Engine runs with ignition switch is turned off 454XL Crusaders

First start up this spring, a couple of odd things that never happened prior years.
When starting Port side alarm sound as soon as battery switch turned on, engine starts but when ignition is turned off engine keeps running. If Battery switch is turned off the engine stops running. Only thing new was replaced the alternator because I damaged the old one while changing belts. Ay Idea Guys.

Your best help here will be from when all information is given to us.
As it is now, we must guess at a few possibilities.

If computer controlled, then you may need to see if a code can be pulled.

If not computer controlled....the first thing that comes to mind for me... would be that your ignition power is being used to excite the alternator's field circuit.

If the alternator is bad.... the ignition system may be receiving a "back-feed" of current (from the alternator)........ in which case, the ignition key switch will not kill this current.

If so.... the ignition system may still be receiving power.




Try to avoid turning your MBSS to the OFF position while the engine is running.
You may further damage your alternator.




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Either the replacement alternator is bad or somebody incorrectly connected the wires to it. This is based soley on your description as the warning horn came on with the battery switch - this indicates +12V DC exists on the ignition circuit independent of the key position.