I believe you must have one. Some coils have one built in, so you don't need the external one.
I had a coil go bad one time- it had a built in resistor. The auto parts store gave me one without a built in resister.
To make a long story short, the coil would work just long enough to get the boat off the trailer, and into the middle of the river (in the current) and quit. The coil will get very hot...
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