To clarify things:
The Thermostat is thermomechanical valve. When the engine is cool it is closed so that coolant moved by the water pump flows through the cylinder heads' water jackets, into the thermostat housing and then has to get back to the pump via the bypass hose. When the engine (& coolant) reaches operating temperature the thermostat's valve opens and coolant can flow into the radiator and from there back to the pump.
An easy way to tell that the thermostat has opened is to feel the water pipe.
The Temperature Gauge Sender is a thermoelectric device that is screwed into the back of the thermostat housing. Its resistance varies depending on the temperature, changing the current that flows through the temperature gauge and thus the position of its needle.
(for reference only - does not apply to 500cc models) The fan switch is another thermoelectric device, this time a basic temperature sensitive switch. It is screwed into the radiator of models that have electric fans to turn the fan on when the coolant temperature reaches 95c and off when it is a few degrees below that.
If your temperature gauge isn't working the first thing to check is whether the wire is plugged onto the sender solidly (it is very easy to bump it and knock it off when you are working on anything under the tank).
Re Haynes: For any thing other than a CX/GL500/650 I would recommend the Haynes as your primary reference source but the Factory Shop Manuals for these bikes are so well written and so well organized that the FSM should be your primary reference and any aftermarket book a secondary reference.
AND it is available for free right away (remember me telling you to download it from the CX Wiki when you first joined?)
