Chevrolet Silverado / GMC Sierra. Service manual — part 701

performs calculations using the measured accelerations. The SDM compares these calculations to
a value stored in memory. When the generated calculations exceed the stored value, the SDM will
cause current to flow through the frontal deployment loops deploying the frontal air bags. Once
the air bags are inflated they quickly deflate through the air bag vent holes. After the air bags
have deployed, the SDM sets a diagnostic trouble code (DTC) and requests the IPC to turn the
AIR BAG indicator ON. The SDM, instrument panel (I/P) module, steering wheel module,
steering wheel module coil and the connecting wires makeup the frontal deployment loops. The
SDM continuously monitors the deployment loops for malfunctions and requests the IPC to turn
the AIR BAG indicator ON if a fault is detected.

Side SIR System Description (Front)

The side Supplemental Inflatable Restraint (SIR) System (front) consists of the following
components:

AIR BAG indicator located in the instrument panel cluster (IPC)

Inflatable restraint roof rail modules front/rear, (left/right)

Inflatable restraint sensing and diagnostic module (SDM)

Inflatable restraint side impact sensors (SIS) (left/right)

Inflatable restraint vehicle rollover sensor

Inflatable restraint wiring harnesses

Seat belt pretensioners (left/right)

The roof rail modules (front/rear) are located in the headliner along the roof rails. The roof rail
modules contain a housing, inflatable air bag, initiating device, and a canister of gas generating
material. The initiator is part of the roof rail module deployment loop. When a side impact of
sufficient force occurs the SIS detects the impact and sends a signal to the SDM. The SDM
compares the signal received from the SIS to a value stored in memory. When the generated
signal exceeds the stored value, the SDM will cause current to flow through the side deployment
loop deploying the roof rail air bag. The SDM, roof rail modules (front) and the connecting wires
makeup the side deployment loops. The SDM continuously monitors the deployment loops for
malfunctions and turns the AIR BAG indicator ON if a fault is present. Each roof rail module is
equipped with a shorting bar located on the connector of the module. The shorting bar shorts the
roof rail module deployment loop circuitry to prevent unwanted deployment of the air bag when
servicing the inflator module.

Inflatable Restraint Sensing and Diagnostic Module (SDM)

The sensing and diagnostic module (SDM) is a microprocessor and the control center for the
Supplemental Inflatable Restraint (SIR) System. The SDM contains internal sensors along with

2008 Chevrolet Silverado 1500

2008 RESTRAINTS Supplemental Inflatable Restraints - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

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several external sensors, if equipped, mounted at various locations on the vehicle. In the event of
a collision, the SDM performs calculations using the signals received from the internal and
external sensors. The SDM compares the results of the calculations to values stored in memory.
When these calculations exceed the stored value, the SDM will cause current to flow through the
appropriate deployment loops to deploy the air bags. The SDM records the SIR System status
when a deployment occurs and requests the instrument panel cluster (IPC) to turn the AIR BAG
indicator ON. The SDM performs continuous diagnostic monitoring of the SIR System electrical
components and circuitry when the ignition is turned ON. If the SDM detects a malfunction, a
DTC will be stored and the SDM will request the IPC to turn the AIR BAG indicator ON. In the
event that battery voltage is lost during a collision, the SDM maintains a 23-volt loop reserve (23
VLR) for deployment of the air bags. It is important to note, when disabling the SIR System for
servicing or rescue operations to allow the 23 VLR to dissipate, which could take up to 1 minute.

Inflatable Restraint Passenger Presence System (PPS)

The PPS is used to monitor the weight of an occupant on the front outboard passenger seat and
communicate the status to the sensing and diagnostic module (SDM) whether to enable or
suppress the deployment of the instrument panel (I/P) module. The PPS consist of an electronic
control module, silicone filled sensor pad, pressure sensor, seat belt tension sensor, wiring
harness, and PASSENGER AIR BAG ON/OFF indicator. The silicone filled sensor pad is
located under the passenger seat foam cushion and is connected by a hose clamped to the pressure
sensor. The weight of the occupant sitting in the front passenger seat is measured as a pressure
change within the bladder by the pressure sensor. The pressure sensor sends a voltage signal to the
PPS module. If the pressure from the occupants weight is less than a specified value, the PPS
module will send a suppress signal to the SDM to disable the I/P module. If the pressure from the
occupants weight is higher than a specified value, the PPS module will send an enable signal to
the SDM to enable the I/P module. The PPS module will notify the customer of the
enable/disable status by turning on one of the PASSENGER AIR BAG ON/OFF indicator
located on the overhead console. The PPS monitors itself for faults and will set diagnostic trouble
codes (DTC) if a fault is detected. The PPS will also notify the SDM of a fault and the SDM will
set DTC B0081 and request the instrument panel cluster (IPC) to turn ON the AIR BAG
indicator located on the IPC.

Inflatable Restraint Seat Position Sensors (SPS)

IMPORTANT: The Passenger Presence System (PPS) is a calibrated unit. When

replacing the assembly all parts in the service kit must remain
together. Do not mix any of the old parts with the new parts. After
repairing or replacing the PPS, the system must be rezeroed in
order to function properly.

