Temp sender for gauge is independent and has no effect on the fans at all it is screwed into exhaust side of head which is most likely guilty culprit can be confirmed by removing wire and earthing gauge should go to full indicating faulty sender.

That fan switch looks strange in that it has four pins it, you haven't changed to aftermarket management have you? Unless you have rewired it's more likely to be wiring than fan switch itself. Try bridging wires at fan switch if still no joy then will be power problem.

Martin