2012 Explorer Keeps Blowing Alternator Sensor Fuse

[Toddy968]

2012 Explorer FWD 3.5 litre

Fuse 58 keeps blowing, Alternator Sensor Fuse
Battery Replaced
Alternator tests good

Vehicle runs normally for extended times. Any suggestions on what else I could check? Clearly for the fuse to keep blowing it’s getting an electrical spike of some sort. Could it be the generator sensor on the negative battery cable?

 

[ProjectSHO89]

There are only two things that I can see that will cause F58 to blow: Either the regulator in the generator is shorting to ground or the YE-RD wire between the fuse and the generator is.

 

[Toddy968]

There are only two things that I can see that will cause F58 to blow: Either the regulator in the generator is shorting to ground or the YE-RD wire between the fuse and the generator is.

Thanks for your reply. When the key is in the off position and I try to replace the fuse it instantly blows. I may end up just dropping it off at the dealer Monday

 

[ProjectSHO89]

Disconnect the alternator connector and repeat. If it still blows, it's a wiring problem, most likely due to a chafed or pinched wire between the fusebox and the alternator connector.

If this is the case, the problem will likely be found by a visual inspection of the harness. You can likely do that by inspecting the harness looking for rubs or the like on bolts, studs, brackets, etc.

 

[Toddy968]

Disconnect the alternator connector and repeat. If it still blows, it's a wiring problem, most likely due to a chafed or pinched wire between the fusebox and the alternator connector.

If this is the case, the problem will likely be found by a visual inspection of the harness. You can likely do that by inspecting the harness looking for rubs or the like on bolts, studs, brackets, etc.

After tracing the harness I found that the harness had come loose from its mounting points on the valve cover and had rested against the oxygen sensor. The wires all appear to be intact with no damage to the shielding. Looks like just the plastic wire loom is damaged. I moved the harness off of the o2 sensor, reinstalled the harness to the correct mounting points, popped a fuse in and it’s now running normally

 

[ProjectSHO89]

I'd suspect that the wiring's insulation was indeed damaged, otherwise it wouldn't have blown the fuse.

If the damaged wiring is left open to the environment, eventually the conductor will corrode and cause a high resistance followed eventually by an open circuit.

 

[goalieThreeOne]

I'd suspect that the wiring's insulation was indeed damaged, otherwise it wouldn't have blown the fuse.

If the damaged wiring is left open to the environment, eventually the conductor will corrode and cause a high resistance followed eventually by an open circuit.

I have an alternate theory as an EE. If the loom was damaged, it's possible that the conductors inside that loom were exposed to higher than allowable temperatures. And while it might have not damaged the outer insulation of those conductors, it's possible that the conductors got hotter than expected. Since those conductors are...conductive...that heat may have travelled all the way through to the fuse itself. A fuse operates on the principle of heat, the higher the current through the circuit the hotter the fuse becomes until it reaches its threshold and burns. The fuse has no way of discerning whether a rise in temperature is cause by excessive current or, hypothetically, a rise in the base temperature of the conductor. So if the conductor is hot, the fuse will "trip early".

When designing electrical systems for buildings, heat rise is the most important consideration when picking conductor sizes, wireway and conduit types, and overcurrent (fuse) protection. I have a book on my desk right now with dozens of tables of conductor and fuse sizing guidelines based on temperature factors.

Just a theory based on some of my professional knowledge.

 

[ProjectSHO89]

Interesting idea but I'd have to rate it as improbable in this instance. I'd point out that the heat would have to travel 1-3 feet up an 18 ga insulated wire with sufficient thermal energy at the fuse to cause it to open due to the conducted heat, all without damaging the insulation. Seems implausible.

After doing electrical and electronics repair for four and a half decades and having had to deal with more than my fair share of short circuits, I'm inclined to go with Occam's Razor first.

I would posit that the insulation damage was likely not seen/observed but is, nevertheless, present.

 

[goalieThreeOne]

Interesting idea but I'd have to rate it as improbable in this instance. I'd point out that the heat would have to travel 1-3 feet up an 18 ga insulated wire with sufficient thermal energy at the fuse to cause it to open due to the conducted heat, all without damaging the insulation. Seems implausible.

After doing electrical and electronics repair for four and a half decades and having had to deal with more than my fair share of short circuits, I'm inclined to go with Occam's Razor first.

I would posit that the insulation damage was likely not seen/observed but is, nevertheless, present.

In my hypothetical, the heat is conducted through the copper itself. When we size conductors, we size them not only for load, but also for ambient temperature up to an acceptable rise above that ambient temperature. Wire insulation is not a perfect thermal barrier. Since he reported that moving the loom back into place fixed the condition, I'm inclined to think the insulation was "fine". I suppose we don't really have a way of knowing without having the harness in front of us to review in detail.

 

[Mbrooks420]

In my hypothetical, the heat is conducted through the copper itself. When we size conductors, we size them not only for load, but also for ambient temperature up to an acceptable rise above that ambient temperature. Wire insulation is not a perfect thermal barrier. Since he reported that moving the loom back into place fixed the condition, I'm inclined to think the insulation was "fine". I suppose we don't really have a way of knowing without having the harness in front of us to review in detail.

@Toddy968 was the car hot when you attempted to replace the fuse and it blew, or had it cooled?

 

[Toddy968]

@Toddy968 was the car hot when you attempted to replace the fuse and it blew, or had it cooled?

The first fuse I put in the car was cold, didn’t pop. Took it for a drive and it popped about 15 minutes in. Kept trying to replace the fuse when it was hot and it kept blowing instantly, I even stuck a 25amp instead of a 10 amp just to see what it would do and that instantly blew. The car sat over night and after tracing the wires I found it was pressed against the forward O2 sensor. After inspecting I saw no visual signs of damage but non the less the harness will be replaced. I put the harness back on its mounts, installed a new 10amp and ran it for several hours without popping the fuse again

 

[ProjectSHO89]

Since he reported that moving the loom back into place fixed the condition, I'm inclined to think the insulation was "fine".

It suggests to me that moving the harness took it away from the point where it was shorting, thus removing the short to ground.

Occam's razor.... Keep that in mind as an engineer.

 

[goalieThreeOne]

According to NEC 310.15(B)(2)(a), a swing from (for example) 110 deg to 150 deg requires a .375 adjustment factor. Meaning that 10A through the conductor can look like as much as 26A to the fuse. I don't know the actual temps involved but it just illustrates how ambient air can affect a conductor wildly.

Given that he describes that the issue presented itself after the engine warmed up this sounds more plausible. As I have over ten years as an electrical engineer and I make my living by understanding and applying these concept, I don’t have to rely on an idiom to provide knowledgeable answers to questions I have conceptual understanding of.

 

[ProjectSHO89]

Okay, how much thermal is energy needs to reach that 25A (max he tried) in order for it to open?

Idioms help keep one grounded in reality, that is, both what is plausible and possible.

 

[Mbrooks420]

According to NEC 310.15(B)(2)(a), a swing from (for example) 110 deg to 150 deg requires a .375 adjustment factor. Meaning that 10A through the conductor can look like as much as 26A to the fuse. I don't know the actual temps involved but it just illustrates how ambient air can affect a conductor wildly.

Given that he describes that the issue presented itself after the engine warmed up this sounds more plausible. As I have over ten years as an electrical engineer and I make my living by understanding and applying these concept, I don’t have to rely on an idiom to provide knowledgeable answers to questions I have conceptual understanding of.

I don’t believe that’s how it works. You derate the fusing so it doesn’t melt the insulation on the conductor at it’s rated fusing.

If it was like you described it’d essentially derate itself.

Also, the fuse would see the same amperage as the rest of the circuit.

 

[goalieThreeOne]

I don’t believe that’s how it works. You derate the fusing so it doesn’t melt the insulation on the conductor at it’s rated fusing.

If it was like you described it’d essentially derate itself.

Also, the fuse would see the same amperage as the rest of the circuit.

I'm taking the conductor derating concept and working it in reverse. If we know that the thermal envelope of a conductor allows X amps at Y ambient degree degrees and we know that to stay within that thermal envelope at Y2 degrees we can only allow X2 amps using a derating factor Z, then we are able to roughly calculate the additional heat rise of X amps at Y2 degrees by the inverse of Z, 1/Z. Fuses don't see amps, they see heat so it doesn't care where the heat comes from. The purpose of fuse selection, indeed all thermal overload protection, is to prevent excessive heat rise, as in conductor temperature that exceeds the listed thermal envelope of the conductor. Fuses are not sized to protect loads. Fuses are sized to protect conductors from overheating and causing a fire. Perhaps my wording was unclear and it should say something more along the lines of "10A in a conductor at 150 degrees would exhibit the same total heat as 26A at 110 degrees". These are essentially the same calculations one would use to solve the opposite problem which is that "What size conductors would we need if the application called for higher ambient temperatures". In this instance we'd take our table of conductors, derate them, then select the appropriate oversized conductor (and therefore OCP, or fusing) for that fixed load.

 

[Mbrooks420]

I'm taking the conductor derating concept and working it in reverse. If we know that the thermal envelope of a conductor allows X amps at Y ambient degree degrees and we know that to stay within that thermal envelope at Y2 degrees we can only allow X2 amps using a derating factor Z, then we are able to roughly calculate the additional heat rise of X amps at Y2 degrees by the inverse of Z, 1/Z. Fuses don't see amps, they see heat so it doesn't care where the heat comes from. The purpose of fuse selection, indeed all thermal overload protection, is to prevent excessive heat rise, as in conductor temperature that exceeds the listed thermal envelope of the conductor. Fuses are not sized to protect loads. Fuses are sized to protect conductors from overheating and causing a fire. Perhaps my wording was unclear and it should say something more along the lines of "10A in a conductor at 150 degrees would exhibit the same total heat as 26A at 110 degrees". These are essentially the same calculations one would use to solve the opposite problem which is that "What size conductors would we need if the application called for higher ambient temperatures". In this instance we'd take our table of conductors, derate them, then select the appropriate oversized conductor (and therefore OCP, or fusing) for that fixed load.

I understand why fusing is used. I explained that in my first sentence.

I do not believe the fuse is going to see the heat you are describing. Enough heat to open a 25A fuse immediately would have smoked the insulation.

Also, If you really think about it, it wouldn’t open the fuse immediately, as it’d have to heat soak to do the things you are describing.

A dead short is almost certainly causing this problem.

 

[goalieThreeOne]

Okay, how much thermal is energy needs to reach that 25A (max he tried) in order for it to open?

Idioms help keep one grounded in reality, that is, both what is plausible and possible.

I gave one example, but I don't have access to enough detail to tell you what operating temperature that circuit was original designed for when its fuse and conductor where selected. Without a basis to start from, I can't calculate anything precise.

Fun story about that idiom. It's origin came from the medical profession. It's purpose is to guide treatment. As an example, a patient would come in with an ailment like heartburn. Doctor prescribes Prilosec, and gives the patient the advice to avoid spicy foods. After all the "simplest" explanation is that the patient ate something that didn't agree with his stomach. So the symptoms are cured, and the physician has no reason to investigate further. Patient doesn't care because his heartburn is gone. But what if it wasn't spicy food, and it was actually the case of overactive acid pumping in his stomach. Doesn't change things. Treatment is the same. Nothing you can do differently about it unless medication stops working. They teach this in order to put physicians in the mindset of not "chasing zebras", aka wasting the patient's and physician's time or energy on something that isn't likely to be the case.

But what that phrase was never intended to do was have a place in root cause analysis. Here we have solved the patient's heartburn, we have alleviated the symptoms, but we aren't necessarily closer to identifying the root cause. Calling a solution the "simplest" doesn't rule out any other theories. Occam's Razor is not an acceptable response to a mission critical RCA, investigations and analysis must be performed.

Fortunately, I think OP can satisfy all of us by posting some pictures of the old harness when he replaces it. We can see exactly what happened.

 

[goalieThreeOne]

I understand why fusing is used. I explained that in my first sentence.

I do not believe the fuse is going to see the heat you are describing. Enough heat to open a 25A fuse immediately would have smoked the insulation.

Also, If you really think about it, it wouldn’t open the fuse immediately, as it’d have to heat soak to do the things you are describing.

A dead short is almost certainly causing this problem.

What would and would not smoke the insulation depends entirely on the conductor size. In fact we design OCP with a safety factor in mind so that the OCP trips well before a conductor can start to burn/melt.

 

[Mbrooks420]

What would and would not smoke the insulation depends entirely on the conductor size. In fact we design OCP with a safety factor in mind so that the OCP trips well before a conductor can start to burn/melt.

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.