2001 grand cherokee alarm system

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• 52132 Member Posts: 62

  December 2007

  The first documented case of car theft was in 1896, only a decade after gas-powered cars were first introduced. From that early era to today, cars have been a natural target for thieves: They are valuable, reasonably easy to resell and they have a built-in getaway system. Some studies claim that a car gets broken into every 20 seconds in the United States alone. In light of this startling statistic, it's not surprising that millions of Americans have invested in expensive alarm systems. Today, it seems like every other car is equipped with sophisticated electronic sensors, blaring sirens and remote-activation systems. These cars are high-security fortresses on wheels!
  The Basics
  If you want to think about a car alarm in its simplest form, it is nothing but one or more sensors connected to some sort of siren. The very simplest alarm would have a switch on the driver's door, and it would be wired so that if someone opened the door the siren would start wailing. You could implement this car alarm with a switch, a couple of pieces of wire and a siren.
  
  Most modern car alarm systems are much more sophisticated than this. They consist of:
  
  An array of sensors that can include switches, pressure sensors and motion detectors
  A siren, often able to create a variety of sounds so that you can pick a distinct sound for your car
  A radio receiver to allow wireless control from a key fob
  An auxiliary battery so that the alarm can operate even if the main battery gets disconnected
  A computer control unit that monitors everything and sounds the alarm -- the "brain" of the system
  The brain in most advanced systems is actually a small computer. The brain's job is to close the switches that activate alarm devices -- your horn, headlights or an installed siren -- when certain switches that power sensing devices are opened or closed. Security systems differ mainly in which sensors are used and how the various devices are wired into the brain.
  
  The brain and alarm features may be wired to the car's main battery, but they usually have a backup power source as well. This hidden battery kicks in when somebody cuts off the main power source (by clipping the battery cables, for example). Since cutting the power is a possible indication of an intruder, it triggers the brain to sound the alarm.
  Pressure Sensors
  Another way to detect breaking glass, as well as somebody opening the door, is to measure the air pressure in the car. Even if there is no pressure differential between the inside and outside, the act of opening a door or forcing in a window pushes or pulls on the air in the car, creating a brief change in pressure.
  
  You can detect fluctuations in air pressure with an ordinary loudspeaker driver. A loudspeaker has two major parts:
  
  A wide, movable cone
  An electromagnet, surrounded by a natural magnet, attached to the cone
  When you play music, an electric current flows back and forth through the electromagnet, which causes it to move in and out.
  This same system can work in reverse, which is what happens in a basic pressure detector. Pressure fluctuations move the cone back and forth, which pushes and pulls the attached electromagnet. If you've know how electromagnets work, you know that moving an electromagnet in a surrounding natural magnetic field generates an electrical current. When the brain registers a significant current flowing from this device, it knows that something has caused a rapid pressure increase inside the car. This suggests that somebody has opened a door or window, or made a very loud noise.
  
  Some alarm-system designs utilize the car's built-in stereo speakers as pressure sensors, but others have separate devices that are specifically designed for detection.
  
  Pressure sensors, glass-breakage sensors and door sensors all do a pretty good job of detecting someone breaking into a car, but some thieves and vandals can do a lot of damage without ever making it inside because these sensors only detect if someone is actually breaking into a car. Those little speakers on the dash board also operate as pressure sensors. In other words, someone has to actually break into the car to set off the alarm (it's normal) Hope this helps

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• tidester Member Posts: 10,059

  December 2007

  Great explanation - thanks!
  
  tidester, host
  SUVs and Smart Shopper

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• debc68 Member Posts: 3

  December 2007

  I think I had that problem once too. I was told by a friend that if you've every locked or unlocked the jeep with the key instead of the FOB or locked or unlocked it manually from inside.....things could be "out of sync". He told me to use the key to unlock the doors and make sure all the doors were in the "unlock" position inside. Then use the FOB to lock/unlock a couple times. Also, on my jeep, I have to push the FOB button twice. The first time locks and the second time sets the alarm. Hope some of that helped.

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• 52132 Member Posts: 62

  December 2007

  I have a 2001 Grand Cherokee and DO NOT have to press the FOB button twice. I just press it one time and it locks, flashes lights, and honks for a second. I am not saying that all systems operate the same. Maybe you have a aftermarket system.

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