Truck on runs when key is turned to on/start position

I have a 2006 Freightliner Columbia Detroit 60. This truck will not stay running unless the key is held in the on position. Once i let the key go the truck shuts off. Fires up no problem. Could it be a fuse? Ignition? Starter? Fuel pump? I have no idea. Any advice?

 

Ok, need to understand this better. Are you holding the key in the "start" position and then when you release it to the "run" position it stops? If so you are forcing the starter to stay engaged and this is what is turning the engine. You will burn your starter out in no time. If the engine is properly firing and dies when you release the key switch, it might be just a bad ignition switch.

 

What was the problem, my truck is doing the same. Already put new ignition switch