After years of not having a tach,I finaly fixed it. It was a shorted capacitor that burned out a supply limit resistor on the board. it was burned so I had to guess it was about a 10ohm. works great.

Speaking of tachs, does anyone have or know where to find a schematic for the board attached to the tach in and 87 300E?

Why don't you try this fast fix IF you have the Tach Amp on the driver's side fender the round black cap. The 87 may have the tach driven off the flywheel though.
If you have the tach Amp on the fender well, simply take the amp out of the cap, scrape off the silicone carefully so as not to cut any traces or wires then jam some non conducting material(i used a plastic sponge(Piece of it), into the cap first and then the amp , then screw the cap and amp back in place. it works, it puts pressure on the solder joints and revives a dead tach.
My tach was dying on my 300SD and I read this tip on this forum last week and thought . Yeah Right that'll work(sarcasm). and I had to eat my words. it worked like a charm and revived my tach. Been good for a week.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.