was overheating now the fan only works w/ a/c on ?

I have a 97 VW Jetta GL 2.0 I had a crack in the flange on the end of the head (drivers side of the motor if your standing in front of the car).I replaced the flange but I removed the two sensors from the old flange and put them in the new flange, should I have replaced the sensors ? Now the car still gets hot but the fan only comes on when the A/C is on if I turn A/C off fan goes off turn A/C on car stays cool turn off A/C fan goes off car slowly gets hot. Did I mess something up ? Should I have changed the sensors when I changed the flange? Why does fan only work with the A/C on ? Any answers would be great I feel like I am going in circles thank you