2002 Ecotec Cooling Fan Problem

Hello everyone, I have a 2002 cavalier coupe ls sport 2.2 ecotec with 5 speed transmission. the radiator cooling fan only comes on when the air conditioning is turned on. i swapped the relays for the heater blower motor so i know that the relay is ok. the tempature gague in the dash is working so i can assume that the sending unit is ok. However, i dont know where the sending unit is, does anybody know?? When i come to a stop the temp gague climbs to about 230 degrees and the fan does not come on, if i turn the a/c on, the fan comes on and the coolant temp comes down to normal. I could really use some help here. any imput will be appreciated. Thanks

Hey guys 26 views and no response. i really need help.
How do i diagnose this problem??