Tach is dead on my 87'300d turbo,I have experienced the problem in the old 5 cyl diesels and the "cigarette butt" fix worked beautiful,however I dont know how to handle this issue in the 6 cyl diesel 86-87.Anybody had the same problem?

What else isn't working? AC compressor? Cruise control?

Check the archives for the rundown on checking the AC compressor which includes checking the engine rpm signal at the Klima relay.

Sixto
87 300D

i have the same problem , and my as and cruise control don't work, some more nfo on the matter would be great
thanks

Overvoltage protection relay (or fuse), behind the battery, relay with the fuse on top.

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Gone to the dark side

- Jeff

I replaced the over-voltage relay behind the glove compartment (on the R wall) and no problems since, good luck Bryan