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Tel 01952 247007 for car electrics breakdown callout to Telford , Shrewsbury ,& Shropshire areas. www.autoelectrics.net
Why fit a car alarm ?
A car is often one of the most valued possessions owned. Car alarm systems are fitted to discourage unauthorised tampering
or theft from a vehicle.
Alarms provide an audible and possibly visual warning when activated.
Many manufacturers now incorporate a factory fitted alarm as standard.
Types of protection
The vehicle alarm may offer Perimetric protection or perimeter activation, this means the alarm control unit is wired into
micro switches on doors,bonnet,boot,fuel flap, etc . Often the same switches used for the courtesy light , so the alarm will
sense when a door etc is opened.
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.
A glass breakage detector may be fitted.
Tilt switches (inclination sensor) which will set the alarm off if the vehicle is raised, eg by a tow truck ,or by somebody
jacking the car up to remove wheels, can be fitted.
A panic function may also exist allowing the alarm siren to be activated by pressing a button.
Passive arming
The alarm system may be switched on automatically when the
occupants leave the car , this is referred to as Passive arming.
Immobiliser function
A vehicle immobiliser is designed to prevent unauthorised starting and driving of a vehicle. The Imobiliser may be wired to
the vehicle alarm system so that the alarm siren & lights flash if activated.
Vehicle immobilisers 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 Immobilisation
This type of immobiliser is integrated into vehicle electronic system control units during manufacture.
A coded signal is required before certain functions will operate.
There are three common types of this system . The first type uses a remote control fob (often known as a 'plip') which
requires the user to press a button to send a coded radio or infrared signal to a receiver unit mounted inside the vehicle
which then passes a code to the engine 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.
The second type is a coded key system where the ignition key contains a coded device which is read as the key is inserted
into the ignition barrel (transponder key).
If the correct coded key is used a data signal is sent to
the relevant vehicle control unit to allow 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 start up.
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.
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
or immobiliser.
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 battery fitted to provide power to the system should the vehicle starter battery be
disconnected.
Central door locking function
A central door locking system allows 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.
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.
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.
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;
Lack of fuel,discharged starter battery,faulty charging system,faulty starter motor, wiring problems,engine management
or fuel injection component failure, mechanical faults,to name a few which can lead to a disabled vehicle.
Computer diagnostics of manufacturers integrated 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 test may reveal stored fault
codes which can prevent engine starting, eg 'key code not recognised','ecu function blocked by immobiliser','fuel system pressure
sensor fault',etc.
Function tests of door locks and switches etc may be completed using computerised equipment.
Mechanical Anti theft devices
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 wheel covers, lock bars, brake and clutch pedal locking bars,handbrake locks,wheel clamps, fuel
system shut off devices, driveway bollards etc.
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