You might have a blown fuse on the overvoltage protection Relay which is located behind the glove box in my 1984 300D Turbo. If the F14 fuse is loose or blown it too will cause the tac to fail. It powers a EGR Control Unit in the 1984 300D Turbo with California Emissions. On the "Federal" unit the tac is fed from an engine speed sensor that is supposed to be located between the transmission and the engine. Under the car, I think. Never seen one but that is where I read that it was on mine. According to a schematic the F12 fuse (8 amps)feeds red brake warning indicator and the clock and tac. If your clock is working then the connections from the Tac might be bad at the connector (C166). C166 is a 2 wire connector with a brown/white and a white wire.