The crank sensor has three wires - black read and yellow, the tach seems to be getting data from two wires read with black and green with black.
Can someone tell me what happens in between.
I'm transplanting the engine into a 67 Land Rover and wold like to take the tach with me.

I'll be curious to hear the answer to this. Last I heard is that there's some sort of German Voodoo associated with transferring these tachs. That's one of the reasons I went with the photo tach. I think the M-B tach uses a mag pickup and needs some sort of a signal conditioning circuit but I'm not sure about that.

Dan

From what i got so far the 3 wires go to the VDO amplifier, And the amplifier i got with the donor is faulty as far as i can tell because the tach worked on and off when the car ran. A replacement would be around $50 which can get me a whole unit that mounts to the alternator..

I looked at the one you have and it is nice but i'm keeping all the stock(Land Rover) gauges and a digital one would look odd between them.

Path is:

Pick up--> tach amp--> tach

Here is a guide I wrote on how to wire it up, I transplanted one into my 380SE that I converted to diesel

http://www.peachparts.com/shopforum/diesel-discussion/319522-guide-how-get-tach-working-w126-gas-diesel-conversion.html

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

Oh, and you can pull the tach amp apart to fix it. They often get bad solder joints on them. The only Pain in the ass part is the circuit board is coated with a layer of silicone. You cave to very carefully scrape it all off before you can re flow the solder joints.

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

with the guide all i need is to paint the needles white, find some trim and i'll have a nice looking tach with a clock

glad to help

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

In my 84 & 85, the tach pickup is on the transmission bell-housing and goes thru the emissions computer. Earlier cars used the front engine damper pickup, with a little amplifier that clips onto the "diagnostic port" (tach was an option?). I have that front pickup on my cars, but it just goes to the diagnostic connector (for dealer instrument?). This is all from memory, so might be a bit corrupted.

Mine's 83 and the sensor gets triggered by a lil rod on the balancing disk. I wonder if it has anything with determining TDC.

It is called a TDC sensor because it picks up the rod that is at the TDC position, and is used for the tach only, not for timing. The nomenclature is confusing the way Mercedes uses it.

The YL and BK wire carries the signal from the sensor to the tach amp, the third wire is the shield. You should get 81 ohms across the YL BK wires.

If the tach worked intermittently, it could be corroded contacts at the tach amp or most likely the tach amp itself is intermittent. You are transplanting the tach module from the Mercedes cluster over to the Rover? There's little chance of success otherwise.

__________________
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

I'm getting 74 ohms, is it close enough?

Close enough- good to go. Difference is probably in the tolerances and ohmmeters used.

__________________
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