I fitted a temperature gauge a while ago and wired it to the spade connector at the back of the speedometer.
It has recently decided to go off scale, am I right in suspecting the little voltage reg behind the speedo?
It aint the engine which even in these hot climes is as cool as a cucumber.
Which begs the question of course......."why bother"?
Earthing out is the most likely explanation. Is it still off-scale when you unplug from the sensor? If not - then the sensor has failed -or just maybe the end has become coated with enough 'rust' to form an earth path. remove and clean! If still off scale - look for a short on the connecting wire.