The 84 factory manual in pdf is on MontyMcV's site, look in the sig line in any of his posts for the link.

For a quicker fix, I looked at my copy for you. The tach wire goes to the coil- terminal, and is white with blue and black stripes. It goes thru the firewall to the back of the combo meter to the top center position in the same color. It's wire #79 in the wiring diagram in Section 8 of the manual (the front harness diagram).

The tach applies the pulses to a coil that moves the needle against a spring pressure. Faster pulses move it further. Use a 9v battery to test the meter. Connect - battery terminal to vehicle ground. Very quickly, apply 9v+ to the tach wire at the coil and observe the needle for a quick jump. DO NOT leave the current applied to the wire for more than an instant. Repeat test at the male side of the cluster round connector top center terminal. Look for the same needle jump. From there, inspect the traces on the instrument circuit board from that terminal of the round connector to the gauge. You can use the battery again at various places on the trace to see if trace is broken, or you have a cold soldered joint.

Also check the ground of the circuit board.