Don't think I have ever seen a 240D with a tach, but the 300SD did use the balancer triggered crank sensor for the tach signal through 1984. The 1985 unit (still a 617.95x motor) used a crank senor off the flywheel located in the bellhousing. Engine speed at this time became a signal used in A/C and emission control and sensors similar to the developing gas technology were used.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician