2012 Explorer Keeps Blowing Alternator Sensor Fuse

[goalieThreeOne]

Again, I understand what’s being protected. I get fuses. I get over current protection. I understand fusing. It’s to stop the insulation from burning. (And, for the record fusing is also used to protect loads. Any good PLC output system fuses by the expected device draw, and not just the wires ampacity rating).

Do I have to say it again? I get fusing.

It’s not at all the wire size, it’s the insulation type. If enough heat soaked in there to open the fuse, and trip a 25 amp fuse immediately, the wire INSULATION would be damaged.

For the sake of discussion, and only for the sake of discussion let's imagine that I have a 300MCM conductor fed by a 15A fuse. That conductor is baking in the desert and the ambient air is 150 degrees F (using the numbers I' already looked up since my book is back at the office). Nominal load is ~10A DC. Accounting for heat rise, that conductor is now approaching a temperature that imitates the same heat rise of a 26A load in a standard. The fuse blows because it's met its threshold. But the conductor, including the insulation were never in any danger of overheating at any point because that conductor was oversized. Same thing here. If Ford used a 14 GA when a 22GA would have met spec, but still fused it at 10A, the conductor is in no danger whatsoever, but the fuse blows. You can't make the assumption necessarily that the fuse was specc'ed 1:1 to the conductor. In all likelihood, it would be to Ford's benefit to oversize conductors that sit in a hot engine bay for hours on end.

I could make another silly example. If I put a 25A fuse in-line with my battery and cranked my car up, the fuse heats up past its threshold and burns out nearly immediately. But the conductor was never in any danger. This is only an extreme example to illustrate my point.

 

[Mbrooks420]

For the sake of discussion, and only for the sake of discussion let's imagine that I have a 300MCM conductor fed by a 15A fuse. That conductor is baking in the desert and the ambient air is 150 degrees F (using the numbers I' already looked up since my book is back at the office). Nominal load is ~10A DC. Accounting for heat rise, that conductor is now approaching a temperature that imitates the same heat rise of a 26A load in a standard. The fuse blows because it's met its threshold. But the conductor, including the insulation were never in any danger of overheating at any point because that conductor was oversized. Same thing here. If Ford used a 14 GA when a 22GA would have met spec, but still fused it at 10A, the conductor is in no danger whatsoever, but the fuse blows. You can't make the assumption necessarily that the fuse was specc'ed 1:1 to the conductor. In all likelihood, it would be to Ford's benefit to oversize conductors that sit in a hot engine bay for hours on end.

Why does it blow immediately and takes no heat soak?

Why is it not too hot to touch?

Because this is a short circuit scenario, not some wild heat soaked wire traveling to a fuse.

 

[goalieThreeOne]

Why does it blow immediately and takes no heat soak?

Why is it not too hot to touch?

Because this is a short circuit scenario, not some wild heat soaked wire traveling to a fuse.

In OP's case he mentioned that it was already hot when he swapped in the 25A. He said he started from a cold engine using a 10A fuse and it took some time to blow. In the first case, the wire was already at high temperature. The second case it took some time for the conductor to heat up.

I'm not sure if he mentioned what he physically touched. I could imagine that the wire loom might be able to cool off much quicker than the inner conductors simply due to air flow. Perhaps it cooled off rapidly enough in the time that it took to pop the hood. Maybe he had gloves on. The insulation is likely at one temperature based on the ambient air, but the conductor is likely to be hotter because it's got additional heat rise from current flowing through.

I suppose we could argue all day, and I don't want you interpreting this as me telling you that you are wrong. I'm simply trying to explain my reasoning for an alternate theory. Hopefully OP updates us with pictures from the harness. I'd be glad to be proven wrong, if only for the sake of satisfying my curiosity.