I have been reading from my search and still can't seem to establish just what part I am looking for here.
The window motor just occasionally chatters (no window movemenet at all) indicating that the motor gear is not engaging the (blank).
What is the blank?
Now the regulator is the whole assembly but is there a smaller part that is just this part with teeth on it that enegages the motor?
I see on Fastlane a guide jaw, could this be it? I hope because it is cheap...

Anyway, this is on the 81 SD that was supposed to be a donor to the 84 but it drives so nice I am now convinced to fix the thing up.

Makes me sick though how the PO treated it.

BTW - On my 123 Cd the window lift crank handle is called the regulator? Someone should shoot the folks that put together these CDs.
Just kidding...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Front window or Rear window would be helpful information. From your description, it sounds like the window regulator is bent/broken and is only partially engaging the motor. The motors on these windows are pretty robust, sounds like you need a new regulator. Generally the regulators can't really be rebent back into shape (at least I wasn't successful at it) and you'll need to replace. I found the dealer price (less my 20%) was a better deal than the aftermarket units. However Fastlane will probably be able to get you the part more quickly. Most of the dealers are not stocking these items for the older cars.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)