20 amp fuse is blown. replaced starter twice and replaced starter relay help.

Need more info! Does it crank? Are both batteries good, What is your voltage? If it cranks, what is your FP? Do you have fuel to the injectors, to the VP, to the fuel filter, to the fuel pump? Have you checked for codes? What year of truck?

Hmmm... Starter circuit is rather simple. The key triggers the relay in the PDC and has a fuse for going down to the starter. So for testing purpose unhook the brown wire on the starter and cover the end. Turn the key to start and check the starter fuse is it blown? If not then the starter has something internally wrong with the solenoid. But if its is blown there is a short in the brown wire between the relay and the starter solenoid.

Good conductive connections... batteries, cables, grounds? Need a flow chart & systematically eliminate possible causes.

I would check your main harness that runs by your left foot. O wait ill back up do what mike said first check grounds and everything before going to my step. I was up hunting and my truck wouldn't start ended up the main wiring harness had melted together by my foot.(to many POS roads) My truck keep blowing the 30 amp ignition fuse in the engine bay and the 20 amp in by the door. I also found that my head light switch the plug was melted where it had been hot from being loss from vibrating. I believe the ignition power travel through it also. The They have head light switch now you can get with the pig tail included.