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how to disable or lower sensitivity of stock alarm system

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how to disable or lower sensitivity of stock alarm system

Old 10-09-11, 12:12 AM
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how to disable or lower sensitivity of stock alarm system

My car (1991) has the stock security system, and it goes off all the time. I need to either disable it or lower the sensitivity of it. can anyone help me? I tried tyo search but i didnt get much luck.
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Old 10-09-11, 11:30 PM
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There is no "sensitivity" to the stock alarm on a 91. The alarm is intigrated into the CPU. There are sensors on each door lock, the hatch lock and the hood latch. When every single one of those is locked the single wire for the stock alarm going into the CPU becomes UN-grounded. When this happens the alarm arms itself. If that wire going into the CPU gets grounded out at any point before the alarm is "disarmed" then the alarm will go off. You can disarm the alarm by puting the key in the driver side door or the rear hatch lock and unlocking. The wire going into the CPU would be grounded by someone breaking the window to your car and unlocking it from the inside. Because the door lock moved (grounding the wire to the CPU) without a key turning the lock first this event would trigger the alarm.

The stock alarm go wacky for several reasons.

1. The CPU goes bad. It arms the alarm at it's choosing and it grounds itself out at its choosing. Therefore the car alarm could just trigger itself in the middle of the night.

2. You wiring is getting old and you have corroded connections. Frequently the hatch and hood are closed. If the doors are unlocked but the corrosion is killing the connections the grounding out of the CPU could be temporaraly lost from the door locks (even though they were never locked) and the alarm could arm itself with the doors unlocked. Once the signal gets back through it gounds the CPU wire and bam, alarm goes off.

I would suggest grounding out the alarm at one of the hatch latch points and see if that stops the wacky alarm. Your idiot cluster will think the hatch is open but that doesn't matter. If that stops the wacky alarm then your CPU is probably okay an the issue is related to grounds going in and out on there own because of corrosion on the wires and connecting points.
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Old 10-12-11, 09:31 AM
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thanks man i really appreciate it!
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Old 10-14-11, 02:34 PM
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No problem. Please give us an update when you figure out what is going one. It helps a lot when other owners have the same problem and are trying to reconcile there problems to the advise given for correcting your problem.

Also note that my description is specific to all series 5 FC's, 1989-1991 only. The series 4 FC's 1986-1988 have a seperate control modual for the alarm that is not intigrated into the CPU. It can simply be unplugged to disable the stock alarm. This is very bad for security as someone with my knowleged could get to it in a matter of seconds if breaking into a s4.
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Old 10-15-11, 12:23 PM
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yeah i noticed the hatch light is already on 100% of the time. could this mean that someone already tried to fix it?
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