I have a *new* '82 300sd that I bought about a week ago and have been going through undoing all the crazy things the PO did to it, but this one absoloutly has me stumped.
The tach wasn't working, I pulled the instrument cluster and found the plug off, plugged it in , still no tach.
Now there is a small black approx 6" ground lead attached to the parking brake mount that the other end of the lead with a female spade connector on it , was connected to nothing ( just hanging there ) so I tried to feel around for a ground spade somewhere under the dash where it would reach and found non, but the oddest thing, a few times I touched it to the spring clip on the lower dash panel ( one of the clips with a threaded hole in it to attach the lower knee panel under the dash) and the tach would start working, and not just a random current spike , but accurately following the rpms.but it won't work all the time, and there doesn't appear to be any metal in that panel.
Does anybody know where this wire is actually supposed to connect, and is there supposed to be a seperate ground wire comming off the back of the instrument cluster ( possibly where this is supposed to connect to??)?

Thanks--------Robert

I'm pretty sure the proper grounds are in the normal connector to the unit. It is common for the ground to fail in the cluster itself. The individual units of the cluster carry the ground between themselves by being screwed together. For some reason this sometimes don't get it and I imagine many have added extra ground wires.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician