I have a disabled child from my previous marriage in IL. If I give up all rights once she is 18 will I still have to pay support

If the child was identified as disabled in the divorce, the support could be extended as long as the child requires it. But, if the support stops, according to the order when she is 18 or when she finishes high school, that is when the support stops. However, if the child is disabled and requires continued support, don't be surprised if your ex files for extended support.

At 18, you do not give up all rights. Until the court terminates your rights, you will always be that child's parent. At 18, all that happens is that you no longer pay standard support. After 18, many divorced parents pay support for college or for their disabled children.