I have a Moen shower knob that broke...

Try removing the screw in center, and pull handle off. Check to see if maybe just inside of handle is stripped, or if it has a plastic insert with splines that are stripped. Thus just replace handle or insert or both. You can try turning it with vise grips or pliers carefully when handle off to see if tap itself is still good. My guess is a replacement handle and/or insert and you'll probably be good to go. Any hardware store should stock parts, reasonably priced.

You need to install a new moen cartridge its a single handle posi-temp cartridge you should be able to get 1 for free if you are the original owner by calling 1-800-buy-moen more than likely when soldering in the new valve a little bit of solder broke free into the cartridge or hard water. When the water gets turned off and then turned back on it can break free alot of particles that can get jammed up in these cartridges or any fixture for that matter you will need a cartridge puller to perform this job with ease.

If you pull the silver knob on the tub spout up, the diverter located located inside will redirect water up to the shower head and when lowered it will return the water to the tub spout. It sounds as thought the diverter is not working properly. Hope this helps..

Remove the handle and see if it will turn off without the handle and if it will reposition the handle alignment and your good to go. If no the cartridge will need to be replaced.

should have a small gear like plastic part that needs to be adjust as well as adjusting the cartridge..once the handle is off

you will need to replace the cartridge, along with the handle.
plastic gets brittle over time, but adding a difficult to turn cartridge will cause it to break more frequently.

make sure when you replace the cartridge, to clean everything well. a small wire brush will do, no need to scrub, unless there is a buildup of scale. just a light brushing inside the valve.

clean and dry, then lubricate inside the valve, and the new cartridge with non-silicone based lubricant. this will make your valve work like new.

I could not find an exploded parts diagram for the Benton suite from Moen. I did find the bayhill which is very similar. Please see link below of exploded view. That should help you piece it back together.

Your control cartridge needs to be replaced. If the water is turned on by pulling on the knob you need Moen service part #1225B. If the water is turned on simply by turning the knob counterclockwise you need serviic part #1222. Try to turn the water on briefly after either cartridge is out to flush any debris that might be still in the body of the valve. Get the cartridge beforehand. the come with a handy removal tool.

Hello
Take the plastic knob off.Turn the brass stem with a pair of pliers until you get it in the proper position.

Off faces down
Cold faces left
Hot faces right

Once in the position screw pastic knob back on.

drill out screw with an EZ-Out bit and remover kit sold at hardware.