Alarm – Design

Design and Function

Central electronic module (CEM)

The central electronic module (CEM) (4/56) is the master unit for the alarm.

The control module communicates with directly connected units, via the LIN and the controller area network (CAN). The control module has a number of sensors connected to it and determines what actions should be taken.

Switches in the lock units

A contact breaker is mounted in the lock units on the side doors and trunk lid/tailgate to indicate if any of them are open or closed. On vehicles with an alarm there is a single switch on the hood. The switches are closed when the door or tailgate/trunk lid is shut.

When the alarm is activated, the central electronic module (CEM) checks every 6 times / second that the switches are closed. If the central electronic module (CEM) detects that a switch is open the alarm is triggered.

If the central electronic module (CEM) detects that a switch is not closed when a lock command for locking the central locking system and alarm activation is received, the vehicle tilt sensor and movement sensor will not be activated. When the last door or hatch is closed, all alarm functions are activated, including movement sensor/vehicle tilt sensor after the initial time delay of 11 seconds.

Siren

The siren is used as a sound source for the alarm function. The siren is mounted in the right front wheel housing, above/behind the wheel. The siren is available with or without an internal vehicle tilt sensor. The type of siren mounted in the vehicle is market and customer dependant.

The siren sounds if the power supply is cut or if ground is cut or if communication with the central electronic module (CEM) fails while the alarm is activated. The siren contains an internal battery which is charged when the ignition is on. The battery cannot be replaced by itself. The alarm cause can be read from the diagnostic tool. This states what caused the alarm.

If the vehicle is equipped with a siren with a vehicle tilt sensor, the sensor is integrated into the siren. The vehicle tilt sensor reacts to changes in the angle of lean or motion of the vehicle and transmits a signal to the central electronic module (CEM). If the vehicle tilt sensor indicates that the vehicle is being lifted the central electronic module (CEM) must send an activation signal to the siren. In this case the siren cannot trigger an alarm by itself.

The siren is powered via a fuse from the central electronic module (CEM) The siren is grounded in the engine compartment.

Movement sensor (IMS)

The movement sensor (IMS) is located in the overhead console (OHC). The movement sensor (IMS) is used to detect movement in the passenger compartment when the alarm is activated.

When the alarm is activated, the central electronic module (CEM) checks the sensor.

The sensor communicates serially with the central electronic module (CEM).

When the sensor is activated it sends out ultrasound into the passenger compartment. The sound waves are reflected back to the sensor by the vehicle interior and if the sensor detects change in reflections or movements in the passenger compartment then the alarm is triggered. The sensor is also sensitive to air turbulence and therefore all windows and roof hatch, if appropriate, must be closed so that the sensor shall function optimally without the risk of false alarms.

Lock and alarm LED.

The lock and alarm LED is located on top of the dashboard. The LED is in the same housing as the sun sensor and twilight sensor. The LED is directly connected to the central electronic module (CEM). It is used to indicate alarm status.

When the alarm is activated, the LED will flash approximately once every two seconds.

If the alarm was triggered during the alarm cycle, the LED will flash rapidly, about 3 times per second, at unlocking. This continues until the remote control key is inserted into the starter unit or the starter button is pressed (only applies to vehicles with keyless locking system installed).

Vehicles without alarm will still be equipped with lock and alarm LED. For these vehicles the LED will be used to indicate lock status.

Replacement of Siren control module (SCM) via the normal horn

If the alarm system indicates break-in normally the Central electronic module (CEM) shall request activation of siren control module (SCM). If this cannot be done due to, e.g., disconnected siren control module (SCM), then the Central electronic module (CEM) activates the normal horn instead as replacement of siren control module (SCM).

Remote key/Volvo Personal Communicator

The remote control key is used to activate and deactivate the alarm. The vehicle's alarm is fully activated (provided that the function for reduced alarm has not been activated) approximately 11 seconds after the vehicle has been locked from the outside or via automatic alarming (for Belgium). The alarm is deactivated when the vehicle is unlocked. There is a button on the remote control key for triggering the panic alarm function in order to attract attention in an emergency situation.