When a Moen shower faucet handle grinds and is difficult to turn, or is leaking, it is time to disassemble the faucet and inspect the valve cartridge. Taking a Moen shower faucet apart is mostly a matter of removing a series of screws with a screwdriver. The valve cartridge is located just behind the handle of the faucet and is easily removed for inspection. A quick look at the valve cartridge will tell you if it can be repaired or if it needs replacing. The repair process is the same for shower faucet or tub and shower faucet cartridge.
Locate and turn off the main water supply to the house. Remove decorative caps from faucet with a screwdriver, using care not to damage the plastic.
Remove the faucet handle by unscrewing the screw that secures the handled with a screwdriver and pulling the handle off. Use a screwdriver to unscrew the trim and remove it.
Locate the pin on the top front end of the valve and pull the pin straight up with needle nose pliers to remove it. The pin holds the cartridge in place.
Slide the plastic removal tool onto the valve and push it back as far as it will go. Use pliers on the plastic removal tool and turn to loosen and remove the cartridge. If you have no plastic removal tool, you can buy one at your local hardware store.
Inspect the valve cartridge for worn or broken "O" rings and corrosion. If the "O" rings are in good shape and there is no corrosion, clean the outside of the cartridge with a brush, wash and dry the cartridge, and lubricate the "O" rings with a silicone lubricant or petroleum jelly. Use a brush to clean the sleeve that holds the valve cartridge. Reinstall the cartridge and faucet handle. Turn the water back on and test the faucet.
If it works correctly, remove the handle, install the trim and replace the handle. If this did not solve the problem, replace the old valve cartridge with a new valve cartridge.