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I believe that the shoulder straps were new to Ford trucks in the 70's. Do you know if the 77 had locking shoulder straps? Neither of mine lock, so it looks like it's almost just decoration!
If they are supposed to lock, any recommendations on how to fix?
If they do not, any recommendations on installing a spring system that would enable them to lock?
I have two young children and I want to make sure which ever one sits in that seat has a proper working shoulder strap.
If you're simply yanking on them really fast to get them to lock, it won't happen. There has to be forward momentum for the weight in the retractor assembly to engage the teeth of the retractor. It's difficult to check their functionality at a stand-still.
I don't know of a way to prove that they do work (other than the obvious); I can only say your original method doesn't necessarily prove they don't work. Hopefully someone else can chime in with more advice. I should have also mentioned that this is how they are in my '79 - I'm not sure if a '77 would be any different, being older.
only one thing I can think of,
Got a rough country road nearby? go bomb down it and aim for the bumps instead of going around them... I live on a 1.5 lane gravel road, lots of pot holes and ruts... can't go bombing down it without the strap locking/unlocking repeatedly as i bounce against/away from the strap.
Might try a brake test, stop hard and see if it catches, sometimes mine will for that.
This is in a 78 though, but i doubt they'd be there is they wouldn't catch.
And there is a thought; what was the cutoff for these trucks actually getting shoulder straps? my 76 doesn't have them.(and I am glad of it, but thats personal pref.)
On every vehicle I have ever owned with shoulder harnesses from my 1977 F-150 to my Ford Focus daily driver, if I pull the belt out quickly it will lock. Its annoying sometimes if I lean forward quickly to look for traffic they will lock in everyhing I've ever had and driven (I was a GM tech so I've driven a lot of vehicles). Then I let up and lean forward again and they still lock. Many times I just unbuckle them so I can actually lean forward and see vs moving forward very slowly so they dont lock because then my traffic opening would be gone.