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Hi Folks,

I'm fitting a bosch lower-temp-trigger sensor to lower the fan actuation temp, but am not clear on which sensor to replace as there are two and one is for the instrument cluster. Is the fan control sensor on the left side and denoted as #3 on the attached graphic? Advice appreciated!
Best,
Peter
 

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Doesn't work that way with the ST4s (or any of the ST series I believe). Fan is turned on by the ECU. If you managed to change the temp sensor to one that somehow reads lower, not even sure you can in this case, you will effect what the ECU sees as engine temp which will also effect the fuel map. Not the way to go.

There are two sensors on the ST. One is for the digital temp readout on the dash. The other is for the ECU. No dedicated fan sensor like you'd find in the older Superbikes. Those actually have three different sensors on the same journal. ECU, Fan and a small thermistor for the temp gauge.

If you want control of the fan, you would need to add a manual switch that will apply a ground to the coil of the fan relay. The ECU controls relays and such by turning on a ground path for the voltage that is already sitting there. Splice in a manual ground and the fan relay will energize. Probably easier and less expensive than changing a temp sensor.

I have not made the modification, but there are many who have. I remember a few threads on the subject offering which relay and wire to splice in the ground.
 

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On my ST2 I bought a 4x4 lightbar fitting kit on Amazon which came with a waterproof switch, relay, fuse and a complete harness.
Pretty easy to install...

... this did however point to a problem with my rear temperature sensor which is my next job.
 
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