1997 Pontiac Transport Cooling fans will only work when AC

I have tested the fans manually with my battery and a good ground, they passed. I have also checked the three relays and coresponding fuses whitch all seem fine as well. The engine has recently been getting up to 100' on the coolent temp guage when I am at idle (ie, intersection). The two electric cooling fans only seem to engage when the air conditioning is turned on. At first I thought it might be the engine coolent temperature sensor, however upon replacement the fans still only engage when the ac is on. Any advice or possible way to overcome this problem would be greatly appreciated.