Mounts to back of tachometer - Will require zeroing of the needle on the gauge with the tachometer energized - 95% of all problems are in the printed circuit board. It's also advisable to check the tachometer filter. Located between the engine wiring harness & the distributor, just bypass it to check functionality.
Instructions @ youtube address below
www.youtube.com/watch?v=bA3khKLw2Ys