I have a 1999 buick century i have changed the starter but when i try to connect the battery it turns the engine over why..

Your hooking up the wires wrong.

You may have the big positive wire hooked onto the bottom post or it is positioned over to touch the small post on the starter solenoid. The battery cable is to hook on the top big post. Never the less wires are not right down on the starter.

Thier is no way to hook it up wrong that i can see. Thier are 2 big wires that hook to the starter and one small wire that hooks to the solenoid...

Ok must have ben tired. Then what it could be is that there is power on the small wire on the starter. Power should only be there when you turn the key to start. I bet if you take the small wire off of the solenoid, and then hook up the cable to the battery that then it would not turn over. Thus indicating the ignition switch, or the starter relay.

Cool ty i will try that..