Since yours is a converted 280E and there are spliced/ crimped wires to what you call the tach amp holder (I call it a diagnostic port- I'll explain later), I assume whoever did the conversion did not swap over the engine harness from the diesel donor.

If it were my car and I want to add a tach, I would not bother trying to use the existing wiring. I would add new wires: mag pickup to tach amp, tach amp to tach in cluster. You just have to supply switched 12V power to the tach amp and the tach.

The 77 280E elec diag. from the W123 service manual 2 CD set that I have indicate there is a TDC sensor and a diagnostic plug. The 78, 79 280E elec diags are multi use, i.e. for diesel + gas engines and more confusing. The 80 280E elec diagram is plain wrong (a 240D diagram) so don't bother looking at it.

I think the TDC sensor and diagnostic plug (in both diesel and gas models) is strictly for factory test equipment, i.e. to read RPM only. My 85 300D has the diagnostic plug which has wires to the TDC sensor and has power and ground and no where else (no tach amp output pin like on pre 85 300D models, which use a tach amp to drive the tach). The tach on 85 is driven off a sensor on the flywheel/EGR computer. On the 85, the TDC sensor and diagnostic plug is again, IMO, for factory test equipment to measure RPM only for the digital RIV injection timing method.

If you decide to use the diagnostic plug, you will need to add a pin for the tach amp output and run that up to the tach, or get a tach amp socket from pre 85 turbo diesel models.

6" long pig tail on the TDC sensor is long enough to splice if you take it off. Use shielded cable to minimize noise pickup. Old RCA video or audio cable should do nicely. Instead of splicing, go to the JY and get both the TDC sensor and tach amp holder from a pre 85 turbo diesel?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked