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How does automatic belt tightners work?

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Old Oct 29, 2009 | 04:47 PM
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daytonabeach
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S212 2014 220 CDI
How does automatic belt tightners work?
Im wondering, everytime i release my seatbealt i hear a click sound on the door side where the belt hangs, but what does it actually do?

I cant figure out any difference with or without...
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Old Oct 29, 2009 | 05:45 PM
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Air Marshall Eldritch
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John Deere
Many cars have a little tiny pendulum hanging inside the unit that forces the belt to lock up if forward motion deceleration is hard enough to make the arm swing forward. Not sure about MB's method though.
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Old Oct 31, 2009 | 05:54 AM
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That's the locking part. The tightening part is done by the SRS system setting off a small explosive charge in a small cylinder that yanks the belt backwards keeping you onto your seat in case of a crash.
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Old Nov 1, 2009 | 01:59 AM
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Originally Posted by Mister Brenton
Many cars have a little tiny pendulum hanging inside the unit that forces the belt to lock up if forward motion deceleration is hard enough to make the arm swing forward. Not sure about MB's method though.
the pedulum engages a pawl that grabs a tooth on a geared ratcheting mechanism that then keeps the belt from extending any further. The method is pretty common amongst all the car mfg. In the rear seats in most cases, extending the belt all the way also engages the pawl so that a car seat can be secured as the belt is retracted.
Last edited by E-420; Nov 1, 2009 at 01:02 AM.
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Old Nov 1, 2009 | 04:15 AM
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S212 2014 220 CDI
But what does the clicking sound when releasing the belt indicate?
I cant notice any difference when puling the belt when its on, the pendlum has been in cars for decades, so that cant have anything to do with it...

I still cant figure out what an automatic belt tightner do in comparison to a conventional pendlum system
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