replaced starter,solonoid,and ignition swith on my 00 f150 and starter wont disengage. same problem i had in the beginning

Disconnect the starter from the flywheel but leave it electrically connected. Support the starter on an stand/box/etc. so you can see the gear. Have someone turn the ignition key to the "on" position, but not the start position. If the starter engages you have a short somewhere. You can do the same thing with a multi-meter if you know which wires to check.

Check the fuses....with a test light....try to find the one it says..( start).....if the fuse is ok,....maybe the anti thieft system is doing that thing close the doors put the alarm and turn it off again!! ..if still not working.....check cables from battery + and - to the starter.....good luck

Did this condition begin with the old starter in the vehicle? Or did it begin after you replaced something? If it began after replacement of a part it may be wired wrong. If not, check all the wiring to make sure something is not chaffed/corroded.