Rob,

In short this is why: http://www.pinrepair.com/em/index1.htm#reels

The contacts on all the score reels when at zero should all be open, one (or more) of the contacts are closed

Have fun 

As Tony said in the reply above the motor will continue to run when resetting the game until all the score reels return to ZERO.  As one of your coils is burnt out you will need to manualy set this score reel to zero.  This can be done by pushing in and leting out the coil plunger much like it would do if the coil was operating, do this until it clocks around to the zero position.  Once all reels have returned to zero the motor should stop turning.  If not check the contacts on the score reels that Tony is reffering too.  With the machine turned off if you clock the reels around by hand when it reaches zero you should see the contact open.  This tells the machine it is in the zero position.

Good luck