'99 Oldsmobile Alero cooling fans won't turn on

The cooling fans on my '99 Olds Alero won't turn on when the car heats up. I have tested the fans themselves and they both work fine. I tested all the relays and checked the fuses and everything is working fine. I thought that maybe the engine coolant temperature sensor was bad so I replaced that but it didn't fix the problem. The weird thing that I noticed is that when I disconnect the engine coolant temperature sensor the fans come on when the engine is hot. If someone could help me out with this problem I would greatly appreciate it. Thanks.