Engine keyed shutoff
 

I spent about 4 hrs. yesterday maintaining my new (to me) 1978 300D. I installed a new alternator adjustment bolt ( removed and reinstalled the alt. and bracket), installed three new belts, dropped the oil pan and replaced the gasket, filter, etc.

The car started easily, as normal, but would not shut off when the key was turned to the off position. It did this twice in a row.
I did not work on the throttle linkage, or do anything I am aware of to the ignition system. Can anyone advise me about trouble shooting the problem. Thanks in advance.

Paul