Vehicle alarms may also offer interior protection or volumetric security. Volumetric protection uses ultrasonic or microwave
detectors which can detect changes to the air volume within the vehicle cabin
as a person moves.
Tilt switches (inclination sensor)
If the vehicle is raised, eg by a tow truck ,or by
somebody jacking the car up to remove wheels, a tilt
sensor will activate the alarm
A push button may be fitted allowing the alarm siren to be activated by pressing
the button in an emergency. The panic button may also
activate emergency vehicle tracking devices on certain
The alarm system may be switched on automatically when the occupants leave the car , this is referred to as Passive arming.
A vehicle immobiliser is designed to prevent unauthorised starting and driving of a vehicle. The Immobilizer may be wired to
the vehicle alarm system so that the alarm siren & lights flash if activated.
Vehicle immobilizers exist in a variety of types:
Electrical Circuit Isolation
This type of immobiliser is designed to physically interrupt various vehicle circuits which will prevent the vehicle engine
from starting. Aftermarket alarm and immobiliser units often offer this type of protection.
Vehicle Electronic Control Unit
This type of
"factory fitted" immobiliser is integrated into vehicle electronic system
computer control units during manufacture disabling
ignition or fuel injection functions of engine
management computer to prevent vehicle operation. A coded signal is required before certain functions will operate.
Faults occuring with this type of system may require
touch key Immobiliser
featuring Automatic arming Two circuit
Immobilisation PIN code over ride
Illuminated touch key receptacle status indicator
Anti tamper Security housing
The touch key system is self
arming when the vehicle ignition is switched off. The
active immobiliser function isolates various circuits to
prevent the vehicle from being started by unauthorised
persons. To deactivate the vehicle immobiliser a special
touch key is placed against a dash mounted receptacle.
This type uses a remote control fob
transmitter (often known as a 'plip') which
requires the user to press a button to send a coded radio
frequency rf signal to a receiver unit mounted inside the vehicle
which then passes a data code to the control computer. On receiving the correct code the computer allows the engine to be started when the ignition key is used.
If an incorrect code is received certain functions are disabled preventing startup.
||Autowatch 172 Ri
featuring Encrypted remote Control Self arming Immobilisation
to prevent unauthorised vehicle startup PIN code over ride
Central locking output
Illuminated touch key receptacle status indicator
Anti tamper Security housing
Indicator / Light flashing
System upgrades available to full alarm with battery
Transponder Vehicle Keys
This type is a coded key system where the ignition key contains a coded device
or microchip which is read as the key is inserted into
the ignition barrel (the ignition barrel is surrounded
by a pick up coil)
If the correct coded key is used an encrypted data signal is sent to the relevant vehicle control unit to authorise full function.
If an incorrect key is used ,either with the wrong code or without a code chip , the vehicle control unit will disable various functions to prevent
unauthorised start up.
New keys may be
added to the system or keys replaced or deleted but may
require a security code to allow programming of keys.
programming was once a main dealer only job but there is
now equipment available with key programming functions
available to independent vehicle repairers and
instances the vehicle manufacturer may provide an
alternative method to bypass the vehicle
immobiliser for emergency starting if transponder keys
or remotes have been lost . EKA Emergency Key Access
details will often be advised within the vehicle
Key Code Access pad
A further variation of the coded immobiliser system uses a keypad which requires the driver to enter a secret code by
pressing the keypad buttons.
The key code access system is particularly useful in
cases where vehicle keys are stolen because the vehicle
still needs entry of a pin code to deactivate the
Thatcham Security Rating
The Motor Industry Repair Research Centre at Thatcham,Berkshire,UK, carries out testing of both new vehicle security features
and provides a rating system for vehicle alarms and immobilisers.
Link to Thatcham vehicle security page at www.thatcham.org
Insurance Approved Alarm / Immobiliser System
Usually this term applies to immobiliser systems which meet one of the thatcham category specifications and have a thatcham
approval number. Some insurance companies will provide discount on the insurance cost if your vehicle is fitted with an alarm
Alarm arming activation
The alarm may be armed or activated by means of a door lock switch, or a remote control device.
Remote controls used infrared (IR) or radio frequency
signals. The remote control sends a coded signal to the alarm control unit to activate. Rolling code or random encrypted
code technology may be used to prevent this code being picked up by a scanning device . The rolling code will change each
time the alarm is activated or deactivated.
