Design

Design

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.

Door open sensor




The door open sensor is a switch mounted in the lock units of the side doors and the tailgate/trunk lid. 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.
The C30 and C70 do not have door open sensors for the rear side doors as they do not have any rear doors.
When the alarm is activated, the central electronic module (CEM) checks every 3 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 the central locking and alarm activation command is received, the vehicle tilt sensor and mass movement sensor module (MMS) will not be activated. Similarly, no attention will be paid to signals from a door open sensor not closed on activation.

Siren control module (SCM)




The siren is used as a sound source for the alarm function. The siren is mounted under the plenum on the passenger side. The siren is available in left-hand and right-hand drive versions, 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 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. This battery has a capacity for 10 alarm cycles. The battery service life is limited and depends on the local climate. The siren must be replaced regularly. The battery cannot be replaced by itself. The alarm cause can be read from VADIS. 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 of the vehicle and transmits a signal to the central electronic module (CEM). The aim is for the alarm to be triggered if somebody attempts to steal a wheel for example.
While the vehicle tilt sensor is mounted in the siren it is counted as a separate unit. If the vehicle tilt sensor indicates that the vehicle is about to be raised, the vehicle tilt sensor sends a signal to the Central electronic module (CEM), which in turn, transmits an activation signal to the Siren control module (SCM). The Siren control module (SCM) 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.

Mass movement sensor module (MMS)




The mass movement sensor module (MMS) is located in the roof under its own panels on the S40/V50 and C30.
There is a Mass movement sensor module (MMS) in the S40/C30. The sensor is located in the middle of the roof.
There are two sensors in the V50. On is centrally located on the roof and the other in the roof above the cargo compartment.
There are two Mass movement sensor modules (MMS) in the C70. One is located under the cup holder in the center console and the other under the central rear seat.
The mass movement sensor module (MMS) is used to detect movement in the passenger compartment when the alarm is activated. The aim is to detect if somebody attempts to reach inside the vehicle to steal bag or similar.
When the alarm is activated, the central electronic module (CEM) checks the sensors.
The sensors communicate serially with the central electronic module (CEM). If two sensors are installed they are connected in parallel to the central electronic module (CEM).
The sensor that triggered the alarm can be detected by the Central electronic module (CEM) by differences in the data messages that are received from each Mass movement sensor module (MMS).
When the sensors are activated they transmit microwaves into the vehicle. The microwaves bounce back from the interior of the vehicle and the sensors react to changes in these reflected waves.
The Mass movement sensor module (MMS) functions a slightly different way in the C70. The sensors work as well with the roof open or closed.
If the roof is open, there is no surface that the microwaves can directly bounce off. However, the waves spread in the air and eventually bounce back to the object that is further away, for example, the roof of a car park or a tree that the vehicle is parked under.
To reduce he risk of a false alarm in this mode, the sensor measures the time from when it sent a pulse until it receives the same pulse back. If the reflection time is too long, i.e. the pulse has bounced on something further away than 1.5 meters, ( 4.92 feet) upwards from the sensor, the sensor will refer to the pulse changes.
On the other hand, if the time is within specified values, the pulse changes are interpreted as a movement in the vehicle and the alarm will be triggered.
Using microwaves makes the system more robust and avoids false alarms. Microwaves cannot be changed by different temperatures in the layers of air in a vehicle left in the sun for example.
The sensors are powered and grounded via the central electronic module (CEM).

Alarm LED




The 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 central electronic module (CEM) checks all units and sensors. If everything is OK the LED starts to flash once a second.
If the alarm is triggered the LED starts to flash quickly, approximately 6 times a second. When the vehicle is unlocked, the LED continues to flash until the ignition key is inserted into the ignition switch.

Switch for reduced alarm




The switch for reduced alarm is in the front lighting panel in the roof. The switch is only found in vehicles with deadlocks for central locking and/or an alarm system that consists of sensors (i.e. level above perimeter guard).
The switch is used to temporarily deactivate the vehicle tilt sensor and Mass movement sensor module (MMS) functions. When the function is activated the doors are not deadlocked when the vehicle is locked. The function can only be activated when the ignition has been switched off and up to a minute after the ignition key has been taken out of the ignition switch. If a longer time has passed the ignition must be turned to position II and back to 0 before the function can be activated.
The switch has an integrated LED to indicate when the function is activated. A message is displayed in the display in the driver information module (DIM).

Glass breakage loop (only C30)




On the C30 there is a glass breakage loop installed in the rear window if the vehicle is equipped with a Mass movement sensor module (MMS). In the cable harness for the tailgate there is also a cable that is designed as manipulation protection for the glass breakage loop. Both wires are directly connected to the Central electronic module (CEM) and are monitored when the alarm is activated.
The glass breakage loop is grounded to the body. When the alarm is activated, the central electronic module (CEM) checks that the glass wires are still intact at regular intervals. If the control module detects a break in the wire, the control module will trigger the alarm.
The manipulation wires are used to make grounding the cables in the cable harness for the tailgate more difficult, thus attempting to by-pass the glass breakage loop in the rear window.
The wires are not supplied with voltage and are only connected to the Central electronic module (CEM) at one end. The Central electronic module (CEM) monitors the cable when the alarm is activated and detects if the cable is to be grounded to trigger the alarm.