The tach gets its ground by a strap that touches the other elements in the cluster. Eventually it finds ground through one of the wires on the circular connector on the fuel-oil-temp gauge unit.

Some people add an extra ground wire, attaching it at the bottom where the ground strap is, and running it directly to the main ground junction behind the cluster.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Well I've got a tach already to fit in the cluster, I'll still have to look into an optical tach as I need some sort of reference to set it up. Something like that would be neat as I could transfer it to another project down the line once I've finished with this.

Hmmmm well I think it might pay to add another ground connection while I can. My fuel and temp gauges wiggle a bit whenever I tap the dash so grounding could be an issue there.

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD