we have a 2007 chevy 3500 hd pu with a crank stop crank stop no start

That's normal... The ECM controls starter CRANKING time once key is turned to start... If a no START condition exist, the CRANK timer will stop crank signal -- after a pre-determined amount of time (a few seconds) even if you let go of key.... find out why engine will not START.. Fuel pressure and ignition system needed to be tested... The reason you've never noticed this feature before is the engine has always started right away!