I'm sorry but I am not good at explaining things but I will try again to explain how the tach gets its ground.

Internally the tach ground is connected to the metal plate that holds the tach to the instrument panel.

From that metal plate the ground goes through the metal bar on the speedometer circuit board. The actual connection is at the lower left speedometer assembly mounting screw. See photo 2 of post 4 above.

The metal bar is connected to pin 3 of the 8 pin connector on the speedometer circuit board.

The plug that goes into the 8 pin connector has a brown wire that connects to pin 3.

That wire goes to the ground point behind the instrument panel along with many,many other brown wires.

Is this any better?

PaulM

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84 500 SEL (307,xxx miles)