940 SE Engine will not shut off

I stopped to put air in tires, my horn just went off all on its own. I quickly pulled the wires under hood to shut it off. Went less than a mile down the road, stopped at a store and turn the key off and the engine would not shut off. Went home and pull the coil to shut it off. Had to unhook the battery to turn off dash lights. I have check the ignition switch it seems fine. The car cranks every time just will not shut off on its own. Please help. :confused: