belt tensioner
9. Front passenger’s side impact sensor
10. Occupant Position Detection System unit
11. Right side seat belt buckle tensioner
12. Left side seat belt buckle tensioner
13. Left side seat belt tensioner
14. Driver’s side impact sensor
15. Driver’s side airbag
16. SRS control unit
17. Data link connector (DLC) (16P)
18. Cable reel
19. Memory Erase Signal (MES) connector (2P)
20. Driver’s under dash fuse/relay box
21. Driver’s airbag
4

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SRS Circuit
SRS Circuit (RSX)
1
2
3
4 5 6
7
8
9 10
11
12
15
16
18
19
13
14 17
Driver
side 1st
Driver
side 2nd
Pass.
side 1st
Right
Left
Right
Left
Driver
side
Pass.
side
2nd
SRS UNIT
SIDE
IMPACT
AIRBAG
INFLATOR
DRIVER’S SIDE IMPACT SENSOR
FRONT
IMPACT
AIRBAG
INFLATOR
SEAT BELT
TENSIONER
CABLE REEL
PASSENGER SIDE
FRONT PASSENGER’S
SIDE IMPACT SENSOR
Components*
1. SRS indicator light
2. OPDS sensor
3. ‘Side Airbag Off’ indicator light
4. OPDS unit
5. Side impact sensor
6. Microprocessor and diagnostic circuit
7. Front impact sensor, right
8. Front impact sensor, left
9. Side impact sensor
10. Microprocessor and diagnostic circuit
11. Power supply circuit
12. Frontal impact sensor
13. Side impact sensor
14. Memory circuit, non-volatile
15. Data link circuit
16. Microprocessor and diagnostic circuit
17. Data link circuit
18. Trigger circuit
19. Front impact safing sensor
* See glossary of terms on the following page

7. SRS Theory and Operation
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Glossary of Terms
1. SRS Indicator Light: An indicator light on
the instrument panel that comes on for a few
seconds indicating that the system is operat-
ing properly or remains on indicating a fault in
the SRS system.
2. OPDS Sensor: A group or series of sensors
in the front passenger seat back foam pad
that determines the height and position of the
occupant. Also may determine when an
object such as a briefcase or bag has been
placed on the passenger seat.
3. Side Airbag Off Indicator: An indicator light
on the instrument panel that comes on indi-
cating when a child or small-statured adult (or
package) is in a position where he might be
injured by side airbag deployment. It alerts
the driver that the side airbag will not deploy
in this condition.
4. OPDS Unit: Receives passenger seating
position via seat back sensors, then transmits
that information to the SRS control unit.
5. Side Impact Sensor: Mounted on the left or
right side of the vehicle, detects impact and
transmits that information to the SRS control
unit.
6. Microprocessor and Diagnostic Circuit:
Monitors side impact sensor operation and
receives impact information from the side
impact sensor. Transmits data to the data link
circuit.
7. Front Impact Sensor, Right and Left:
Mounted in the vehicle’s front end crush
zone, detects impact and transmits impact
deceleration to the SRS control unit. .
8. Same as 7.
9. Same as 5.
10. Same as 6.
11. Power Supply Circuit: Voltage supplied
directly from the battery and ignition switch to
the SRS control unit..
12. Frontal Impact Sensor: Contained within the
SRS control unit, acts in conjunction with the
front impact sensor indicating to the proces-
sor that a collision has occurred.
13. Side Impact Sensor: Contained within the
SRS control unit, acts in conjunction with the
side impact sensor and side impact micropro-
cessor and diagnostic circuit indicating to the
processor that a collision has occurred.
14. Memory Circuit: Stores diagnostic informa-
tion related to SRS systems if a problem
arises or one or more airbag components
deploys. (Non-volatile code. Disconnecting
the battery will not erase stored memory in
SRS control unit.)
15. Data Link Circuit: Circuitry within the SRS
unit which links the various sensors to the
microprocessor and diagnostic circuit.
16. Microprocessor and Diagnostic Circuit:
Monitors the various SRS components for
failure or deployment. Processes input infor-
mation from the front and side impact sen-
sors. Indicates to the trigger circuit that a
collision has occurred.
17. Same as 15.
18. Trigger Circuit: Receives signal from the
microprocessor that an accident occurred,
sends trigger signal to the appropriate air-
bag(s) and/or seat belt tensioners to deploy.
19. Front Impact Safing Sensor: Contained
within the SRS control unit, acts in conjunc-
tion with the front impact sensor indicating to
the processor that a collision has occurred.

