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Fuse blows when battery disconnects

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Hi all,

Did some research and haven’t come up with a good answer to this one.

A several days ago, I was idling in the parking lot with the AC blowing because it was HOT out. Suddenly, the AC stops blowing, the airbag light comes on, and the idiot tone comes on.

Worried something bad was happening, I turned off the engine and started looking around for what the issue could be. Couldn’t find anything, so tried to start the car… turned 5e key and got all the standard lights and tone in the ON position… then nothing when I turned the key to START.

checked the starter fuse, and sure enough… fuse is blown.

I then remember reading a post about the HVAC blower being on the same circuit, so turned off the AC, used one of the other fuses, and the car started. Took the fuse out in case it was a problem, and drove home.

At home I had a spare on hand, replaced it and didn’t have a problem.

Today, while driving home, same thing happened, AC dies, airbag light comes on, tone. Drive home, try to jiggle cables to see if I’ve developed a short somewhere, but this time, as soon as the new fuse went in, it popped again - with nothing turned on! And by pop, it’s a visible flash and audible pop when a 30A fuse blows!

Disconnected the battery, did some more jiggling of the starter cables and wiring harness, disconnected the AC/heat specific fuses and reconnect the battery.
plugged in the last fuse I had, and everything seemed to be ok.

Then I disconnected the battery and saw the arc flash and pop. BIG ARC FLASH that melted the top cover of the fuse!

Couldn’t believe it, so did it again with my fuse from the exterior lights, and sure enough, as soon as the battery disconnects, the fuse BLOWS!

What on earth would cause that circuit to blow when disconnecting the battery?

I’m usually good at the electrical stuff so I’m really puzzled by this!
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interesting and a 30 amp to add drama... details - what year TJ which trans, the fuse is it in PDC or JB and do you know which # it is?

Anything AFTERMARKET added except tires, esp electrical - mega watt trail lights, a Chernobyl audio amp.
 

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If it's blowing as the battery is disconnected, I'd suspect the battery cable is moving the loom near the battery and shorting out a PDC fuse.

If we know the year and fuse number we can tell you what wire colour you'd be looking for in the loom.
 

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What year is your Jeep?

You're blowing the ignition switch fuse under the hood, right? According to the below link, depending on year that fuse can be 40 or 30 Amps. Do you have the correctly rated fuse?

'97-'06 Jeep Wrangler TJ Fuse Box Diagram (knigaproavto.ru)

For a fuse to blow when the battery is connected/disconnected there must be a constant draw on that circuit. Like a short. If you removed all of the fuses associated with the ignition switch (airbag, HVAC, Neutral switch, or whatever is on your jeep) then I would look toward the ignition switch itself or the starter relay.

I don't understand what you mean by your "outside lights".

Having a fuse blow when you disconnect a power source could make sense. When there is demand on a circuit and you briefly interrupt the power source, there can be a surge in current draw greater than the fuse's current and time ratings. You can't get a clean disconnect/connect in one millisecond on your battery terminal. Think about those little sparks you see whenever you connect a battery. That's from things that always run off the battery (like parts of the radio and ECU). Never experienced it on a car, but have in other electronics. Just a theory.
 

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Discussion Starter · #5 ·
All, thanks for the replies.

It’s a 2006 LJU with the 42RLE auto transmission. It’s the #6 fuse in the main power distribution box next to the battery. It’s not the loom itself as I’ve played with that to no effect and I have a quick disconnect at the battery that I use when I park the Jeep for long trips that I’m gone for more than 10 days on.

When I get home next week, the plan is to pull all the fuses from sub-circuits coming off the ignition to see if anything changes.

I’ll keep y’all in the loop.
 

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Good info. That fuse (6, 30a) besides powering the starter solenoid through the relay also powers the ignition switch for the circuits for fuses 5-8 in the fuse block behind the glove box. So only need to deal with those. However, when the key is off those circuits are disconnected from the battery (& fuse 6). Plus, those fuses should be 10a or 20a fuses so if any of those circuits were shorting to ground any of those fuses would blow before the 30a fuse at #6.

I would agree with Jonny that moving the cables is probably causing the short. That fuse has battery power all the time so if that red wire going to either the starter solenoid or through a splice on to the ignition switch get shorted to ground it will blow & the better the ground contact is, the hotter the short.


The wire going to the relay is confined to under the PDC but when the spliced red wire leaves the PDC heading to the ignition switch it is more venerable to damage & shorting to ground as well as the yellow/grey wire which goes from the relay to the starter solenoid. However that wire only has battery power when the starter is engaged so a short there would only pop the fuse when cranking the engine. So if it has popped when driving it is more likely to be shorting in the red wire going to the ignition switch which has power even if the key is off.

The most common area to get such a short is in the harness leaving the PDC when it passes near the battery. That is a known area where battery acid can damage any of the various wires in that harness. That would be where I would look first.
 

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Discussion Starter · #7 ·
Well, y’all were right, the harness right between the PSU battery was crushed. Apparently when I replaced the battery about 2 months ago, the new one has a bit more of a lip on it and it’s been slowly working it’s way through the loom.
Motor vehicle Automotive tire Wood Gas Auto part

So now, the question is whether this is a pick through and tape with electrical tape fix, or am I looking for a new harness?
 

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Thanks for posting findings & the photo too. It may help others when searching with similar issues.

I would open it up & inspect the actual damage & hopefully just a fairly easy repair rather than finding a harness which might be in worse condition than yours.

Make a good splice where (if) needed but rather than or in addition to the tape use shrink tubing to get a better, tighter seal to avoid corrosion working its way in especially since it is near the battery where acid can be a problem.
 

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I’d pick through and use liquid electrical tape individually on the conductors. Tape alone will trap moisture and cause more damage than good down the road. After the liquid tape had dried then you should replace the split loom and re rout the wiring.
Edit- assuming the wires hav only exposed conductor and no cut conductors. If a couple strands are cut on a particular wire I’d call it ok- if a wire is too damage you may need to pull it pack in the loom each way- then splice in a new section of wire between the two ends. Soldering is best practice with a western union splice. If that’s not possible I use gel filled dolphin connectors, then liquid tape, then heat shrink over that assembly after it’s dried.
 
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