Wired up my Gen II Coyote to the Autometer Coupe Platinum Gauges as per the Coyote install instructions— tapping into the blue/red wire in the #4 coil wire. At idle, the tach seems to twitch between 0-500 rpm, then when I rev the engine it actually stops twitching and holds steady at zero— almost seems like it's working backwards.

I checked the dip switches on the gauge and they are properly set for 8 cylinders. I also checked the wiring again— tapped into blue/red wire on #4 coil wire. I've read that maybe I need to buy the Autometer 9117 tach adapter, but it makes no mention of this anywhere in the install manual, so I'm hesitant to throw down $100+.

Any other suggestions? Thanks!