In the Venture there should be 2 fans, one with the A/C comprtessor, and one with eth engine temp like the posters before me have said, and your problem is probably one of two things, either a temp switch or your a little bit paranoid, like my wife when her Malibu fan doesn't come on when she thinks it should. BUT in rare cases it could possibly be a bad fan motor, this is very very rare. to test the fan motor, pull the plug of the temp sending unit(s) and jumper the two electrical connectors, the fan should come on. if it doesn't you know whats next, but the fan in the Venture goes off when you turn the engine off, Modern general Motors vehicles kill everything except the lights, radio and door locks when you turn the key off.
Also check the fan relay, probably not gone bad, but it might have shaken loose of these nice smooth roads we have in this country
It would seem that big Brother REALLY is watching you :)