The RPM gauge in my 300D never worked since I bought it, I don't know if it is just stuck or something else.

How do I go about troubleshooting it, the 1st thing to do would be checking unplugged wires etc, where are those wires located... and does it get any input from the vacuum system ?

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'85 300 D Diamond in the rust

pull down passenger side kickpanel, relay on side btn glove box and wall has a 10a fuse. check here first.

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1983 300d~250k Sold
1985 300d~185k
1999 Volvo V70 wagen 109k
2001 Dodge Ram 1500 78k

i have among others an 84 126. my tach sometimes also reads zero. i tap the plastic in front of the tach with my nuckle. and the bugger comes to life. for what its worth.

probably the tach sending unit. do a search. Good luck, and be prepared to be disappointed, or spend lots of $$

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Paul
2004 E500 4matic; 72,000mi

I've seen this relay in my car, what is it for???

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I've never had to fool with mine so I don't know a lot of details but the '85 MY has an electronic tach and some of the electronics are in the passenger footwell. IIRC the sensor works off the crank on the front of the engine. Others here will know more.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold