Shower control turns 360

My wife and I just bought a house that is around 30 years old. The master bath tub faucet has three separate control valves/knobs. The middle valve/knob turns the shower head supply off and on. It works, but it does not stop anywhere. You have to turn it round and round until you find the "on" position. What can I do to fix this?
Thanks in advance
ATL
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Probaly there is a stop under the handle and it is worn off if these taps are that old I would probaly replace them as soon as you can afford to . Without seeing it suggest you go to whoever in your area see if parts avaible. Good luck
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Thanks Jim. When you say "taps" are you speaking of the entire faucet or the stems, etc.? In other words, is what you're suggesting a repair job that can be done without going into the wall or is it a replacement job you're talking about?
Thanks ATL
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ATL wrote:

Not all 3-handle controls are alike; get a mfr name or some identifier. As a last resort, remove the diverter stem. IIRC, some designs didn't even have stops.
t'other Jim
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what's to fix? You should only have to turn it 180 degrees to get from one mode to another . "round and round" is a waste of time.
s

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