Well, the tach started working for a few minutes again. It did not accurately measure the rpm but it worked. It bounced and not in relation to the engine speed but it worked.

What could this be? Could it be a loose wire in the instrument cluster? I have replaced the innards of the black cylinder which sits on the wheel well; that did nothing. I also tried the cigarette filter in the cylinder and that did not work.

Anyone have any idea? Sometimes it will come on when I hit the high speed button on the AC too.

Trying to attach the video. 1981 300SD https://www.youtube.com/watch?v=_0SRmcULuXw

Not sure I posted this youtube video properly. Might need help from a mod.

Stopped working again.

when you screw the amp off the socket.You can see,where the pins go.Over time they break away,and head south,or down into the fender.The trick is to push them up,one by one.Secure wood,rubber,plastic to keep them from slipping back.And using superglue to give amp,a fighting chance.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Only a handful of things that will cause problems with the tach.

1: Bad sensor
2: Bad amplifier
3: Problems in the tach
4: Wiring faults

The sensors seem to be pretty reliable. Failure is not something that is reported often.

Amps are a known weak point. Have you ever installed a known-working one?

The tach itself has some electrolytic capacitors in it that can dry out and cause erratic or poor operation. The tach itself is also grounded through a metal strap on the speedometer. If it has developed oxidation or a poor connection, you have ground issues.

Wiring is always a possibility, especially if something has chafed or broken off in a connector somewhere. Don't discount fuses, especially if you still have the crappy aluminum amalgam ones installed.

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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!)

Is the amp the gizmo inside the black cylinder? I bought a rebuilt one maybe ten years ago from a forum member and installed it - but no luck.

Got mixed up. I was thinking about the cruise control and not the tach so ignore the earlier post