I have a 300SD W126.

Where is the OVP Relay?

Do you have a picture?

Where is the tach amp located?

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Big Fred

(RIP Sept '10) 83 300SD Turbo Diesel (aka "Rocky")
(RIP April '10 - Accident) 86 Chevy M1009 CUCV (Blazer) - 6.2L Diesel (aka "Ogre")

If it's the 83 in your sig line, the OVP is on a newer car

Yes, but you don't want to know what I look like

It's the round plastic thing behind the left headlight. Unscrew, lift off the socket, gently pry the PCB with connector pins out of the plastic housing, even more gently scrape off the silicone covering the hidden side of the PCB, wad insulating (foam peanuts, cigarette filters, etc.) material into the plastic housing to press on the PCB when you put the PCB back in place. That should get your tach working for a while if not a long while.

Someone offered or is offering an improved tach amp design in the parts forum.

Sixto
87 300D

Yep, the SD has no OVP relay. Its either the tach pickup sensor at the front pulley (my problem was this) or the tach amp on the fender. After replacing both mine has been working great for a long time now....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

On the inner fender, drivers side.
My money is on a failed amp, a common problem. The one you see in the pic is new, the tach works now.

Attached Thumbnails

 

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83 SD

84 CD

I used to pry cover off and use washers.One day I will drop tiny amounts of sodder in the holes.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt