seat belts

I have an 87 ford ranger. The shoulder seat belts don't activate if I give them a quick tug. All of the Ford seat belts I tried at the wreaking yard acted just like mine. If you pull on them sharply the
belt does not stop.
My question is this; do ford seat belts activate in some other way than a quick pull on the belt? Do they have an inertial slider in the mechanism or some other means of activation? Or,are my belts broken and all those I tried in the wreaking yard?
If broken is there an easy way of fixing them?
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The actual lockup mechanism is in the de-reeler area, and the belt locks up by sensing & acting upon vehicle deceleration (inertia).
Yang wrote:

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I could be wrong but I think there is a pendulum mechanism that causes the belt to lock when the vehicle either decelerates of is placed on a steep in/decline.
--
R. J. Talley
Teacher/James Madison Fellow
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