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2008 RESTRAINTS Supplemental Inflatable Restraints - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

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The seat position sensor (SPS) is used to determine the proximity of a front driver position with
respect to the frontal air bag. The SPS interfaces with the sensing and diagnostic module (SDM).
The state of the SPS allows the SDM to disable stage 2 of the frontal air bag for a front seat that
is forward of a forward/rearward point in seat track travel. The SPS is a Hall effect sensor that is
mounted on the outboard seat track of the driver seat. The seat track includes a metal bracket that
shunts the SPS magnetic circuit creating two states of seat position. The shunted state represents a
rearward seat position. The non-shunted state represents a forward position. The SPS provides 2
current ranges, one range for the shunted state and a second range for a non-shunted state. These 2
states are inputs to the SDM. State 1 (shunted) being the rearward threshold and state 2 (non-
shunted) being the forward threshold. When the SDM receives input from a SPS that state 1
threshold is reached (seat is rearward) the SDM will not disable stage 2 deployment, if required
by the deployment sensors. When state 2 threshold is reached (seat is forward) the SDM will
disable stage 2 deployment on the side the seat is forward. The SDM monitors the SPS circuit
and if a fault is detected the SDM will set code B0079 and defaults to disabling stage 2 frontal
deployment.

Inflatable Restraint Passenger Seat Belt Tension Sensor

The seat belt tension sensor is used to enhance the Passenger Presence System (PPS) when an
infant car seat is properly restrained on the front outboard passenger seat. The seat belt tension
sensor is a 3-wire potentiometer mounted on the lower seat belt anchor and provides an input to
the PPS module. When an infant car seat is properly restrained on the front passenger seat, the
seat belt is tightly secured through the infant car seat. The seat belt pulls on the tension sensor and
changes the voltage signal to the PPS module. The PPS module uses the voltage signal to help
determine if a tightly belted infant car seat is installed. The PPS uses the inputs from the seat belt
tension sensor and the PPS pressure sensor to determine if the instrument panel (I/P) module
should be suppressed or enabled. The PPS monitors the seat belt tension sensor circuits and sets
DTC B0071 if a fault is detected.

Inflatable Restraint Vehicle Rollover Sensor

The vehicle rollover sensor is used to supplement the side Supplemental Inflatable Restraint
(SIR) System. The sensing and diagnostic module (SDM) uses the input from the vehicle rollover
sensor to assist in determining the severity of a vehicle rollover or near rollover condition. If the
SDM determines a deployment is warranted, the SDM will cause current to flow through the
deployment loops deploying the inflatable restraint roof rail modules.

AIR BAG Indicator

The AIR BAG indicator, located on the instrument panel cluster (IPC) is used to notify the driver
of Supplemental Inflatable Restraint (SIR) System malfunctions and to verify that the sensing and

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2008 RESTRAINTS Supplemental Inflatable Restraints - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

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diagnostic module (SDM) is communicating with the IPC. When the ignition is turned ON, the
SDM is supplied with battery voltage and requests the IPC to flash the AIR BAG indicator 7
times. While flashing the indicator, the SDM conducts test on all SIR system components and
circuits. If no malfunctions are detected the SDM will communicate with the IPC through the
serial data circuit and request the IPC to turn the AIR BAG indicator OFF. The SDM provides
continuous monitoring of the air bag circuits by conducting a sequence of checks. If a malfunction
is detected the SDM will store a diagnostic trouble code (DTC) and request the IPC to turn the
AIR BAG indicator ON. The presence of a SIR system malfunction could result in non-
deployment of the air bags. The AIR BAG indicator will remain ON until the malfunction has
been repaired.

Inflatable Restraint PASSENGER AIR BAG ON/OFF Indicator

The PASSENGER AIR BAG ON/OFF indicator located on the overhead console is used to
notify the driver when the Passenger Presence System (PPS) has enabled or disabled the
instrument panel (I/P) inflator module. The PPS air bag indicators will also inform the driver of
any PPS malfunctions. When the ignition is turned on, the SDM is supplied with battery voltage
and commands both PASSENGER AIR BAG ON/OFF indicators ON for 5 seconds. The PPS
module conducts tests on the PPS components and circuits while both ON/OFF indicators are
ON. If no malfunctions are detected the PPS module will turn the PASSENGER AIR BAG
indicator ON or OFF depending on the status of the PPS. If a malfunction is detected, the PPS
module will store a diagnostic trouble code (DTC), default the PPS to the OFF state and
communicate with the sensing and diagnostic module (SDM) that a DTC has been set. When the
SDM detects that the PPS has set a DTC, the SDM will set either DTC B0056 or B0081 and
request the instrument panel cluster (IPC) to turn the AIR BAG indicator located on the IPC ON.
This is done to notify the driver of any PPS malfunctions. The presence of a Supplemental
Inflatable Restraint (SIR) System malfunction could result in non-deployment of the air bags. The
AIR BAG indicator will remain ON until the malfunction has been repaired.

Dual Stage Inflator Modules

Dual stage inflator modules contain a housing, inflatable air bag, 2 initiating devices, canister of
gas generating material and, in some cases, stored compressed gas. The 2 initiators are part of the
inflator module deployment loop. The inflator modules have 2 stages of deployment, which varies
the amount of restraint to the occupant according to the collision severity. For moderate frontal
collisions the inflator modules deploy at less than full deployment which consists of stage 1 of the
inflator module. For more severe frontal collisions a full deployment is initiated which consists of
stage 1 and stage 2 of the inflator module. When the vehicle is involved in a collision of sufficient
force, the sensing and diagnostic module (SDM) will cause current to flow through the
deployment loops to the initiator. Current passing through the initiator ignites the material in the
canister producing a rapid generation of gas and the release of compressed gas, if present. The gas

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