The cooling fans should switch on whenever the engine temperature requires this. Even when the engine is turned off.

The after-cooling is switched on when ignition is off, hood closed (switch), and intake temperature is over 87,5C (sensor-switch on top of the intake closes) or... if the coolant temperature is over 85C (NTC resistor based temp sensor) at the bottom of the radiator. The cooling fans should engage at approx. 6,5volts and the flaps opened completely. This is managed by the temperature control unit inside the pass.side cover next to the seat, and the power stage in front of the engine (with the fins, just before the AC dryer).

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Dash instrument Coolant Temp sensor at the water pump (right to the Temp2 sensor)
25C -> 380 ohm
60C (1/4 of scale) -> 60 ohm
85C (1/2 of scale) -> 50 ohm
 

Measured with a potentiometer and a thermometer the instrument cluster shows:

100 ohm -> 60C
60 ohm -> 85C
50 ohm -> 90C

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An older version of the sensor has  2 contacts, one is 4.5mm (temp switch) and one is 6.3mm (NTC sensor) Both act against chassis-ground. Applicable MY ???

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I had a similar problem 2 years ago;  it was caused by the Coolant Temp sensor at the radiator. Here's some data that helps diagnosing:
Check the temp sensor by:
Remove connector 1 and II at the control unit next to the pass. seat (under the cover).
Between connector II Pin 1 and connector I Pin 7 a resistance must me measured, based on actual temperature, but something between 1000 and 4000 Ohm.
 

More precisely:
60C = 3862 Ohm +/- 150 Ohm
85C = 1582 Ohm +/- 54 Ohm
100C = 967 Ohm +/- 36 Ohm.

I would replace the sensor. Cost about 45,-,
Part number Nr. 928 606 149.00. Also needs a seal ring, 2,50.
It is located at the radiator, driver side, front, lower half of the radiator core.

It is a fan control device and (if existing) a flap control device working together with the ventilation and cooling controller. WSM 1, page 19-11
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Coolant temperature sensor readings at bottom of coolant radiator:

74C - 2300 ohm
77C - 2066 ohm
79C - 1931 ohm
85C - 1582 ohm
90C - 1330 ohm
95C - 1128 ohm

At 79C both Fans would run 50% capacity, at approx 7.5 volt. When temperature rises the voltage increases to 9.5 volt and at 95C the controller lets the fan run at 12v
regards

Theo

http://928gts.jenniskens.eu