The glass doesn't slide up and down the rails, it slides inside the black weatherstripping that is inside the rails. Usually what happens is grit and grime builds up on the sliding jaws. On some models, I have seen where the motor mounting bracket gets bent from the window being raised all the way and then some, so the pot metal flexes and eventually (over the years) bends.

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Never thought about this but I suppose you could grease the rail which houses the sliding jaw. I have replaced several sliding jaws in my 81 300SD over the years and did have to straighten out one arm on one regulator. But never thought to grease the rails. Do not think it would hurt anything and might help the plastic sliding jaw to move back and forth.

Consider a teflon spray lubricant to reduce friction on the sliding jaw.

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Fred Hoelzle