Recently there are some strange issues with my 1987 300SDL
This morning upon starting the engine, the tachometer did not work.
After the engine became fully warm in about 10 minutes, there was a sudden noticeable surge in the engine speed and the tachometer began working again. Then a moment later the same surge happened and the tach stopped working. So this goes on back and forth. The two seem connected obviously.
I don't think there is anything wrong with the tach. What could be causing the sudden engine surge has me most concerned and how is it related to the functionality of the tach. Any knowledge on this matter would greatly be appreciated.

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1987 300SDL

Doing a search brought me to this page:

300SDL Tach Issue

I'd investigate the Klima and OVP relay. Start with the OVP relay as they tend to get wonky and are cheap the replace.

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-Typos courtesy of my mobile phone.

Klima has nothing to do with tach. OVP sure does, and if it's flaky, it'll be causing the EDS system to drop the idle to ~500 RPM and then surge and rise back up to ~650-700 when the EDS kicks back in.

Don't just blindly replace the OVP. Find out if it's actually doing its job. Measure the battery voltage while the engine is running, if you're >14V, you need to replace the regulator in the alternator and the OVP is actually saving your car's electronics.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)