The fan has it's own control module that controls the speed of the fan depending on the value(voltage) send by the ECU.The fan will run on full speed in ONLY 2 occasions:
-ECU has a fault code related to cooling,engine or transmission safe(limp) mode activated
-communication between the fan and the ECU is lost(usually bad or dead ECU)
Even if you remove the solar sensor and everything else is Ok ,the fa will not run at full speed.