Without knowing what model you have, this is just a guess. The 240's from 1986 up have electronic speedometer system. Inside the works is a little plastic gear, not designed to last as long at the rest of the car. The gear breaks, and the odometer quits working. This is a very common 240 problem.

Review recent history and if you drive 250/300 miles a week, start changing oil every 3 months, and estimate other maintenance stuff similarly.

The speedo can be repaired. The gears can be found, but not easily. Then you remove the instrument cluster and disassemble the speedometer. Remove/ install the gears, and reassemble everything.

If this is not your cuppa tea look about the brickboard for shops that do the work. I know there is one on the west coast and one in Austin, TX. The shop in Houston is too expensive for me, around $200.

Good Luck,

Bob

:>)