Fuse # 40is blowing truck will not start, replaced serveral times

Just need to figure out what is on that fuse and trace those wires until you find the short, sound like its not touching metal or other wires all the time or it would blow as soon as it was replaced and the ignition turned on, could be a bare wire on bad connector or connection, or bad electrical part.

Remember if it coincides with turning something on when it blows, may help find it, Like a switch bad ect. Do you know what is on that fuse? Is it under the hood or dash?

check your batteries first.with a volt meter .each should have about 12.7 volts each both on posts and on terminals pull seperate cables from one battery and check other then check other battery if no volt drop on cable terminals they are clean. remove cable and test each battery with load tester to find out if you have one battery weak or both bad. if batteries are weak they will cause low voltage and starter will draw more amps than normal.if good go to starter and remove starter relay wire. ( starter will not engage with this wire off) try to start with new fuse installed if does not blow fuse . remove starter and repair or replace.