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SRS Operation
The main circuit in the SRS unit senses and
judges the force of impact and, if necessary,
ignites the inflator charges. If battery voltage is
too low, or power is disconnected due to the
impact, the voltage regulator and the back-up
power circuit respectively will keep voltage at a
constant level.
Driver’s and Front Passenger’s Airbag(s)
For the SRS to operate:
The frontal impact sensor(s) must activate and
send electronic signals to the microprocessor.
1. The microprocessor must compute the
signals and send them to the airbag inflators
(and seat belt/buckle tensioners if equipped)
TECH NOT E
Depending on the severity of the collision on
vehicles with dual stage inflators and dual thresh-
old inflation, and dependant upon whether the
seat belt buckle switch is ON or OFF, the micro-
processor sends the appropriate signals to the
airbag inflator(s).
2. The inflators that received signals must ignite
and deploy the airbags. (If equipped, seat
belt tensioners and buckle tensioners deploy
at the same time as the airbags.)
TECH NOT E
Side Airbag(s). See module RSC04, OPDS Test-
ing, Diagnosis and Initialization for additional
information.
1. The side impact sensor(s) must activate, and
send electronic signals to the micropro-
cessor.
2. The microprocessor must compute the
signals and send them to the side airbag
inflator(s). However, the microprocessor cuts
off the signals to the front passenger’s side
airbag if the OPDS unit determines that the
front passenger’s head is in the deployment
path of the side airbag.
3. The inflator that received the signal must
ignite and deploy the side airbag.

9. SRS Theory and Operation
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04/15/02
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TECH NOT E
See module RSC02 Front Airbag Operation and
RSC04 OPDS Testing, Diagnosis and Initializa-
tion for additional information on front and side
airbags.
Self-Diagnosis System
A self-diagnosis circuit is built into the SRS unit.
When the ignition switch is turned ON, the SRS
indicator light comes ON and goes OFF after
about 6-seconds if the system is operating nor-
mally.
If the SRS light does not come ON, or does not
go OFF after 6-seconds, or if it comes on while
driving, this indicates an abnormality in the sys-
tem. The system must be inspected and repaired
as soon as possible.
For better serviceability, the SRS unit memory
stores a Diagnostic Trouble Code (DTC) that
relates to the cause of the malfunction. This infor-
mation can be read with the Honda PGM Tester
when it is connected to the DLC (16P). The DTC
can also be read by the “Flash Method” from the
“SCS” menu of the PGM Tester with the tester
connected to the 16P DLC.
TECH NOT E
Before beginning work on any component in the
SRS system turn the ignition switch OFF. Next
disconnect the battery negative cable then the
positive cable. Wait at least three minutes. Dis-
connecting the battery and waiting three minutes
allows the capacitors in the SRS control unit to
dissipate the stored energy, preventing acciden-
tal airbag discharge. On older vehicles install a
short connector (RED) on the airbag side of the
connector.

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Before removing the airbag or SRS related
devices (the SRS unit and the cable reel), dis-
connect the airbag connectors from the airbags
to prevent accidental deployment.
Refer to the “SRS” precautions section of the
appropriate service manual.
The DTC’s in the SRS control unit memory are
not erased even if the ignition switch is turned
OFF or the battery cables are disconnected from
the battery.
Sensor Location and Operation
As vehicles continue to evolve, so does the abil-
ity to provide the best passenger protection pos-
sible. Certain vehicle designs require using
sensors that are mounted on the front side frame
(left and right) just behind the bumper. These
sensors provide input to the SRS control unit in
the event of a frontal impact They work with the
SRS control unit to discriminate between crash
and non-crash events.
The front sensors are mechanical. When their
mass moves as a result of impact force, the cam,
which is combined with this mass, pushes the
contacts together and makes a connection. This
action sends a signal to the SRS control unit.
Both front sensors consist of a contact spring and
weight, and must be replaced after a collision
only if they are damaged.
The sensor also contains an integrated resis-
tance monitored by the SRS control unit for fail-
ure diagnosis. The SRS control unit can set a
DTC for each sensor in the event of a malfunc-
tion.
TECH NOT E
Early SRS systems used impact sensors located
on the cowl (driver and passenger front foot well),
radiator support, or inside the SRS control unit.
MAIN SENSOR
(SRS CONTROL UNIT)
FRONT SENSOR
(BOTH SIDES)
FRONT SENSOR
CAM
CONTACT
WEIGHT
(MASS)

11. SRS Theory and Operation
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SRS Harness
All harnesses associated with the SRS system
are identified by the yellow connector and yellow
sheathing surrounding the harness.
• Never attempt to modify, splice, or repair SRS
wiring. If there is an open or damage in SRS
wiring, replace the harness.
SUMMARY
Being able to locate and identify the various com-
ponents and sensors as well as understanding
their overall function in the SRS system’s opera-
tion is the first step in troubleshooting and fixing
the car right the first time.
In order to gain a proper understanding of the
supplemental restraint system or to approach
testing and repairs in a logical, thorough and pro-
fessional manner, it is important to be familiar
with these processes.
This module provides information which assists
individuals with accurately diagnosing items in
the event of a component failure. As safety sys-
tems become more numerous and complex,
understanding them will only grow in importance.
The SRS system serves to help protect the vehi-
cle occupants against serious or even fatal inju-
ries. Understanding how the SRS system and its
related components operate assists a technician
with proper diagnosis and repair.
TRAINING CENTER NOTE
This knowledge module is a prerequisite to a
skill, or “hands-on”, module you will take at your
training center. Before starting that skill module,
you will be asked by your instructor to validate
your knowledge on the subject of this module. To
save yourself some time, and to refresh yourself
on the content of this module, we suggest that
you review the content prior to your arrival at the
Training Center. This will assist you when taking
the skills module, and save some time for you,
your instructor and your dealership.
SELF-EVALUATION
When you have studied the Worksheet, proceed
to the Online Test Center and complete the Self-
Evaluation for this module