I want to add a tachometer to my 300TD - up until now it just had the big clock in the instrument cluster.

I've gotten a tach and tach amp at a salvage yard, now all that remains is how to hook it all up... I hope it's possible!

The plug for the tach amp on the driver's side fender wall is there; there is a gray wire leading somewhere into the engine that has three connections to it, and on the other half there are two more (in positions #2 and #6) that lead from the socket toward the firewall.

Stripping back the insulation on those two wires I've discovered they're power and ground wires (ground to #2 and power to #6). Nothing else leaves the tach amp socket for the firewall, but the tach amp cylinder itself has a prong at the #3 position, so presumably that's where the tach signal comes from ( I'm guessing here).

At the back of the tach gauge itself are two prongs... I have the socket for this but of course I have no idea what to wire to it. Presumably the wire from the #3 position at the tach amp socket would go to one, what about the other?

Am I totally off? Does anyone have any idea? I wish I had a wiring diagram or something for the tach! Has anyone else attempted this before?

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)