Chevrolet Repair: car will not shut off, old chevy, intermittent problem

Question
building a old chevy put in all new wiring kit with a new 350 engine every thing works great except once in a while when i turn off the ignition key to off the car will keep running till I disconnect the battery checked wiring in new ignition switch and it all looks well please help thank you

Answer
Hi Robert,

I am thinking of all the possibilities that could cause this.  It depends on what kind of car and what year.  
The reason is, I don't know if this is fuel injected, computer controlled, or otherwise.

The ignition switch's job is to provide a connection from the battery to the ignition system.  It also directs current to other systems (radio, blower, etc).  Of course, at it's very end, it runs all current to the starter.  Once the key is released (engine started), this contact is broken and the other systerms are energized.   When you turn the key off, to kill the engine, on a modern car you are usually stopping current to the fuel pump.  On an older car you are stopping the current to the coil.  If the car won't turn off, then which ever system is still getting it's current.  This is due to a short, or the ignitiion switch is out of alignment.  Another extreme possibility is, the timing is off.  

It is an intermittent problem.  So I have to rule out a dead short.  Timing would also be more consistant.  The fact that you state you have a new ignition switch, I would start looking there.  

The switch has a slider inside it.  As you turn the key, you are actually turning a long rod, that enters the ignition switch slider.  If you missed the hole the rod goes into, you may have jammed the rod into the body of the slider, behind the hole, and it will be out of alignment.  Thus the slider will not be able to break the contacts when the key is off.  If you did hit the hole, you may have the ignition switch mounted a little 'off' on the column, and the slider cannot reach the end of travel.  Usually the mounting holes are oblong.  With the key in the off position, push firmly on the ignition switch body, toward the key off position, as you tighten the mounting screws.  This should assure you are positioned correctly.

If this happens often, drop the column, and turn the car on and off.  When the car doesn't turn off, push the rod toward the key off position.  If it does shut off with a little help, you know what the problem is.  

I hope this helps.

C J S