My car has no tach in the instrument cluster but it does have what appears to be a tachometer pickup, my question is, why is that there? is it part of the cruise control? did my car originally have a tach? could the pickup i have be hooked up to a tachometer? It is a cylindrical thing above the harmonic balancer with a wire leading back to the left fender wall where there is a big cylindrical plug to the rest of the wire which joins the mass of wires going through the firewall, there is nothing that fits the description of the tach amp, but it seems like this must be a tachometer pickup.

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83 240D four bagger stick diesel 276K

That does indeed sound like a tach pickup. You say its wired into the harness? I am not familiar with W123 240D so I am not really qualified to answer what purpose it might serve. However I did add a tach to my '79 diesel, swapped the instrument cluster and tach amp from a turbo diesel - mounted it on the left fenderwell and fed 12V to it thru a small fuse. I connected these two with a short length of cable and hooked it up to the sender on the transplanted '85 engine and Voila! I now have a tach. Is it possible someone installed a later version engine in your car? Did 240D come with a tach? I don't know the answer to these questions. But I do not think the sensor is used with cruise, as that need driveshaft speed. The cruise control pickup is a small motor located on the back of the speedometer.
FWIW, the '85 300D does not have a tach amplifier in the engine compartment, it is integrated into the ECU (engine control unit) and supplies RPM info to the computer in that module. Perhaps your 240D has something like this? If it does its the first I heard of, and kinda neat if it does (let us know what you find out!).

Dieseldiehard
1971 220 (gas) 4-spd manual 104041 or is it 204041?
1979 300TD w/ ’85 turbo engine 288975
1983 300D 237300
1985 300TD 205437

I'm almost certain now that it is either part of the cruise control, which isn't working. Or as someone mentioned in another post, used for diagnostics. so i'm wondering if it could be hooked up to a regular tach gauge from a 300D

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83 240D four bagger stick diesel 276K

Yes, to answer the question "i'm wondering if it could be hooked up to a regular tach gauge from a 300D" - just do like I did.
It is not a lot of trouble actually, the new tach/clock will replace the existing clock now in the right side of your cluster.
Finding a working one is the hardest part. I bought one at a junkyard that was no good. Another had a good tach but the clock was dead. There is a fix for the clock, its simply replacing the small electrolytic cap located on the circuit board, they dry up and short.
Dieseldiehard

Would the tach amp have to be found at a junkyard? or can it be acquired from Partshop or anywhere else?

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83 240D four bagger stick diesel 276K

I bought a tach amplifier at http://www.ips-parts.com/
See attached picture.
A new one cost about $75 - unless you select a dealer to make them rich, in which case I bet they won't sell it for under $100 ?
A good junk yard might have one but you should ask them for a guarantee that it works, these get flaky (the tach stops working or is intermittent). One solution to the flaky problem is to stuff something under the cap so the pins get forced into the socket tighter, I found that solution to be temporary.
Anyhow the socket is what you will probably better find at a wrecking yard, a dealer is likely to have a problem getting one, or if they can its likely to be fairly steep. You will also need a bracket to mount the socket, I made one out of scrap aluminum, mounted on the fenderwell. Sorry I don't have a picture of my DIY setup handy - maybe tomorrow?
Dieseldiehard

here is the picture:

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