Ok, I'm reading all that I can on these and I must be totally missing something.

3-prong deal off engine into amp, 3 wires out of amp, green/yellow black/red and some brownish color. Brown is ground (I know that much.) The best I can tell the green/yellow goes straight through to the back plug for the tach. Not sure about the black/red, it goes into the fuse box and I can't really see if it goes somewhere else after that. The plug I got off the tach has 4 wires on it but 2 prongs, wires are green and green/yellow one side and black/red and blue/black (blue/black goes to a light)

Ok so wtf? I've tried just about every combo I can think of. I just had wires run from the battery for ground and 12v to try to get the tach to work then I was going to wire it up. Can someone tell me how to do this? I've searched and found a few threads on this but different color wires were described, so sorry.
-Nate

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'82 240D, 4-speed, 617 turbo engine

I'm not sure I understand what you are trying to do?

Is you Tach broken?? or are you tring to install one??

Is this on your 240D??

I'm trying to wire up a tach....

240d, put a 617 turbo engine in it (see sig) and want to hook the tach up I got from the donor car.
-Nate

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'82 240D, 4-speed, 617 turbo engine

anyone know this? I'd like to get it done this weekend. If I missed something while searching can someone direct me? I've searched tach install, tach wiring...variations like that, it just doesn't seem like I'm REALLY missing anything but I just can't get it to work. Thanks.
-Nate

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'82 240D, 4-speed, 617 turbo engine

I'm on the same dillemma as you. I'm trying to retrofit a diesel tach on to a turbodiesel car that didn't have one, but already has the pickup sensor mounted near the center pulley (btw, all 84-85 euro & usa turbodiesels have this)... but i just need to know where does the wiring from the rear of the tach connect/route/mount to?? I believe the tach plug on the rear of all late '83-'85 tach/clock gauges have a 3-prong plug...while the '82-'83 have a 2-prong plug in the rear and connects diirectly to the tach rpm sending unit (this is the TALL black cansiter on top of the driverside fenderwell). I would like to trace the 3 prong plug back all the way to where the wires end up at in the engine area (or tranny area? or fuse box? or ECM??). I have the wiring diagrams (from the shop manual, and they differ from year to year on the OM617.95x) and they only help so much... Anyone???

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Okay, here's what I found out... this is for the tach unit that has the TWO prong plug in the rear (NOT the 3 pronged one):
black/red wire goes to fuse box at fuse #12
black solid wire goes to fuse box at fuse #14
brown solid wire is chassis/ground
black/green goest to that "tall canister tach sending unit"

This is the easiest if ever someone wants to retrofit a diesel tach to a diesel car that didn't have one... BUT remember in order to do so, the magnetic pickup sensor (located by the center pulley, and this has the thick grayish wire which plugs into the bottom of the "tall tach rpm sending unit canister thingy"... ) must already be installed in your car. If it is missing, then chances are you will not be able to just "install" one in that spot since the bracket that holds that magnetic pickup sensor is bolted onto the engine block!! And I don't think it's a good idea to start drilling holes in the block just for mounting that bracket... I never tried it and so I never will neither...

I believe all post-'83 Euro-version turbodiesel models already has the magentic pickup sensor in place from the production assembly line - I read this in a German W123 literature. And my car is proof! It didn't have the tach option (Euro versions couldn't) but the pickup sensor is there in order for the Duetsche MB Techs to diagnose RPM problems on the OM617.952-equipped cars... And I know that all USA-version turbodiesel models should have that magnetic pickup sensor, as well as the tach/clock gauge already installed...

The tachs with the 3-prong plugs on the rear are however more complicated... I hear the the rpm sending unit (or amplifier as others call it) may be built-in inside the tach itself - hence the extra prong for the plug... and when I dismantled one months ago I remember seeing those wires leading to the tranny area too, as well as the fuse box, magnetic pickup sensor, etc.... It was difficult because the entire warning harness (when dismantled) is like spaghetti!! The wiring diagrams (from factory shop manuals) also show the difference between '82-''83 300D-T from '84-'85 300D-T cars, in such that what I explained above can be translated from the wiring diagrams itself. if you wish to have a copy of this, PM me and I'll scan the wiring diagrams which depict both differences - and email them to you...

Hope this helps... if I learn any more info - then I'll repost my findings again... but this project can get quite tedious as you can imagine. WHEW!

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My Mercedes-Benz Homepage and W123 Owners Photo Gallery and W111/W112 Coupes & Convs Gallery
2005 Northeast GTG and 2005 Mid-Atlantic GTG and Summer '09 Central FL GTG on YouTube

ok...my sending unit has 3 wires coming out of it black/red and green/yellow. I traced the black/red to the fuse block. And at the tach the black/red and green/yellow appear again going into the 2 prong tach unit, however the unit has 2 wires coming out of it. Basically the only area that I'm totally confused about is what goes on between the fuseblock and the tack that allows unit to get power, or is it powered there? This black wire is new to me, but since it didn't appear at the tach or at the sender I don't know what to think. Thanks for the help though, I'll tinker around with it some more.
-Nate

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'82 240D, 4-speed, 617 turbo engine

How about if you do have power at the gauge, but no movement of the gauge? I have a new amplifier in there as well as a new gauge pod, and it still won't go! I am guessing maybe an issue with the actual sensor...is there a way to test if the sensor is working? It looks difficult to reach down in there to mess with/or replace it.

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