Hi,

Have you checked that the strength of your coolant/antifreeze mixture is
sufficient? When did you last replace the coolant?  What you describe
happens if the mix is weak.

 When you start driving the thermostat is not open as the coolant is cold,
so no flow goes to the radiator. The cold air passing over the radiator is
enough to freeze the weak mixture, and when the thermostat finally opens
the coolant does not circulate because of ice/slush in radiator. So only
the 3-4litres of coolant in the engine are available for cooling, and this
is not enough without the radiator so temp needle climbs and it IS boiling
over at the rear.  I don't think you have an electrical problem, in fact it
sounds like that is all working well.  When you let it sit for a while the
radiator is no longer wind chilled and flow starts slowly, eventually
warming up everything and it works like normal. Any coolant pushed out into
the overflow tank will be pulled back in when cooling, and that is why you
didn't need to add coolant, even though the light came on.

If you start driving too soon you may also freeze the water in the front
heater, from windchill, although this is a bit harder to do as there is
normally flow through there immediately in winter (since you have the
control open).

bye, Tim
'87 Syncro Westy
'85 DoubleCab