Installed new coil, now the tach acts funny

yellow rose said

What ignition box? Most of those boxes have a terminal or a wire to trigger the tach. You don’t hook the tach to the coil on those. Usually.

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It’s the orange ignition box, I don’t recall the manufacturer. However, I used my old tach with no problems (other than the occasional bouncy needle, which I could never figure out), and then installed the new tach with zero problems. Swapped coils and now this.

I should add that I have a non-Rallye cluster with no factory tach. I’m using an external aftermarket tach.