WHY WOULD THE 100 AMP BATTERY FUSE BE BLOWING?

  • Tiny
  • Jack Jennings
  • 1999 Nissan Altima
  • 142,000 miles

Why would the 100 amp battery fuse be blowing. I know it has a short, but what feed off of this. None of the smaller fuses blow.


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Saturday, October 6th, 2012 AT 9:06 PM

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  • Tiny
  • caradiodoc
  • Expert
  • 25,641 posts

That circuit usually leads to the generator's output terminal. Measure for voltage there. If it's missing, disconnect that wire, then measure for continuity to ground from the wire and from the output terminal. If you read continuity to ground on the output terminal, replace the generator. If you read it on the wire, look for a place it's rubbed through and grounding out.

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Saturday, October 6th, 2012 AT 9:19 PM
  • Tiny
  • KHLow2008
  • Expert
  • 41,994 posts

The 100 A fuse is the main fuse for many components but since othere smaller fuses are not blowing, the one that mainly feeds off it would be the charging system, meaning the alternator. Disconnect the alternator main wore and tape it up and test if it shorts.

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Saturday, October 6th, 2012 AT 9:26 PM
  • Tiny
  • Patrick Jones
  • Member

I'm having a problem with my 99 nissan altima's alternator and battery system after I accidentally touched (with a wrench) the power terminal of the alternator while the battery was connected causing a spark. Something keeps blowing the 100amp fuse that is apart of the cable system connecting the alternator with the battery. It blows when I attach the positive end of the battery cable. I think a short to ground would cause the fuse to blow and the only major things in this system that is capable of blowing a 100amp fuse are the cables, alternator, 100amp fuse and the battery. I disconnected everything and I am not getting a short to ground in the cables. With the alternator removed from the car, the positive power terminal on the alternator has continuity to its case (ground). The same happens if it is connected in the car except it also has continuity to car ground. I took the alternator into Auto Zone and their tester checked it out fine. If the alternator is fine and the cables are not shorted to ground what could possibly causing that fuse to blow?

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Wednesday, September 14th, 2016 AT 7:10 PM
  • Tiny
  • Patrick Jones
  • Member

The car will still start up and run but a lot of things aren't working including windows, a/c, blinkers, all of the dash lights gauges. The things that do work include the lights, emergency flashers and radio.

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Friday, September 16th, 2016 AT 6:15 AM
  • Tiny
  • caradiodoc
  • Expert
  • 25,641 posts

You need to start a new question with the car information, (engine size) pertinent to your car. Your post can only be seen by those of us in this conversation. That does you a disservice because none of the other experts will get to see it or have a chance to reply. In this case I can offer some general advice that pertains to any car.

If I followed that correctly, you've stumbled onto the cause of the problem. You just don't see it yet. That large terminal on the back is the output terminal. Current flows through it and back to the battery positive post. You said you found a direct short to the case of the generator, and that will definitely blow the fuse. There's only two things that can cause that, and only one of them can be intermittent.

First of all, every AC generator, ("alternator" is a term copyrighted by Chrysler, but that's not important now), has a minimum of six diodes, and some have more. A diode is a one-way valve for electrical current flow. There are actually two sets of three diodes per set, and due to how they are installed, they are backward to current flow when the generator is not working. Being in the circuit backward blocks current flow from discharging the battery when the engine is not running. It is not uncommon for one diode to short. When that happens, one third of the generator's maximum current rating is all you'll be able to get during a full-load output test. 30 amps from the common 90 amp generator is not enough to run the car's entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

The more serious problem occurs when two diodes short. One has to be in each set of diodes, then you have a direct short to ground, ... And a blown whopper fuse. Very often a single shorted diode will overheat very quickly and burn open. That removes the potential for a dead short if another one shorts, but it still leaves you with insufficient output current. The current you DO get has to come through the remaining diodes, and if they're running wide-open it won't be long before one of them gets hot enough to short. Regardless of the chain of events, if two diodes are shorted and neither one burned open, you have a dead short. That failure is never intermittent. When they're shorted, they're shorted. There is no way testing on or off-car is going to show the generator is working.

The second, and more likely cause of your problem is in the cable connection, specifically how it is bolted on. Given the history of accidentally grounding that point with a wrench adds to the likelihood the terminal is damaged and when things move or wiggle just the right way, the terminal shorts to the housing. That short CAN be intermittent. More importantly, that cable isn't at the store when the generator is being tested.

What I would do is stuff the generator back on the engine, bolt the output cable onto the terminal on the rear. Do this with the battery negative cable removed! Then, since 100-amp fuses are expensive, grab an old 20-amp spade-type fuse and hold it between the negative post on the battery and the negative cable clamp. If the circuit to the generator is still shorted, that 20-amp fuse will blow with no undue excitement and save the expensive 100-amp fuse. If the 20-amp fuse doesn't blow, there's no short and it's safe to reconnect the battery cable.

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Friday, September 16th, 2016 AT 10:33 PM

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