I would suggest that you try hooking to the real TD signal.

The engine speed signal TD is a square wave pulsed output created by the ignition controller from the engine input (crank sensor or inductive pickup in the distributor). Many tachs including all of MBs will not recognise primary voltage (what you have hooked to) as engine speed.

Most aftermarket tachs will accept coil primary but I presume yours must be looking for a real TD signal.

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