240D models don't come with a tach. But I believe the '82 to '85 300D
> turbo diesel model has a tach. If so, get one from a wreck and install
> it in place of the 240's clock.
>
> The diesel does have a pick up on its balancer, the pulse goes to the
> diagnostic cap on the left fender. Then you need to figure the wiring
> scheme from there to the new tach.
Mistaking the clock for the tach is common in diesels since they move at about the same rate.
Chas Hurst