When you tried booting the system with no RAM and the fan ran at low speed, then with the LCD sensor disconnected and still no RAM, the fan ran at high speed, that would indicate to me the SMC is properly detecting the LCD sensor and there is no issue with the LCD sensor. But I could be wrong. I just went through all this myself last week on my iMac which I bought used and had no LCD sensor. When I finally installed my sensor, the fans went quiet. If your computer is working just fine other than the loud fan, another sensor may be the problem.