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Q:

Grand Marquis wont shut off on 1990 Mercury Grand Marquis

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Tried disconnecting battery & it still wouldn't shut off
(1) Answer
Most likely the ignition switch or actuator system is defective. Possible ignition primary circuit shorted to battery voltage. The alternator keeps the engine running with the battery unhooked! To stop the engine you can unplug the ignition coil connector, then unhook battery to keep it from being discharged until repaired. You can also just pull the coil wire (from the dist cap not the coil) but be careful, it still bites if not done correctly! It might make you mad but it wont hurt you!! Lot of volts but near 0 amps.