A problem can occur if either the remote control fob or vehicle battery fails causing the code synchronisation between
remote transmitter and control unit receiver to be lost. This may require a defined procedure to resynchronise devices.
Battery back up
The alarm unit may have an additional
internal battery fitted to provide power to the alarm system should the vehicle starter battery be
Central door locking function
Central door locking systems allow all vehicle doors to be locked from one point.
Vehicle locks are moved from the lock to unlock position and vice versa by motors or solenoids fitted within each door.
Aftermarket central door lock (CDL) motors are either two wire or five wire types. The 2 wire type operates on a power
supply & ground to lock and connection reversed to unlock.
The 5 wire central door lock actuator also contains switches to activate / deactivate the system.
A typical 4 door power lock system would use 5 wire actuators on the front doors , and 2 wire actuators on the rear doors.
A further development of central door locking is deadlocking or double locking.
Double locking uses the CDL motors to disconnect the door lock linkage from the lock preventing an intruder from opening
the vehicle door by smashing a window and pulling the lock button from inside the vehicle.
Shielding plates may also be fitted to further increase security & prevent access to lock linkages & motors.
Some car alarm systems may be wired into a central door lock module to enable remote locking.
Full closure alarm system
A total closure alarm system may be connected to both central door locking and electric windows / sunroof. The full closure
function may close all windows and door locks when activated.
Car stereo anti-theft system
The car alarm may also be connected to the vehicle in car entertainment system. If the entertainment system is disconnected
the vehicle alarm may be activated and also immobilise.
The car stereo may also require a code to be entered after power supply disconnection (battery removal) and reconnection
before it will operate.
If a car radio code
has been lost it may be possible to obtain the code from
the supplying dealer or have the radio decoded.
Remote Paging, Vehicle tracking and Text alert.
Vehicles may be located and position tracked using a GPS device ( similar to a mobile phone signal ) .
Top of the range car
alarm systems may provide a text alert to a designated mobile phone if the vehicle alarm is activated. Some SMS messaging alarm systems allow the owner to send an activation code from a designated
mobile telephone to disable or enable the vehicle alarm system.
systems can also offer benefits to fleet and service
companies by providing logistics data and remote asset
offer real time vehicle position location over the
internet allowing service companies to reduce labour and
Removal of alarm and vehicle immobilisation due to
If the alarm or immobiliser is to be removed or bypassed this should be agreed with your vehicle insurance company beforehand
as it may affect your policy.
Diagnosis & Troubleshooting
My car won't start , the immobiliser is faulty , the alarm siren goes off without warning.
A few reasons why an immobiliser may fail:
Remote plip batteries can become discharged.
Spurious radio interference may affect some immobiliser devices causing failure to decode.
Control units may develop faults often due to water ingress ,vibration, or component age.
Failure of wiring looms
Vehicle battery or memory back up discharges causing loss of control unit memory , often known as control unit amnesia
because the coding between remote and receiver loses synchronization. This can happen to vehicles fitted with a body control
module ( BSI module ) if the recommended battery removal procedure is not followed.
Because your vehicle refuses to start does not necessarily mean that you have an immobiliser fault.
Vehicles can fail to start for numerous reasons including; Battery failure
Fuel shortage : Fuel system component failure eg,
fuel pump etc
Fuel quality : contaminatd or incorrect fuel
Discharged starter battery due to faulty charging system, wiring
or fuel injection or ignition component failure Mechanical faults
Computer diagnostics of manufacturers integrated
factory alarm systems
Vehicle alarm & central door lock systems can be complex with a variety of switches,modules and sensors.
Diagnosing the cause of breakdown can be assisted with computerised equipment . A scan tool
diagnostic test may reveal stored fault
codes which can prevent engine starting, eg 'key code not recognised','ecu function blocked by immobiliser','fuel system pressure
Function tests of door locks and switches etc may be completed using computerised equipment
Mechanical Anti theft devices
Primary and Secondary immobilisation can be performed by the use of mechanical devices which prevent operation of vehicle controls or
movement of the vehicle.
Examples are : Steering lock
Steering wheel covers, lock bars, brake and clutch pedal locking bars,handbrake locks,wheel clamps, fuel
system shut off devices, driveway bollards etc.
Commercial Vehicle Cargo Protection
methods to protect vehicle cargo and personnel include