Page 339 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Electrical systems 321

Figure 3.128 Alarm switch type sensor

Building the alarm system as an integral part of the vehicle electronics has made
Key fact
signifi cant improvements. Even so, retrofi t systems can still be very effective. The
main types of intruder alarm used are: When the alarm system is an integral
part of the vehicle electronics it is
switch operated on all entry points
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more effective.
trembler operated
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battery voltage sensing
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volumetric sensing.
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There are four main ways to disable the vehicle:
ignition circuit cut-off
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fuel system cut-off
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starter circuit cut-off
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engine ECU code lock (now the most common).
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A separate switch or transmitter can be used to set an alarm system. Often,
such a device is set automatically when the doors are locked. Some types have
electronic sirens and give an audible signal when arming and disarming. They are
all triggered when the car door opens and will automatically reset after a period of
time, often 1 or 2 minutes. The alarms are triggered instantly when an entry point
is breached. Most systems are in two parts, with a separate control unit and siren.
Most systems now come with remote ‘keys’ ( Fig. 3.129 ) that use small button-
type batteries and may have an LED that shows when the signal is being sent;
they operate with one vehicle only. When operating with fl ashing lights most
systems draw about 5 A. Without fl ashing lights (siren only) the current draw is
less than 1 A. The sirens produce a sound level of about 95 dB, when measured
2 m in front of the vehicle.
Most factory-fi tted alarms are combined with the central door locking system.
This allows the facility mentioned in a previous section known as ‘lazy lock’.
Pressing the button on the remote unit, as well as setting the alarm, closes the
windows and sunroof, and locks the doors.
A security code in the engine electronic control unit is a powerful deterrent. This
can only be ‘unlocked’ to allow the engine to start when it receives a coded
signal. Many manufacturers use a special ignition key, which is programmed with
the required information ( Fig. 3.130 ). Even the correct ‘cut’ key will not start the
engine.