@Ian.V here is some info for u in how it all works, An RCM is a Restraints Control Module
The RCM is installed on the top of the transmission tunnel, in line with the B pillars, and controls operation of the
SRS. The main functions of the RCM include:
Crash detection and recording.
Air bag and pretensioner firing.
Self test and system monitoring, with status indication via the
SRS warning lamp and non volatile storage of fault information.
A safing
sensor in the RCM provides confirmation of an impact to verify if air bag and pretensioner activation is necessary. A roll-over
sensor monitors the lateral attitude of the vehicle. Various firing strategies are employed by the RCM to ensure that during an accident only the appropriate air bags and pretensioners are fired. The firing strategy used also depends on the inputs from the safety belt switches and the occupant monitoring system.
An energy reserve in the RCM ensures there is always a minimum of 150 milliseconds of stored energy available if the power supply from the ignition switch is disrupted during a crash. The stored energy is sufficient to produce firing signals for the driver air bag, the passenger air bag and the safety belt pretensioners.
When the ignition is switched on the RCM performs a self test and then performs cyclical monitoring of the system. If a fault is detected the RCM stores a related fault code and illuminates the
SRS warning indicator. The faults can be retrieved by the Land Rover approved diagnostic system on a dedicated link between the RCM and the diagnostic socket. If a fault that could cause a false fire signal is detected, the RCM disables the respective firing circuit, and keeps it disabled during a crash event.
SRS OPERATION
General
In a collision, the sudden deceleration or acceleration is measured by the safing
sensor in the RCM and by the impact sensors. The RCM evaluates the readings to determine the impact point on the vehicle and whether the deceleration/acceleration readings exceed the limits for firing any of the air bags or pretensioners. During a collision, the RCM only fires the air bags and pretensioners if the safing
sensor confirms that the data from the remote
sensor(s) indicates an impact limit has been exceeded. The RCM also monitors the vehicle for a roll-over accident using the internal roll-over
sensor and high speed CAN bus messages from the anti-lock brake system (ABS) module and the steering angle
sensor.
The RCM incorporates the following impact thresholds to cater for different accident scenarios:
Front impact, pretensioners.
Front impact, driver and passenger air bags stage 1, belt unfastened.
Front impact, driver and passenger air bags stage 2, belt unfastened.
Front impact, driver and passenger air bags stage 1, belt fastened.
Front impact, driver and passenger air bags stage 2, belt fastened.
Rear impact.
LH side impact.
RH side impact.
Roll-over.
The front impact thresholds increase in severity from pretensioners, through to driver and passenger air bag stage 2, belt fastened.
Firing Strategies
The seat belt pretensioners are fired when either the pretensioner impact limit or the roll-over limit is exceeded. The RCM only fires the pretensioners if the related safety belt is fastened. For the front passenger pretensioner to fire, the seat must also be occupied by a large person, i.e. someone over a given weight (NAS only).
The driver and passenger air bags are only fired in a frontal impact that exceeds the stage 1 threshold. Both stages of the inflator in the driver and passenger air bags are fired. At impacts between the stage 1 and 2 thresholds, the delay between the firing of the two stages varies with the severity of the impact; the more severe the impact the shorter the delay. At stage 2 impact thresholds and above, the two stages of the inflator are fired almost simultaneously. The passenger air bag is disabled unless the front passenger seat is occupied by a large person (NAS only), or the passenger air bag deactivation switch is on (all except NAS). The time delay between firing the two stages of the inflator in the driver air bag is increased if the driver seat is forward of the seat position
sensor switching point.
If there is a fault with a safety belt buckle
sensor, the RCM assumes the related safety belt is fastened for the pretensioner firing strategy and unfastened for the driver and passenger air bag firing strategies. If there is a fault with the occupant detection system, or if there is a fault with the passenger air bag deactivation switch, the RCM increase the time delay between firing the two stages of the inflator in the passenger air bag.
If a side impact limit is exceeded, the RCM fires the side air bag and the side air curtain(s) on that side of the vehicle. If the side impact limit on the front passenger side of the vehicle is exceeded, the RCM also evaluates the input from the occupant classification system, and fires the side air bag only if the front passenger seat is occupied by a large person (NAS only).
The side air curtain(s) on both sides of the vehicle are fired if the roll-over limit is exceeded.
If multiple impacts occur during a crash event, after responding to the primary impact the RCM will output the appropriate fire signals in response to any further impacts if unfired units are available.
Crash Signal
When the RCM outputs any of the fire signals, it also outputs a hard wired crash signal to the Engine Control Module (ECM) and changes the high speed CAN bus output message from 'no crash' to 'crash condition'. The high speed CAN bus message is used by the central junction box (CJB) and the FFBH (fuel fired booster heater).
On receipt of the crash signals:
The engine control module (ECM) disables the fuel pump.
Operation of the FFBH is disabled.
The CJB enters the crash mode and:
Activates all of the unlock signals of the vehicle locking system, even if the vehicle is already unlocked. After 3 seconds, the CJB activates the unlock signals again, in case a lock button is pressed during the crash, by flailing limbs for example.
Ignores all locking and superlocking inputs until the crash mode is cancelled, when it returns the locking system to normal operation.
Activates all of the courtesy lamps, except for the approach lamps. The activated courtesy lamps remain on until they are manually switched off at the lamp unit, or the CJB crash mode is cancelled, when they return to normal operation.
Activates the hazard warning lamps. The hazard warning lamps remain on until cancelled by turning the ignition switch from position II to position I or 0, or until the crash mode is cancelled.
The crash mode is cancelled by cycling the ignition switch.just for some information
