A customer called asking for help on an old shower valve. He has three handles: one for the hot water, one for the cold, and a middle knob that shifts the water from the tub nozzle to the showerhead. The problem is that the middle knob didn’t fully shift the water, so he would end up standing under a drizzling showerhead while lots of water flows out of the tub spout.
Many newer showers use a single-handle shower valve and a tub spout diverter, rather than a diverter valve in the wall like you have. Getting replacement parts may not be as easy as going to a home center, so you will likely have to make a trip to a plumbing supply store. Before you replace the stem assembly, you might try to repair it first. The problem usually lies in the washers at the rear end of the stem, but getting access to this part can be tricky.
You will first need to turn off the water to the house and disassemble the valve. You can remove the diverter handle by the screw in the middle of the handle, which is hidden underneath a cap (it just pops off with a screwdriver). Then unscrew the escutcheon cover to reveal the stem assembly.
Getting the stem out can also be tricky. It is usually recessed inside the wall and it has a large nut (called a bonnet nut) that holds it down. The problem is that there is usually very little space between the bonnet nut and the wall surface to get a wrench around it. If the nut sticks out slightly from the wall, you may get lucky and be able to twist it out. However, you will usually need to get a deep-set socket, which can get by the tight clearance and remove the stem.
Use gradual pressure and twist out the stem, but remember this has been in place for years and won’t react kindly to a sudden pounding. If you get really nasty with it, you could damage the old solder joints and really spoil your weekend plans.
With the stem assembly out, you can take it to a plumbing supply house for a new one or you might be able to clean it up and replace the washers. If you feel like repairing it, grab two wrenches and twist the stem free of the bonnet nut. You will be able to see and replace the washer on the bottom of the stem. It is held in with one screw and you should easily be able to find an exact duplicate (or in a pinch you can turn it over for a fresh surface).
While you have the assembly apart, use a small wire brush and clean off any sediment or buildup. You can use vinegar or a lime remover and give it a gentle cleaning.
Reassemble the pieces and screw them back together. You may need to add some Teflon tape or packing to the assembly if it leaks, but when you use your shower again, you should get more water on your head than on your feet.