check one coil at a time,what hhamm is trying to say is you want the other plug wires unhooked so it wont start on you.
if you dont want to ground them out like he said,put spare plugs into the loose wires and make sure the plugs are grounded from their body or threaded portion to a bolt or some where in the block,as stated,running a ground wire to the battery is overkill and unnecessary.
do not remove the plugs in the motor as you can start a fire when a fuel charge is blown out out of the cylinder and ignited while testing.
the reason I test with the plugs in is that you are demanding the most out of the coil while cranking with the load of compression against the starter,this is when a weak spark really shows up.
now,if you dont see a weak spark and think its intermittent,or higher up in the rpm range,hook the muffs up and make sure all of the plugs except the one you are testing are hooked up,start the motor and watch the spark on your tester up through a high idle.
continue testing until you find one that is intermittent or quits altogether.
you may also want to warm the motor up a while,why?? sometimes coils dont start breaking down until they are warm.
my own personal preference is to not have the tester inline with the suspect cylinders plug,why? you are asking the coil to jump two gaps,one in the tester,one in the plug,this can lead to a misread bad spark when in reality its fine.
and yes,you will have someone tell you that a coil should be able to jump an eighth inch gap,this is not always true,it depends on the coils total rated output,and jumping that big of a gap is hard on the coil and the electronics in the ignition system believe it or not.
another way to check for a coil that may be cracked is to spray water on each coil with a mister,you will most often see and hear secondary voltage leaking from that coil or wire,at the least,you will hear the motor start misfiring when the bad coil/wire gets wet.
if I can be of further help,pm me.
