I`ve just put one of these back together, and can tell you what`s happening in yours.
Inside, you`ve got a little white nylon housing, just after the water inlet, with a couple of wires going to it. This is the electronic water shut-off valve, and, assuming your shower`s identical to my W7500 inside, what happens is that with no electrical power applied to the valve, it should spring fully closed, opening when you switch the shower on with the pull-cord. The little shut-off valve should operate solely in an on or off state. It follows that this is where the failure is, as it`s the only device present inside the shower to control water flow.
I`m an electronics engineer, not a plumber (I`ve just moved to a new house with a W7500 which is playing up in other ways, so I had a look inside), but it appears to me to be a simple and fixable fault, in that it`s a relatively low-value part, and the labour time would be a matter of minutes for the whole job. The sole possible issue would be availability of parts - even if the correct factory-spare were not available, I daresay there`d be a generic, compatible part which would fit and function perfectly.
Good luck!
Paul Marchant, January 2007
