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Anti-theft Systems, Page 2 of 2

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How anti-theft systems work

With the widespread and increasing rate of auto theft, automotive anti-theft systems have come into general use. Many auto manufacturers now offer anti-theft or alarm systems as optional equipment. In addition, there are literally dozens of after-market suppliers who manufacture these systems.

Most systems can be installed with hand tools in a few hours, but more complicated systems are best left to professionals.

There are two basic types of automotive anti-theft systems: alarm systems and movement inhibitors.

Alarm systems
See Figure 6

There are two basic types of burglar alarm systems for automobiles: those which actuate the vehicle horn, and those which set off an auxiliary siren or bell. Regardless of which type it is, each one can be broken down into its separate components: the trigger, trigger control and the alarm itself.

Figure 6 Alarm systems cover all entry points of the vehicle.
Alarm systems cover all entry points of the vehicle.

Trigger mechanisms

The trigger mechanism is the device used to activate the alarm. In most cases, the trigger consists of a switch or switches, and a drop relay. A drop relay is a relay that, once activated, will not recycle until reset manually; therefore, the alarm will not stop functioning even if the trigger switch is deactivated.

Motion sensitive switches such as mercury switches, pendulum switches, etc. are excellent means of detecting tampering. The switch may be mounted anywhere in the vehicle, and its sensitivity adjusted to the desired level. The disadvantage of a motion-sensitive switch is the accuracy with which it must be adjusted. The switch must respond to the opening of a door, the hood or trunk, but not to such things as parking on an incline, being bumped by a pedestrian, or traffic passing by. Once the proper sensitivity is determined, it is a good idea to include a timer in the trigger circuit, to shut the alarm off after a certain period if it is accidentally triggered.

Pushbutton switches (such as interior light doorjamb switches) mounted on all doors and the hood and trunk may be used to trigger an alarm. These spring-loaded, normally closed switches may be positioned adjacent to the existing switches on the doorjambs and on the hood and trunk latch plates. A combination system of motion sensitive and pushbutton switches will provide excellent protection. Mercury switches used to activate hood and trunk lights may be used as triggers, in lieu of pushbutton switches, on the hood and trunk.

Trigger control switches

The trigger control switch acts as an on-off switch for the alarm system. It must be arranged in such a manner so that the owner may enter the vehicle without triggering the alarm, but a thief must be unable to detect or disarm it.

The simplest type of control switch is a toggle switch mounted outside the vehicle in an inconspicuous place. Inside the fender well or under the rocker panel are two typical places that this type of switch is mounted. The only drawbacks to this type of switch are that the switch and wiring must be waterproofed, and that someone may find the switch and deactivate the alarm. The most popular type of switch is the locking type that may be mounted anywhere on the outside of the vehicle. These switches use cylindrical "pickproof" locks, and provide excellent protection, in addition to acting as a visual deterrent.

See Figure 7

An alarm may be devised as an integral part of the electrical system to set off the horn, or the warning system may contain its own alarm. To connect an alarm system utilizing the vehicle horn, proceed as follows: Locate the terminal on the horn relay that will sound the horn when it is bridged to ground. Connect one lead from the on-off switch to this terminal. Connect the other lead from the on-off switch to a pulsating (flasher-type) terminal. Connect the open terminal of the pulsating switch to the trigger mechanism. Now firmly ground the trigger mechanism.

Non-integral, self-contained alarm systems may be connected to the accessory position in the fuse box. The alarm itself (siren, bell, buzzer, etc.) must be loud enough to attract attention at a reasonable distance, and should be positioned somewhere where full advantage can be taken of its capabilities (such as behind the grill). A drop relay and/or a timer should be installed somewhere in the circuit. The drop relay will keep the alarm activated, even after the trigger is deactivated, unless all current is removed from the alarm circuit. The timer is used to deactivate the alarm a certain period after the trigger is deactivated, to prevent the alarm running down the battery, and to prevent disturbing the peace after accidental triggering.

One of the best things you can do after you install one of these alarm systems is to mount the "protected by alarm" sticker that comes with the system. A casual thief seeing this sticker isn't going to stick around to see if it's telling the truth or not.

Figure 7 Schematic of simple alarm circuit using the vehicle horn.
 Schematic of simple alarm circuit using the vehicle horn.

