Restraints and Safety Systems: Description and Operation

SUPPLEMENTARY RESTRAINT SYSTEM (SRS)

DESCRIPTION

General
The Supplementary Restraint System (SRS) is controlled by the SRS Diagnostic Control Unit (DCU) which is mounted on the transmission tunnel, to the rear of the gear selector lever. The SRS DCU registers front and rear impacts via an integral accelerometer. Side impacts are registered via two side impact sensors; one mounted on the base of the RH B/C post, one mounted on the base of the LH B/C post.

The drivers and passenger airbags can be deployed in one of two stages. A lesser impact will result in stage 1 deployment. Both the drivers airbag and the passenger airbag contain two squibs. Stage 1 entails the SRS DCU delaying deployment of the second squib for a split second. This results in a less severe deployment of the airbags. A larger impact will result in stage 2 deployment. This entails both squibs within the drivers and passenger airbags being deployed simultaneously.

Front head airbags are also mounted in the 'A' post. These work in conjunction with the door mounted airbags. Rear head airbags, mounted in the '0' post are available as an option.

OPERATION

Power Distribution
Feed from the positive battery terminal (C0192) is supplied to fuse 53 of the passenger compartment fuse box (C0632) on an R wire. Fuse 53 (C0583) is connected to the ignition switch (C0028 on V8 vehicles) by an R wire. When the ignition switch is turned to the 'auxiliary' position, current flows across the switch (C0028 on V8 vehicles) to fuse 44 of the passenger compartment fuse box (C0585) on a PB wire. Fuse 44 (C0586) provides an auxiliary ignition feed to the following:
- The SRS DCU (C0256) on a PU wire.
- The LH seat belt buckle switch (C0100) on a PG wire.
- The RH seat belt buckle switch (C0260) on a PG wire.
- The occupancy detector sensor (C0962) on a PG wire

Drivers Airbag
The drivers airbag can deploy in one of two stages, depending upon the severity of impact. If a less severe impact is detected, stage 1 deployment is carried out. The SRS DCU (C0256) provides a firing signal to the drivers airbag (C1254) via the rotary coupler (C0082) on a YB then S wire. The SRS DCU (C0256) provides an earth path for the drivers airbag (C1254) via the rotary coupler (C0082) on an LG then YN wire.

A split second later, the SRS DCU (C0256) provides a second firing signal to the drivers airbag (C1254) via the rotary coupler (C0082) on an SN then U wire. The SRS DCU (C0256) provides a second earth path for the drivers airbag (C1254) via the rotary coupler (C0082) on a US then SB wire.

When stage two deployment is required, the SRS DCU provides simultaneous firing signals to both squibs as described previously.

Passenger Airbag
The passenger airbag can deploy in one of two stages, depending upon the severity of impact. If a less severe impact is detected, stage 1 deployment is carried out. The SRS DCU (C0256) provides a firing signal to the passenger airbag (C0251) on a UN wire. The SRS DCU (C2056) provides an earth path for the passenger airbag (C0251) on a UB wire.

A split second later, the SRS DCU (C0256) provides a second firing signal to the passenger airbag (C0251) on a WB wire. The SRS DCU (C0256) provides an earth path for the passenger ail-bay (C0251) on a WN wire.

When stage two deployment is required, the SRS DCU provides simultaneous firing signals to both squibs as described previously.

Door Airbags
The SRS DCU registers a side impact via the side impact sensors, which are mounted at the base of the LH and RH B/C posts. The SRS DCU monitors the condition of the RH sensor (C0951) and the LH sensor (C0950) on YR and YG wires respectively. Both sensors are earthed on NB wires. When the SRS DCU registers a side impact above its threshold level, it will fire the relevant door airbag.

The SRS DCU (C0256) provides a firing signal to the RH door airbag (C1499) on a U wire. An earth path is provided by the SRS DCU (C0256) on an N wire.

The SRS DCU (C0256) provides a firing signal to the LH door airbag (C1499) on an N wire. An earth path is provided by the SRS DCU (C0256) on a Y wire.

Head Airbags
The SRS DCU registers a side impact via the side impact sensors, which are mounted at the base of the LH and RH B/C posts. The SRS DCU monitors the condition of the RH sensor (C0951) and the LH sensor (C0950) on YR and YG wires respectively. Both sensors are earthed on NB wires. When the SRS DCU registers a side impact above its threshold level, it will fire the relevant head airbag.

LH Front
The SRS DCU (C0256) provides a firing signal to the LH front head airbag (C1477) on a UY wire. An earth path is provided by the SRS DCU (C0256) on a Y wire.

RH Front
The SRS DCU (C0256) provides a firing signal to the RH front head airbag (C1478) on a W wire. An earth path is provided by the SRS DCU (C0256) on a UW wire.

LH Rear
The SRS DCU (C0256) provides a firing signal to the LH rear head airbag (C1474) on a B wire. An earth path is provided by the SRS DCU (C02.56) on a Y wire.

RH Rear
The SRS DCU (C0256) provides a firing signal to the RH rear head airbag (C0948) on a UG then UR wire. An earth path is provided by the SRS DCU (C0256) on an R then G wire.

Occupancy Detector
The occupancy detector determines if there is a passenger occupying the front seat. If the front seat passenger is not detected, the front passenger airbag, and front passenger head airbag deployment is suspended in the event of an impact. All other SRS components will operate as normal.

The occupancy detector (C0962) is provided an auxiliary ignition feed from fuse 44 of the passenger compartment fuse box (C0586) on a PG wire, and is earthed on an NB wire. The occupancy detector (C0962) provides a pulsed voltage signal to the SRS DCU (C0256) on a WR wire. The signal consists of 7 bits. A signal of '0 00 00 01' informs the SRS DCU the front passenger seat is unoccupied. A signal of '0 11 1' 01' informs the SRS DCU the front passenger seat is occupied.

Seat Belt Switches
The SRS DCU determines if the front seat belts are being used by monitoring the status of the seat belt switches. If the seat belt tongue is inserted into the buckle, the switch contacts close and battery voltage is supplied to the SRS DCU (C02.56) on UW (LH) and UP (RH) wires. If the SRS DCU detects a seat belt is not being used, it will not fire the relevant seat belt pre-tensioner.

Seat Belt Pre-tensioners
If the SRS DCU detects a front or rear impact, it will deploy the front seat belt pre-tensioners. The SRS DCU (C0256) provides a firing signal to the RH seat belt pre-tensioner (C0252) on a UN wire. An earth path is provided by the SRS DCU (C0256) on a UR wire. The SRS DCU (C0256) provides a firing signal to the LH seat belt pre-tensioner (C0254) on a YR wire. An earth path is provided by the SRS DCU (C0256) on a YN wire.

Battery Disconnection
In the event of an impact severe enough to implement stage 2 airbag deployment, a pyrotechnic charge on the positive battery terminal will disconnect the battery. The SRS DCU (C0256) provides a firing signal to the battery (C2250) on an NR then B wire. An earth path is provided by the SRS DCU (C0256) on a 6 then N wire.

Body Control Unit (BCU)
In the event of an impact severe enough to trigger the SRS system, the SRS DCU (C0256) provides a pulsed voltage signal to the BCU (C0661) on an SU wire. The BCU then enters its 'crash' mode, unlocking all the doors, illuminating the interior lamps, and powering the hazard warning lamps.

SRS Warning Lamp
The SRS DCU (C0256) controls operation of the SRS warning lamp (C0230) by providing a pulsed voltage feed on an NP wire.