I'm have an 85 300SD with an intermittent operating tach. Upon buying the car the tach was intermittent at best and not accurate at all. Due to some driver side fender well repair with a knew drain tube and sealing up all the metal from leaking, the tach started working after the area dried out of moisture. After a few months it stopped working at all.
I know that the tach setup used on the 300D's and SD's on earlier models is different than later 84's and the 85's. But how different ?
Currently I'd like to get the tach working on this car but I need to identify the components used and then devising a way to test each one.
Any advise from the expert world would be appreciated.

Thanks

IIRC on the W123 300D the tach pickup is in the adapter plate between the engine and trans. On other years it was in a different location. Not sure about the 300SD however.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

Instagram: @maximed93

I think the 85 set up is quite different. Try removing the computer/control in the RF kick panel, clean all of the contacts including IIRC the OVP??? in that area.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yeah, not sure if your 85 w126 is like my 85 w123 California emissions model. My tach was non functional and I was unpleasantly surprised at how different the circuit was from other older Turbodiesels I’d played with. Mine turned out to be the red blade fuse on top of the OVP relay right behind the glove box.

I don’t have an 85 diagram but my understanding is the tach signal is processed back there unlike my 82 300sd.

I guess short of a wiring diagram you can trace the circuit by wiring color codes back from the pickup as far as you can get to the gauge. It sounds like you have a bad wiring connection or corroded module somewhere. If you can just find the key components like the amp you may be able to clean them up. I believe there are at least two black boxes of electronics on the passenger side footwell.

Maybe this will help? Wrong car but may be similar enough. It seems there is some hardware on the drivers side that could have been messed up by moisture. (At least on a 123). Perhaps your SD has a similar setup.

https://youtu.be/gNoEe2nvrCg

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles