easy way to test a coil, using a extra spark plug, gap it right, and plug it into a spark plug wire and place it on ur uper intake. when u turn the car over (dont start it) the spark should be blue, NOT WHITE, if its white, the coil or distribitor is bad. if the spark is blue, then the coil is still good. the blue spark is much hotter, thats why i say it must be blue. a pro mustang mech told me this and this is how i found out my stock coil was bad.