Any ideas on where to hone in a search on why the 30amp PCM fuse located in the under dash panel would blow. Problem started while truck was running and a simple replace of the fuse would it it back alive again. Now fuse is blowing at ignition on. Truck is 1997 F250LD.

 

There are two ignition noise supression caps.
Usually up front on top of motor.
Disconnect and see if it clears the fuse blowing.
Good luck.

 

RFI caps will take out the fuse for the ignition coil.

If it's the PCM fuse blowing, most often it's due to O2 sensor wiring harnesses getting melted on the exhaust, usually after an incomplete repair.

Identifying the fuse by its number designator goes a long way in figuring out what's wrong.

 

Fuse is number 30 in the Junction Box Fuse/Relay panel. The one under the dash below instrument cluster.

 

Yeah, that's the one that the ignition coils run off of.

 

OK so this is what I have tried so far and I still get the number 30 fuse to blow. Unplugged both left and right noise capacitors. Found them on the top of the manifold toward back on both sides. Before replacing the fuse I also pulled out the PCM Power Relay Number 6 in the Power Distribution Box under the Hood. Based on wiring diagrams this would get me down to just power flowing to the capacitors and coils. Did I get that right or did I miss anything? If I do have that right then I guess it is a coil issue? Does that seem right? If so I guess I can start working to disconnect them all and bring them back one by one.

 

OK so I have now also disconnected all coils, PCM relay and noise suppression caps, fuse blows. So I pulled apart connector C120 right under the mega fuses and the fuse does not blow with connector disconnected. Are there other items in the circuit I can disconnect that I am missing? If not then it must be a wiring harness issue as it runs along the firewall as it goes from right to left.

 

Your doing good, keep at it.
We could only guess from here on.
Good luck.