Temp Light comes on

[iceman]

I have no coolant (or very little) in the reservoir as it's about an inch below the low!, but no high temp light ever and the bike rides fine. I'm taking it to a dealer next week, but this seems to suggest what one dealer advised on the phone - if the high temp light does not light then the main cooling system is ok, but the reservoir part is the problem - strange, because as you say the point of that is to drain into the main cooling system if needed - or so we thought.[/quote]
From a post of mine a while back:
The coolant expands as it gets hot and the radiator cap vents the excess volume to the overflow tank via that rubber line that runs from the radiator neck to the bottom of the tank. Once the motor is stopped and cools down, the coolant volume in the system shrinks and what was excess coolant vented to the tank is now drawn back into the radiator via that same rubber line. This process keeps the radiator(and the system) full throughout the entire heat cycle (cold to hot to cold)
Any clearer?[/quote]
Urm, not sure - if normal use is to vent into and out of the reservoir, yep, get that. So, the reservoir can be almost completely empty (just a trickle in the very bottom which we saw when tilting the bike), no high temp light ever, bike runs fine, just nothing in reserve to top up the otherwise separate and complete main system?

[j.d.b.]

That's correct. The overflow is how the radiator is kept full. If the radiator's full there can be nothing in the o/flow and the motor won't overheat. The coolant sensor is mounted in the system at the motor, itself. It has no knowledge of anything except the coolant temp in the motor as it circulates around the sensor's probe.
BTW: coolant evaporates. The o/flow reservoir is not well-sealed. Just a "splash-lid" and an open ground tube. All things functioning a they should, expect the level in the reservoir to decrease over time and heat-cycling. No worries.