Hi Martin. I am not trying to be a pain, BUT, Seeing as common type alternator has a Battery positive supply and a warning light wire which turns to Battery voltage "ALL" the time that the engine is running, The alternator does not know that the ignition has been turned off until it slows down and stops. ??? At that point the Voltage on the warning light wire then drops off and becomes an earth through the Rotor windings (or regulator on the later models).

If a Rectifier pack fails, it can backfeed and send a positive signal back up the warning light wire, This will keep the warning light on with the ignition off, But seeing as it normally has a feed up the wire all the time that the engine is running anyway, How can this cause the engine to fail to stop. ?

I Have never seen an alternator cause a run on problem, I have seen people blame them on older cars, But the true cause is usually elswhere, i,e incorrect warning light bulb fitted etc. ( too high a wattage allowing current back into the ignition system )

Tony.