Stan... that's a difficult question to answer. A component like a solenoid is both electrical and mechanical.
If it's jammed and can't move, the electrical supply is still there trying to move it.
This "could" result in a change in the impedance in the solenoid circuit.
(Impedance is more or less the fancy word for resistance (ohms).)
This "could" result in the voltage drop you're getting.
Although a faulty solenoid is common, it may of course be another fault entirely. Electric shower problems are difficult to diagnose. It could even be a circuit board problem, but I doubt it.
I don't see any reason why you couldn't replace the solenoid yourself if you're confident. But, I would speak to Triton Technical Help first, together with a look at any exploded parts diagrams that may be available on their site.