I have an '83 300SD.

My tachometer is very intermittant (mostly not working). I have checked and changed the fuses. Don't know quite what else to do. I would like to have the tach working so that I can troubleshoot an intermittant idle problem.

What would be the best way to troubleshoot my tachometer?

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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")

Most likely your OVP Relay. I had the same problem and I reflowed some of the solders on the board and to my suprise it fixed the problem. what model is your mb?

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• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

Thats right,I need to roll up some more tin foil balls.To drop down into socket holes where tach amp unscrews off the fender (drivers side).
I have original paperwork when car was new.Mercedes tried 5 times to fix tack.I roll up little balls of foil.After 6 months they flatten and need more.Keeps the Tach working.

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

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

I had the same problem. Various "manipulations" of the tach amp provided only temporary resolution.

I bought one of techguy512's solid state amps and my tach has been working perfectly ever since.

http://www.peachparts.com/shopforum/showthread.php?t=219949&highlight=tach+amp

Did he use larger pins?

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

The pins are not usually the problem. The issue typically is with the circuit board embedded in the silicone.

Well today I finally fixed it right.
Got my 500 power glasses on,and saw where,everytime I screw amp on base.The wire coming up from sender on crank,slips down.Not making contact with amp.Solution was to hold sender wire firmly in socket.While screwing Tach Amp back on.Then cut rubber to put under base to pervent wire from ever slipping again.

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

Can you give any specifics as to what on the board fails. I have just about exhausted the shim fix at this point and I am now having intermitent tach operation.

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81 300D Turbo 190K sold
83 300sd 319K best $500 I ever spent-daily driverw/ 2 tank WVO set up

If you have plenty of patience, you can remove the silicone material and start resoldering the numerous connections. (I did that with some success; it was only temporary, however.)

I think if I look in my spare parts bin I might have a few old amps around. I might try this tonight.

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81 300D Turbo 190K sold
83 300sd 319K best $500 I ever spent-daily driverw/ 2 tank WVO set up