Typical factory alarm systems

Movement inhibitors

See Figures 9 and 10

The most common systems available inhibit the movement of the brake pedal and/or the steering wheel. Of these, the most prevalent (best known by its trade name- The Club®) utilizes a long steel bar that hooks and locks onto the steering wheel, and prevents it from turning beyond a certain point by wedging against interior components. A similar system is a locking, telescoping steel bar, with a hook at each end. In use, one hook is positioned around a steering wheel spoke and the other around the brake pedal arm. The steel shaft is then telescoped down and locked into position, preventing movement of the brake pedal and limiting movement of the steering wheel.

Both of these devices have the advantage of being easily visible from outside the vehicle, thereby acting as a visual deterrent. Their main disadvantage is that they are somewhat awkward and must be removed and installed each time that vehicle is moved.

Because they are relatively easy to steal and have a high "street" value, airbags are fast becoming the accessory of choice for car thieves. It has been estimated that as much as 10% of all vehicle theft claims last year were for airbags. Shield type devices are available to adjust to fit most steering wheels, one-, two-, and three-spoke. These are used in conjunction with a Club® type device. These devices also serve another important purpose, although The Club® type devices are usually hardened steel which makes them difficult to cut, steering wheels left unprotected cut like butter. These shields cover and protect the steering wheel from being cut and The Club® type device removed.

Figure 9 Typical Club®-type steering wheel wedge bar installed.
 Typical Club& type steering wheel wedge bar installed.

Figure 10 Krooklock®-type movement inhibitor.
 Krooklock& type movement inhibitor.


Electronic vehicle immobilizers are relatively new on the market. They are available with different levels of protection-a one level interrupt version that interrupts either the ignition or the starter; a two level system that may incorporate the visual deterrence of "The Club"® type device with the one level version; a three level version which will interrupt the ignition, starter and the fuel system; and a four level version which uses "The Club"® device with the three level interrupt unit.

Tracking systems

A system has been developed to track a stolen vehicle. Services are available through dealers and some private installation shops which will install an aftermarket tracking device to your car or truck. If you report that your vehicle has been stolen, a signal from a police radio tower activates the transmitter in your vehicle. The hidden transmitter in your vehicle then broadcasts a silent, coded signal. The police tracking computer receives the signal that identifies your vehicle and leads them to it.

Accessories and theft

Identify your equipment
See Figure 11

Once the stereo is stolen, it's not lost and gone, forever, if you take certain precautions. Many stolen sets are recovered, but the tragedy is that the owner cannot positively identify the set, or the police cannot trace the owner through the serial number because the owner did not send in the warranty card. The "That's my set! I recognize that little nick on the front," line just doesn't work unless you report the identifying features beforehand. In a move to ease stolen stereo identification, the FCC, since the late 70's, has required manufacturers to engrave the serial number or other unique identifying number on the chassis of the set.

Police are encouraging people to engrave an identifying number on the stereo, or other component and will often supply the engraver free of charge. Your social security number should not be used for this purpose, because the social security office in Washington will not release the name and address of the social security number's owner-not even to the police. Use your driver's license number, your name and address, or some other number that can be easily and officially traced to you and no one else.

There are also national computer registration programs, which for a set one-time fee provide an identifying number (guaranteed yours and yours alone) and a complete kit for engraving it. The number is registered with the computer service. Police can easily trace the number to you through a toll-free telephone number to the computer service. No matter what number you use, be sure you have a copy of it, and be sure you can prove the number is used only by you. It is also a good idea to register it with the local police.

Figure 11 Engrave your driver's license number or another unique number onto your high-theft-risk items.
Engrave your driver's license or other unique number onto your high theft items.

Safety precautions

Most thieves work fast. Once inside a vehicle, prying at the stereo with a stout screwdriver usually frees it in seconds. A couple of snips with the side cutters and the thief is on his way. Because speed is of the essence, anything that will slow a thief down may be a deterrent. Alarms and mounting brackets are sometimes useful, but alarms can be disabled in seconds (by a professional) and locking mounting brackets are generally pried loose with a stout crowbar, tearing up your dash in the process. Locking barrels over the mounting nuts offer approximately the same resistance, are dealt with in the same crude manner, and gain the same net result. The truth is if you leave your rig in plain sight regularly, you're inviting trouble. To sum the whole thing up:

  1. Don't park in the bad parts of town and leave your radio in the vehicle.
  2. When you do park on the streets or in a lot for short periods, try to park under a light.
  3. When you are going to be gone for a while, remove the set and stow it out of sight. If you don't remove it, at least cover it with something.
  4. Many local law enforcement agencies provide a number etching service. A number (driver's license, or other number) is etched onto your set and logged in police files. It won't keep the set from being stolen, but it may aid recovery.

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©1998 W. G. Nichols - Chilton's Easy Car Care