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Brian, Service Engineer
Category: Car
Satisfied Customers: 1227
Experience:  Master Tech, Bachelor's of Science in Automotive Technology, Ford Factory Trained
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99 expedition: 5.4 blows # 23 fuse..only blows..runing

Resolved Question:

99 expedition 5.4 blows # XXXXX fuse in power distrubuion box. only blows when engine get runing temp
Submitted: 9 years ago.
Category: Car
Expert:  Brian replied 9 years ago.
That fuse feeds a lot of different items, here is a diagram showing how the power from fuse 23 splits off to various areas:


One technique for isolating which branch of the many possible short circuit possibilities is to install inline fuses on each branch. Here is an example picture (not the same circuit you have) to explain:

Basically the technique is to find each wire that uses fuse 23, and then install a separate inline fuse on each of the wires that branch off fuse 23. Use inline fuses that are smaller amperage rating than fuse 23. To install each fuse, cut a branch and then splice the cut ends onto the ends of the inline fuse (or crimp some spade terminals on the cut ends and then connect them to the legs of a blade style fuse).

Then try to duplicate the conditions that seem to cause fuse 23 to blow. Check each inline fuse and one will probably be blown, indicating which branch contains the short. Refer back to the wiring diagrams to see if there are any more branches downstream that would require more inline fuses to isolate, or if the branch just goes to a single item.

Follow the wire indicated by the blown inline fuse, looking for any place where it could be contacting another adjacent wire or rubbing against a metal edge causing the insulation to be chafed away and exposing the strands of wire. If the strands contact anything metal, it will cause the fuse to blow. Repair the chafing by replacing the wire or re-insulating it with tape, shrink wrap tubing, or liquid electrical tape, and take steps to protect the wire from chafing again by using zip-ties and plastic wire conduit (split loom tubing).
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