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The gauge operates on the princincipal of magnetic attraction and repulsion. The base of the needle is an armature between two opposing coils that cause magnetic fields. The sending unit has variable resistance. The two coils are connected so that current can flow through either one. When resistance from the sending unit is low the right hand coil receives more current than the left attracting the armature to the right. When resistance is high the needle moves left.
Does your gauge go to cold when the car is off?
When you turn the key to the on position( but not start) does the gauge move to 1/4 or do you have to start the car and let it warm up. You mentioned the auxiliary fan fan doesn't go on? This is usually associated with the a/c.
If the resistance in your sending unit is too low it will move the needle right toward hot. Most of the time these sending units go bad because of too much resistance usually from corrosion in the jack that would pull the needle left toward cold. Its possible that your sending unit is transmitting too low resistance thus sending the needle to the right.
There has to be a spec somewhere regarding the acceptable range of resistance in the sending that you can measure with an ohmmeter. I never had to replace a sending unit but there not that expensive.
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