I generally source an entire regulator with motor. They are side and door specific but regulators from any of the 81- the end of the 123 or 126body style will work. Check for bent arms or missing teeth that engage with the motor.

You want to clean and relube the tracks. Do this whenever they slow down even just a little.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.