In addition to the tools required to remove the tachometer from the dash, you will need:
1) soldering iron & solder (keep in mind that electronics are heat sensitive, so be careful to use a low-wattage iron or use heat sinks to localize the heat.)
2) electric tape or heat shrink tube (preferred)
1) Disconnect the battery by removing the ground connection from the battery terminal.
2) Remove the tachometer from the dash, disconnecting the white loop of wire and the green wire.
3) Remove the chrome bezel, glass lens, and black inner rim. Be careful not to break off the sheet metal ears holding the bezel to the body of the instrument.
4) Remove the 2 screws attaching the mechanism to the body of the tach at the rear of the container..
5) Holding the tach by its face with the RPM red line straight up and the ignition light at 3 o’clock, carefully examine the rear of the instrument. You will see a resistor (a small ceramic cylinder with colored bands around it) soldered to a “spade” terminal. Immediately to the left of that terminal is a post with a green wire soldered to it.
6) The object of the exercise here is to reverse these two connections. Unsolder the resistor from the spade terminal (don’t use too much heat here) and unsolder the green wire from the post. As you may expect now, solder the green wire to the spade terminal, and the resistor to the post terminal, thus reversing the polarity of the tachometer.
7) Reinsert the tach into the housing and tighten the two screws (remember, Terminator, these are little screws!). Use a drop of nail polish to keep things from vibrating loose. Re-install the rim, clean the lens, and install the bezel (careful with those tabs!). You may want to replace the o-ring at this time.
8) Looking at the dash wiring, you will see a white wire coming out of the harness and making a loop around a plastic terminal on the back of the tach. We need to reverse this loop.
9) Mark the wire 1-1/2 - 2 inches from each side of the loop, cut each wire, and reverse the loop. Solder the splices and tape or shrink-tube (neater and preferred) the bare solder joints.
10) Since (as inconcievable as this may now be), you may sell the car someday, be sure to mark the body of the tach housing to show anyone working on it in the future that it is now a NEGATIVE GROUND instrument.
11) Replace the tach in the dash, reconnect the both the loop and the green wire, and be sure that the housing is properly grounded. Your tach is now configured for use in a negative-ground system and you are free to continue with your polarity change.