1986 ford f250 with a 460 wont start. Why dose the solenoid click once when trying to start.?

Try cleaning your battery post and cables first.If that don't work try starting your truck with switch on(if straight gear make sure it's in neutral with brake on) use screwdriver on battery side of solenoid and touch small post small wire coming from it) where the metal is exposed,usually looks like small nut where goes into solenoid.If the engine turns over like that,probably needs new solenoid.Don't want you to spend money if don't need to so check out other answers to.This has worked for me and other Ford owners.