Fast vs Slow-Blow fuse for FF

I was installing an old Eagle Cuda 350 S/Map fish finder in my wife's yak. The manual says to use only a 3 amp AGM glass inline fuse close to the battery. Thinking 3A fuses were pretty much the same, I replaced their inline connection with an inline ATC standard blade fuse, which I could get in a waterproof enclosure. The first time I turned it on the fuse blew! All polarity in the harness was correct, 12V battery was fully charged and FF worked fine when connected to my 12V ice fishing setup. So in researching I discovered that AGM fuses come in slow or fast blow models, while ATC blades are all fast blow. I cannot imagine what would cause a surge on power-up of a FF, but right now that is my best guess as to what is happening. My options seem to be to use a 5A blade or a glass AGM in a non-waterproof enclosure and neither option seems like a good one. Any advice?

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I'm guessing whatever is causing it to blow will cause it fast or slow. There's a fault somewhere

It's wierd; I connected my ice fishing battery (sepatately fused with 3A blade) through the same power line and the FF lit-up fine. So last night I put the brand new battery (12V 9Ah Mighty Max "Hobie replacement")
on my charger overnight. It was measuring 12.4 V right out of the box, but i thought it couldn't hurt. This morning I hooked it up again through the new 3A blade fuse and everything worked fine. Bad first fuse? Something wierd about the brand new battery?

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Could have been a dud fuse or an accidental short circuit. Did the fuse "blow" or did it just not work straight from installation?

It just did not work when I turned the FF on. There was no fireworks show, smoke, etc to indicate a short.

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You can always place a DC current meter in series where the fuse is located and measure the DC current......it's simple and confirms if or is not an issue with your fish finder.

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Dr.SteelheadCatcher
Warrenton, OR

Thanks for the advice, Doc. I ran for several hours today with no issues, so unless the problem recurs and forces further investigation like you suggest, I will chalk it up to one bad fuse.

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