SRS Airbag info - From RR Workshop Manual. Section 75- page 6.
See 75 -2 and 75 - 3 for components and layout.
Looks like P38s went to 2 sensors fitted at the front (crush cans) from MY99, known as a Distributed System.
MAIN SENSOR
The main sensor is a deceleration detection device which is contained in the DCU. The sensor consists of a spring and weight system which is attached to strain gauges in a Wheatsone bridge circuit.
The ’balance’ nodes of the bridge circuit is connected to an integrated circuit that can instantly detect a change in the monitored resistance.
In the event of a collision, the spring and weight move causing a corresponding change in the resistance of the related strain gauge.
If the change in strain gauge resistance is greater than a preset value, it corresponds to a crash condition of sufficient severity to warrant SRS component deployment.
In this case, the processor provides a signal to initiate airbag and/or seatbelt pre-tensioner deployment.
Deployment will only be carried out if a confirmation signal that a crash condition is occurring is received by the SRS DCU. Crash condition confirmation is achieved by simultaneous actuation of the safing sensor and/or one or more of the front crash sensors in the case of a distributed system.
SINGLE POINT SENSED SYSTEM
This system relies on the DCU’s internal deceleration sensor and safing sensor to provide the control inputs required to confirm activation conditions for SRS component deployment.
DISTRIBUTED SRS SYSTEM
The DCU used for the distributed SRS system is identical to the single point system, with the exception that two external front crash sensors provide additional inputs to the unit for determining and confirming a crash condition in conjunction with the DCU’s internal accelerometer.
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