I know this is an ancient thread, but thought I'd add a fix here since it wasn't resolved.

I've had a very similar experience with an '83 tach. Intermittent operation. Tried the cigarette butt fix to no avail. Bought one of Bob's retrofit amplifiers (really nice) but that wasn't it. It was the hall effect crank sensor all along. Not placement, not trigger, the pickup itself was bad. No physical damage visible.

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1985 300TD 387k
1985 300CD (calif.) sold
1983 300D sold

Took me 9 months to figure out why my tach would not work on my 85 300 SD turned out to be the simple OVP relay even though the fuse looked good on top of relay the relay itself was bad replaced it and tach worked great. I did encounter a problem this past summer where the tach would bounce when it was at about 3000 rpm but lately has not been doing it so forgot about for now. Happy Holidays

Oh it was resolved, I just never updated the thread. It was the amp....had to clean the terminals and replaced it with a brand new one ($85) back in 2005....been working flawlessly ever since.

Wshew....7 years since I posted this thread! Still drivin' the same car though! 106,000 miles later!

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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)

pawoSD - my 81 SD tach has not worked since about 86; when it does it exhibits the same strange pegged or zero reading or uncontrolled bouncing. Then a few months ago I got a rebuilt tranny installed. The tranny would not shift but the tach now worked perfectly. I drove it about five miles back to the tranny shop with the tach working fine. They installed another rebuilt tranny and tach has not worked since. Except for a sporatic 100% peg or a few bounces. So what happened between the first replacement tranny and the second; why did it work with the first replacement and not the second? Almost sounds to me like whatever attaches to the tranny could need tightening up, loosening, or something.


Ideas?

Having replaced my tranny (and engine) in my SD, I can say with some certainty that there isn't any electrical tach signal from the transmission, though that does seem awfully coincidental in the timing.

What I did to get mine working again was to clean the face of the sensor down at the 2 o'clock position of the harmonic balancer, and then took a very small screw driver to the back side of the socket that the tach amplifier plugs into, using it to tighten each of the sockets that the pins on the tach amp touch.

Once I had done this, I've had no issues with the tach other than a couple brief jiggles of the needle.

I suspect that the cleaning of grease and grime off of the front of the sensor did almost as much good as the contacts being tightened, as dirty grease plays havoc with the magnetic sensors on the airplanes that I work on occasionally.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...

The pickup for the tach is on the front of the engine on an 81SD, and the tach amp is on the driver side fender. Transmission swap shouldn't affect it at all unless they jarred something loose or did not connect something. Depends how they did the swap....engine pulled? Or from below?

__________________
-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)

They pulled it from below. I am leaning to something needing tightening down near/on the tranny but do not know what.