re: Alarm Glass Sensor

["RICHARD VAUGHN" <rav3@xxxxxxxxxxx>]

Mike:
I'm not an expert on this, but I installed a factor 'remote entry/alarm' 
on my wife's '93 325ia a couple of years ago. If my memory serves me 
right, there is an adjustment on the inside of the unit(dip switches) to 
set the sensitivity. We have since traded it on a 7 series and let all 
the installation & adjusting documentation go with the car, so I can't 
tell how to set it. We had a problem with the alarm going off for no 
apparent reason. I noticed the lights flashing one day while we were in 
a restaurant, with no one around the car. I remember the instructions 
showed how to set these 'sensitivity dip switches'. BTW, setting the 
sensitivity higher didn't help in our case. I finally figured out, that 
as the car's interior got hot from sitting in the sun, it would cause 
the plastic housing to warp from the heat and 'pop' against the inside 
sensor, causing the alarm to go off. I ended up putting a small piece of 
foam rubber between the case and the sensor and the problem went away. 
Have you tried adjusting the dip switches? Sorry, but I don't remember 
how to set them from min. to max. Maybe someone out there can add to 
this.

Good luck

Richard Vaughn
BMW CCA #132931 Tarheel Chapter
'95 740i
 
>Date: 24 Nov 1998 09:49:45 -0600
>From: Mike Schaublin <mschaublin@domain.elided>
>Subject: re: Alarm Glass Sensor
>
>     <James wrote>
>     
>     Third question, does anyone know how to adjust the stock BMW 
>     alarm's sensitivity?  I tried to hitting the winshield, dropping 
>     pennies on the glass, shaking the car up and down, yelling inside 
>     the cabin, even banging the glass sensor.  No luck, the alarm just 
>     refuses to go off.  Only after I bang the glass sensor against the 
>     dash a couple times, the alarm goes off.  I'm hoping to adjust the 
>     sensitivity level up a bit.  The alarm was already installed when 
i 
>     purchased the car used from someone, so I'm guessing it's 
installed 
>     by the dealer. <end of snip>
>     
>     (The info below comes from my experiences with home alarms - I 
>     believe the same principles apply to car alarms).  
>     
>     Glass breakage sensors won't go off when they are shaken, yelled 
>     at, or banged.  They are specifically designed _not_ to go off in 
>     these events to avoid false signals.  That is why most alarms will 
>     have a separate glass breakage sensor and a motion (vibration) 
>     sensor.  
>     
>     Glass sensors listen for the sound of the initial impact of an 
>     object hitting the glass hard enough to break it.  This will 
>     produce a high amplitude noise in a specific frequencey range - 
>     probably in the high bass range between 100 and 1000 hz (just 
>     guessing here); and I'm sure the frequency will be different in 
>     each car, given body resonance, glass shape and thickness, etc.  
To 
>     get the glass sensor to trigger, you need to produce a sound in 
the 
>     frequency range which the sensor is looking for.  The best, and 
>     probably only way to do this (if the sensor was designed well) is 
>     to break your windshield or side glass.  
>     
>     BTW, since the sensor is looking for a low frequency thud, not the 
>     high frequency shattering sound, you cannot set of a glass 
breakage 
>     sensor by breaking a beer bottle inside the car.  This is why 
glass 
>     sensors on home alarms don't go off when you drop a plate or a 
>     glass on the floor.  
>     
>     Hope this helps,
>     
>     Mike Schaublin
>     '94 325i - still stock. Needs to be sharked & BL/SS/ERK'd.  




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