Fuse F9 is the Junction Block blows. I measured about 4 volts on the downstream side of the fuse holder and 12V on the power side with ths fuse removed. Is this correct? What are the possible reasons for the fuse to blow? With the fuse in place the truck starts and runs until the fuse fails. Thanks for any helpful advice.

 

There is not a fuse F9. I am assuming you mean Fuse 9? Fuse 9 (5 amp) feeds the instrument panel/cluster/overhead console.