Alarm Horn: Description and Operation

Siren Module:

An alarm siren module is part of the export premium version of the Vehicle Theft Alarm (VTA) in the Vehicle Theft Security System (VTSS). The export premium version of the VTA is only available in vehicles built for certain markets where the additional features offered by this system are required. The alarm siren module is located in the engine compartment, on the front extension of the right front wheel house panel below and behind the right headlamp. This assembly is designed to provide the audible alert requirements for the export premium VTA.

The alarm siren module consists of microprocessor-based electronic control circuitry, the siren, and a nickel metal hydride backup battery. All of the alarm module components are protected and sealed within a molded plastic housing. A stamped steel mounting bracket is secured to the module with three stud plates and nuts that fit into slotted holes at the top and each side of the bracket. Two mounting holes in the horizontal surface of the bracket are used to secure the alarm siren module to the wheel house extension with screws. A connector receptacle extends forward from the alarm siren housing, and connects the module to the vehicle electrical system through a dedicated take out of the headlamp and dash wire harness.

The alarm siren module cannot be repaired or adjusted and, if faulty or damaged, it must be replaced.

The microprocessor within the alarm siren module performs the tasks required to provide the siren features and functions based upon internal programming and electronic arm and disarm message inputs received from the Intrusion Transceiver Module (ITM) over a dedicated serial bus communication circuit. Upon receiving a request from the ITM, the alarm siren module will self-detect problems with its internal and external power supply and communication circuits, then send electronic messages indicating the problem to the ITM. The ITM will store a Diagnostic Trouble Code (DTC) for a detected alarm siren module fault that can be retrieved with a diagnostic scan tool over the Programmable Communications Interface (PCI) data bus through the 16-way data link connector located on the driver side lower edge of the instrument panel.

When the export premium version of the Vehicle Theft Alarm (VTA) is armed, the alarm siren module microprocessor continuously monitors inputs from the ITM for messages to sound its internal siren and to enter its auto-detect mode. While in the auto-detect mode, if the alarm siren module detects that its power supply or communication circuits are being tampered with or have been sabotaged, it will sound an alarm and continue to operate through its on-board backup battery. If the alarm siren module is in its disarmed mode when its power supply or communication circuits are interrupted, the siren will not sound. The alarm module will also notify the ITM when the backup battery requires charging, and the ITM will send a message that will allow the backup battery to be charged through the battery voltage and ground circuits to the alarm module only when the ignition switch is in the On position and the engine is running. This will prevent the recharging of the alarm backup battery from depleting the charge in the main vehicle battery while the vehicle is not being operated.

The alarm siren module receives battery voltage through a fuse in the Power Distribution Center (PDC), and receives ground through a ground location on the left inner fender shield in the engine compartment. These connections allow the alarm siren module to remain operational, regardless of the ignition switch position. The hard wired inputs and outputs for the alarm siren module may be diagnosed and tested using conventional diagnostic tools and procedures. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the internal circuitry or the backup battery of the alarm siren module, the ITM, the serial bus communication line, or the electronic message inputs to and outputs from the alarm siren module. The most reliable, efficient, and accurate means to diagnose the alarm siren module, the ITM, the serial bus communication line, and the electronic message inputs to and outputs from the alarm siren module requires the use of a diagnostic scan tool.