I've decided to start a new thread on this issue because:

A. I was not previously involved in this project and
B. Since this has been an ongoing problem that has not been diagnosed by a shop, I'm exploring what I assume to be non-traditional diagnostic procedures

The truck in question is a '93, F250, 4x4, SCLB with a 351 and auto transmission. Dr. Monstaa uses it to get to my shop nearly every week day and had been having problems with the maxi-fuse in position "I" of the under hood fuse box blowing regularly. He thought he had it fixed by a local shop, but as of today it started doing it again, in the yard at my shop. To the point that it's undriveable (3 times in 5 minutes)

I did some diagnostics using the photo's provided by subford that I'll re-link to here as long as he doesn't ask me to remove them (thanks subford):







Based upon the wiring diagram provided by subford, it's clear to me that there is nothing wrong with the fuel pumps as suspected in the previous thread on the subject. The thing blowing the fuse either has to be related to the red wire (and its branches) coming from the PCM relay or it has to be related to the yellow wire coming from the fuse going to the PCM.

We did the basics of wiggling wires while it was running trying to create a failure and we were unable to do so with that method. The failure only seems to occur while the truck is moving. The only way I know of to figure out where the failure is actually occurring is to narrow down the possibilities the next time an actual failure occurs. Therefore I would like to:

1. Put an inline fuse on the red wire right after the fuse box

2. Put another inline fuse on the yellow wire that comes from the "I" fuse and goes to the PCM.

3. Remove the fuse that is in position "I" and replace it with a device (such as a wire with spade connectors on each end) that will not short before the two inline fuses short.

If I do this, the next time a fuse blows, it will be either on the red wire or the yellow wire and not the fuse that supplies both of those wires. In this manner I will be able to slowly diagnose (by adding inline fuses as required) where the problem actually is rather than having failures be isolated events.

This will likely require cutting the wiring harness in several locations which is quite unfortunate and something that I would definitely prefer not to do. However, I know of no other way to figure this out in a timely manner. Does anyone else have any suggestions that would help me to avoid doing what I intend to do. Does anyone have any huge opposition to the process that I intend to use since I would imagine it's not a very traditional process?

Thanks in advance, and thanks again to subford for providing the necessary service information to start a good diagnostics process.