Reasons this could be happening?

 

We're going to need more info than this. Does it blow as soon as you turn your key to auxiliary? run? Does the truck run for a while before it blows? What does the fuze go to?

 

Amen! A problem well-stated is a problem half-solved.
What truck?

 

1985 Toyota pickup I'm not sure what it goes to but it will run for a while then out of no we're it will blow

 

How long does the truck run before it blows? Consistent? What if truck just idles?

If same schematic as 86 4Runner. 80-Amp fuse supplies:

• Engine fuse (15A),
• Defogger (15A for truck), and
• Wiper (15A).

If there is a short in any of those the lower-rated fuses should blow first. UNLESS one of those fuses has been bypassed.

A few things to check which would not cost you a penny:

• Make sure fuses above are not bypassed and have correct rating (double check your schematic, because I'm looking at my 1986 schematic)
  
• Inspect wiring to back-up switch which could be rubbing/shorting against tranny. See it here.
  
• Remove fuses to wiper and defogger and drive like usual. Or leave idling and see if 80-A blows again.

Let us know what you find out.

If above does not solve problem, I would suspect something in charging system. Could something in alternator be shorting to ground - like a bad rectifier or bad winding? A bad rectifier should smoke though.

Could battery be bad that it takes too much charging current?

 

The wire that went into my fuse box had a exposed spot and I wrapped it In heat wrap and I no longer have the problem thanks for the input

 

There you go. Cool that you fixed it. Clean robust wiring is a must if you don't want your truck to give you trouble.

 

Glad you found your short. .