Assuming that is it, which I think it is as it's on the salt reservoir, I've had the valve out.
It appears normally closed, with a plunger going into an inner pipe. So I'm imagining that when the coil is energised, the plunger comes back, allowing water to flow? The plunger doesn't appear to be stuck back, so I'm not seeing that it is stuck open, unless I'm not understanding the operation.
I haven't got anything here to test the solenoid, I'll have to bring something home from work or take it to work, as I'd like to energise the coil and see that the plunger moves, or not. I've measured the coil at 3.5k ohms.