Good morning everyone. I have an 02 F-550 w/7.3. The truck keeps blowing the starter fuse. I put in a new starter and it still does it. I replaced the starter relay and it still does it.I traced out the main battery wire and the solenoid wire and they are good. Is it possible that the clutch switch is the problem? I am at a loss on this one.

 

The electron flow starts at your battery and flows to a junction block and then to a fusible link. From the fusible link it goes to a 50 amp fuse in your central junction box. It then moves to your IGNITION SWITCH. Then to the start pin thru a 15 amp fuse in the same central junction box as the 50 amp fuse to the clutch pedal position switch(CPPS). Then goes to your starter relay, thru the coil of the starter relay and then to a black wire and terminated to ground.
It could possibly be your CPPS or your ignition switch. Do you have a multimeter and some time?

OK. Before you do any of the following I want you to check the wire connection going to your CPPS. Unplug the connector and make sure you do not have any corrosion or rust in the connector and that the Pins in the one side of the connector do not look like they are pushed into the connector too far. It might just be a loose connector or wire termination causing the high resistance which is frying your fuse. You can also check the connection coming from your ignition switch and verify the same thing. Is everything tight and pins not pushed back into a connector and free of corrosion.
Next.....
Remove the positive lead(s) from your battery(ies). Switch the multimeter to ohms and check that the two wires on your multimeter when placed together have a very low reading close to zero, maybe less than one ohm. The black lead on the meter should be in the ground socket of the meter and the red lead should be in the V/Ohms socket in the meter. Now we know your meter is functioning properly. Do ensure you have a fresh battery in your multimeter also. Place the lead on the terminal from the battery and find the terminal that goes from the ignition and leads to the CPPS. You will need alligator clips or another person to turn the key to start to check what you are reading on the meter. At this point you should be reading across the start pin and the power being supplied to your ignition and we are checking for continuity( a low resistance reading ) across the start pin of your ignition switch. If it is above maybe 2-5 ohms on this test then it is your ignition switch that is bad.

Now you can check your clutch pedal position switch. Still with the battery not connected...connect the multimeter, still in the ohms/resistance mode, one lead on the lead going into the CPPS and the other lead going back out of the CPPS. Now depress your clutch pedal and you should read the resistance across the CPPS. Again if you have a high reading here then the CPPS is worn and needs to be replaced.

It all comes down to the fact that something is providing a high resistance connection from the battery to ground and it fries the smaller fuse. You can PM me and get my number to further look at this if you are not familiar with a multimeter. Baltimore is quite a drive for me to help you at this moment as I am in Augusta, GA.
Please Advise