Antitheft and Alarm Systems: Description and Operation

Theft Deterrent Schematic - Typical:

CIRCUIT OPERATION

When the theft deterent system has been turned on, or armed, it will automatically operate alarms. It sounds the vehicle horns, flashes the exterior lights, and interrupts the starter circuit. This happens when any of the doors are opened, or the forced entry switched on the door locks or any of the luggage compartment locks are closed.

ALARMS

The light and horn alarms are operated by the theft deterent relay. The coil of this relay is grounded by the solid state theft deterent controller to turn on the theft deterent relay. When the relay contacts close, battery voltage is supplied to the headlights, exterior lights, and the horn. The controller then open the relay coil circuit to turn off the alarms. They will pulse on and off every second for 2 to 4 minutes after the system has been triggered by the tampering or forced entry.

STARTER INTERRUPT

The starter circuit is disabled by the starter interrupt relay. This relay coil is also grounded by the theft deterent controller when the alarms have been set off. The relay is energized when the ignition switch is in RUN, BULB TEST, or START. The relay contacts then open and break the circuit to the starter solenoid. This prevents the engine from starting.

ARMING

The arming sequence consists of: 1) ignition switch turned OFF, 2) either door lock switch moved to LOCK position with at least one door open, and 3) all doors closed. These provide three inputs to the theft deterent module: 1) battery voltage to terminal K, the ignition on input, 2) momentary battery voltage applied to terminal G, the system arm input, together with a ground connection to terminal J, the tamper input; and then 3) an open ungrounded condition at terminal J, the tamper input. The system is armed.

CANCELLATION

The arming sequence can be cancelled. This is done by moving a front door lock switch to the UNLOCK position before the door is closed to complete the arming of the system. Battery voltage is applied momentarily to terminal M of the theft deterent controller with the door lock switch in UNLOCK. This cancels the arming.

DISARMING

When one of the doors is unlocked with a key, a grounding switch in the front door lock cylinder closes. This grounds terminal H of the controller, the disable input. The system is disarmed and no alarms will operate when a door is opened.

TAMPERING

The alarms are set off when terminal J of the theft deterrent controller, the tamper input is grounded by the door jamb switches or the forced entry switches. The forced entry switches close when there is motion between a door or the trunk and its switch. This happens when a door lock is forced open or removed. The forced entry switches then ground the tamper input and set off the alarms.