my 87 190D is starting to shift rough. It was suggested to check the modulator. Any suggestions or comments?
The car has been very reliable ,262K miles, and never had any transmission problems.

Check fluid first, vacuum leaks second (one of the checks will be if the modulator hold vacuum), I'm pretty certain that there is a vacuum amplifier on that car (the blue flying saucer thing on the driver's side fire wall)then Bowden cable adjustment, then the modulator adjustment would be the logical order

Thanks. The transmission fluid and filter were just changed, which is as far as my ability on this subject extends. Your information is most appreciated. Lastly, how extensive of a job is this, or should I consider selling the car?

As Bb already posted, check the vacuum connections. With low (or no) vacuum, the transmission will shift hard, this is the most common reason that these cars will shift too hard.

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