I replaced the radiator that craked and was leaking, and now the van over heats and the service engine light is on. I noticed that the radiator fan does not turn on. will the fan not comming on cause it to overheat, or is there any other ideas... thanks

Sounds like your fan relay failed. The relay is bolted to the radiator support below the air filter box. There are numerous posts on this subject that you can search for.