Well Im not sure what weve proved today but I managed to get the plug off the sender unit;
......then my mate has a selection of resistors to play with and working off the info in the pdf, he started to attach various combinations in order to provoke a reaction from the gauge.
It is not easy. He had a variable potentiometer and the tiniest adjustment sends it off the scale either way. Im sure one of the big digital range adjusting test meters would have done it.
We could get it to show cold and get the warning light;
......and we could get it to show off the scale hot to illuminate the red warning light;
.....but oh, she didnt like that and decided to deprive me of several safety systems!!!
Eventually, we got it to show this position just above normal but we couldnt get it nearer. We also couldnt get it anywhere between cold and normal which I know it does gradually because I paid particular attention to it on my way to work last night.
It does sweep back without indexing but I dont know if it proves anything. It was fun playing but my other mate had to get his sun tester to clear a raft of codes after she had a little sulk at me.
Conclusion - you need some proper test gear to play that game.
Sorry about some of the images being bleached. Id stopped the car where the sun reflected off the hood back into the lens - we are having an unusually good summer here on the island.