The tach could also have dried out (bad) capacitors. My SDL had that issue. When the car was first started, it would work fine, but after a few minutes the tach would sometimes reset to 0, or randomly peg and wobble from time to time. When I recapped the clock, I did the rest of the instrument panel at the same time. Put a stop to that behavior. The Gen II W126 doesn't have the same black cap in the engine bay, it uses RPM pulses from the EDS system, but the tach itself is similar between the two.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

PeachParts member JamesDean does cruise control repairs.
He generally has rebuilt cruise control amplifiers ready to go, cost on his website is posted as $80 plus shipping.
You supply him with your rebuildable core as part of the exchange.
You can PM him or contact him via email, his site is in his profile on this forum.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Great. Except that I just busted the front bumper on my Land Rover so now I have to put the SD behind and figure out what to do with the LR. Have not had a problem with the LR for several weeks now - odd.

Is that cruise control amplifier just a plug and play gizmo? For a 1981 300SD - Gen I. All I need to do is plug it in?

I just went out and cleaned all my brake lights; did not help the cruise to work. Worth a try.

Yes, it's plug and play.
Just like replacing a glow plug relay.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022