I need to know how to find ground connections for my 85. Problems that I think are ground related: Tach, it worked when I got the car, stopped working, I did some unrelated work under the dash on the P-side and the next day the tach worked fine. Stopped a week later.
Seat Belt light and buzzer, go on ad off at random intervals, seems a little bump sensitive.
Dash Gauges, fuel, amp and one other one. Did a LOT of jumping around when I was trying to narrow down the seat belt issue.
Cruise will sometimes accelerate rather than set the speed. Engage/disengage work OK.

The randomness and wide spread issues make me think I have a ground issue most likely behind the dask cluster or in the cluster itself. Is there and diagrams illustrating the location of grounds for the car? I know BROWN generally is ground for MB right? I'd like to clean or fix any ground issues before replacing any parts.
Lights, dashlights etc work fine. Battery stays charged.

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1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

Tach and cruise are probably unrelated to grounding problems as their failures are well known. The other problems sound like a bad ground to the cluster. Add a ground to the cluster.
If you seach on 'tach fix' I believe there are threads about using a cigarette filter or something to fix that problem. Tons of threads on repairing the cruise amp, including the recent one about baking it in the oven. It may be better to retrofit a Rostra cruise control, the method for which was described and illustrated in a recent thread.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The 85 doesn`t use the tach amp. It gets it`s tach signal off the flywheel. there is a cable with a sensor that fits into the adaptor plate on the drivers side. there is a long round plug on the firewall near the Mono-valve where it plugs into the end that connects to the black EGR box behind the Pass side kick panel.

could be this cable or a connection.

Charlie

I did some searching for a thread from a few weeks ago, guess this wont help? this is your thread.
Tach Failed 1985 W123 300D

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works