OK

The single pin is gauge sensor.
The 4 pin is the later sensor ..it has 2 independent sensors in a single case ..The high fan section is pins 2 [br/yl] and 3 [bu/gy], so that is where you bridge with a resistor for lower high fan cut-in.
All you are doing with the resistor across the sensor is throwing the thermistor inside that sensor out of calibration by adding another resistor accross it..that lowers it/s value at temp and fools the fan input circuit to coming on earlier [ lower cut-in temp]

Works cool and cost almost nothing............

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A Dalton