Theft deterrent systems aim to prevent the vehicle being entered, started or driven. The actuators used to achieve this are the electric door locks and windows, the starter motor relay, engine management systems, transmission shift solenoids, and an audible alarm.
The computer that controls the system monitors input signals from various devices. Switches are located at each door, the hood and trunk openings, and the fuel filler door.
Vibration sensors detect any unusual vehicle movement such as lifting, jacking or towing.
Ultrasonic sensors detect any movement inside the cabin.
Voltage monitoring sensors check against operation of the starter motor, ignition system or fuel pump.
The immobilization system is enabled by pressing the lock button on the remote key, locking the doors manually with the door key, or when a period of time has elapsed, typically 15 seconds, after the engine has stopped. The immobilization system is disabled by pressing the unlock button on the remote fob.
The key fob and theft deterrent computers have to be capable of transmitting and receiving coded information. Operational characteristics vary greatly between manufacturers, and because it is a theft deterrent system, details are a close kept secret.
Most systems use a coded system known as “rolling codes”. The coded data transmitted and received between the key fob and theft deterrent computer randomly changes. This creates a situation where the code used to lock and immobilize a vehicle is different to the one used to unlock and mobilize a vehicle. Manufacturers who use this system have seen a dramatic reduction in theft, noticeably from joy riders. The code is transmitted as a digital number, typically containing 16 digits, giving the possibility of billions of different code numbers. When the lock button is pressed, the control unit and key preset the agreed code, which will be used to unlock the vehicle and deactivate the theft deterrent.
Pressing any button on the remote fob transmits an RF signal, even if the vehicle cannot read it, and changes the rolling code unit from that exchanged between the control module and the fob. The system allows an error factor of 25 steps either side of the agreed code for deactivation before the fob transmits a “False Code”, where the control module will disregard the key.
