I know I need to install it to the positive engine coil wire. Where would I find this wire on my son's Sunl 150 CC ATV?

 

Normally kill switches (including tether kill switches) are wired to a dedicated pin on the CDI for killing the engine, and not to the coil primary.

Does your CDI look like this?



If so, is there a wire on the "kill switch" pin? Is it black with a white stripe? If so, that is where you want to tie into for one side of the tether kill switch. The other side of the switch goes to ground.

I've no experience with SUNL specifically, but most chinese quad wiring is pretty generic. If yours is different, there are still other ways to wire in a kill switch, but I definately wouldn't be trying to short the coil primary. The CDI dumps a charged capacitor on this pin. Shorting it to ground with a switch will eat up the contacts over time.

Note that tether kill switches work backwards from how most people think of how switches do things. When the tether plug is anchored in place it forces a switch open keeping it from killing the spark. When the tether plug is yanked off, the switch closes (shorts) and kills the spark.