My front passenger window will not close right. it goes up all the way then falls down 1/2 to 1 inch. If it does close all the way and you slam the door the window falls an inch. Does this sound like a bad regulator? i took the panel off but can't find anything obviously wrong. Window motor sounds healthy. Closing the window right at the top of its travel it bounces and falls down an inch ( best way i can describe it).

thanks for the input.

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1980 300CD - 130,000 Daily Driver
1984 300CD - 325,000 Dad's summer cruiser

1977 K10 - The Grinch - lifted, locked, winch, 37's. Beadlocks.. 379 t Detroit Diesel
1956 Willys Wagon - buick 225 oddfire v6
1982 SJ 410

When you had the panel off did you drive around like that for a while, operating the window?
My PO had an issue with the drivers side (before my time) where the evidence of a brazed repair was clear after I took off the panel - one of the regulator mounting bolts worked the mounting hole larger and everything would move about when the window peaked and then the next time would not go up high enough to close.
I've ahem had a piece of hardwood stuffed in the door to make the regulator stay in the right location for several years.

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95 E300D gave away
77 300D, 227k, station car
83 300CD 370k, body gone away to the rust gods, engine is in a Yota pickup, going strong
89 190E 2.6- 335k, no more
79 VW FI Bus- 154k summer driver
59 VW Beetle ragtop- 175k
12 VW Jetta- 250k
74 MG Midget-78k

No I have not driven with it off because the car is not registered. It will not close all the way as it bounces when almost up and falls down. I found some interesting things inside the door to. I found some fuel line and a screw driver and some broken plastic prices. It almost sounds like the motor gear skips on the regulator. I can make a video if that would help?

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1980 300CD - 130,000 Daily Driver
1984 300CD - 325,000 Dad's summer cruiser

1977 K10 - The Grinch - lifted, locked, winch, 37's. Beadlocks.. 379 t Detroit Diesel
1956 Willys Wagon - buick 225 oddfire v6
1982 SJ 410

Did you lower the window to see if the bolts that holds the window to the window mechanism are tight.
My drivers window was going slow for months It finally fell apart. My repair story is here:
front window problem

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.