You could take all the tach parts from a 300D turbo (mag pickup, tach amp, harness and tach/clock combo) and install it on your 240D. The only thing I'm unsure of is whether there are mounting holes for the mag pickup and whether the tooth trigger exist on the 77 harmonic balancer. If not, the job becomes harder. You have to add the tooth and figure a way to mount the mag pickup.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked