I have a 2001 Dodge dually. HO Cummins, 24 valve, 6 speed. I can drive and upshift just fine. It will not downshift while the engine is running. Have to turn engine off, put in lower gear, crank engine and go. Clutch disengages when pedal is mashed so it isn't the slave/master cylinder. Any ideas what is going on?

 

What happens when you try to downshift? If it's grinding, then you have synchronizer issues, and it's rebuild time.

 

Look see if the slave cylinder is cracked or the mounting screws are loose. It could still be the clutch pressure plate if its warped or has broken springs but usually this will also show as a bit of an issue going into reverse. Watch a few youtube videos about double clutching to keep you on the road if you need to.

 

You can downshift without the clutch, just pull it in to neutral and bring the rpms up so they match the speed that you would be going and it will slide right into gear. It's called floating gears.