With motor vehicles the internal combustion engine is started using a battery-fed electrical machine known as the starter, with the electrical circuit of the starter able to be completed by an ignition switch operated by an ignition key. The ignition lock is connected to the plus pole of the battery, with the circuit being routed onwards from the ignition lock to the solenoid of the starter and from there to the minus pole of the battery. In a normal operating mode the circuit can only be completed when the ignition key suitable for the relevant ignition lock is available. With an intended theft of the motor vehicle, breaking into the vehicle is in many cases followed by short circuiting the starter switch (or the ignition lock) so that the starter can then be operated. To prevent this from being done many anti-theft systems are offered which are designed to prevent unauthorized starting of the motor vehicle. Such anti-theft systems are also referred to as immobilizers.
With a very simple embodiment of such an immobilizer an interrupting switch arranged in the circuit is used, located at a position in the vehicle known only to the driver. The disadvantages such a device lie in the fact that on the one hand in many cases the driver often forgets to activate the switch and on the other hand that a potential thief very frequently knows where such interrupting switches are installed. In both cases this means that this form of anti-theft device is ineffective.
In German Patent DE 43 30 733 C1 a further simple anti-theft device in a motor vehicle is described via which the motor vehicle can be put into a drivable state using resistive encoding. Resistive including in this case consists of resistors which are arranged in load lines at the ignition lock in a mechanically defined sequence. The evaluation of the drop in voltage at the resistance encodings allows any unintentional short-circuiting of the motor vehicle to be identified. A disadvantage of the arrangement described in DE 43 30 733 C1 is that it only allows the situation to be detected of whether the voltage drop measured at the relevant resistors deviates from the expected voltage drop. With the arrangement described in DE 43 30 733 A1 it is not however possible to establish whether the incident involved was actually an attempt at unauthorized short-circuiting (theft) or a fault in the lines or in the power supply, for example with a short circuit or open circuit. A diagnosis of this fault source compared to a theft is thus not possible.
Modern anti-theft devices in motor vehicles use electronic immobilizers, for example with transponders. With such electronic immobilizers there is data communication between a transceiver arranged in the motor vehicle and a transponder arranged for example in a key or a key fob of the vehicle user. Before the motor vehicle is put into use there is first an exchange of encoded data which ensures that the vehicle use is authorized, by the vehicle owner for example.
Such electronic anti-theft systems are however expensive and thus cost-intensive. Despite this such anti-theft systems are required in many countries for insurance reasons. In other countries in which these insurance conditions to not exist motor vehicles are frequently not fitted with such electronic anti-theft systems. There manipulation of the first switch and/or resistance network.