Alternator or mega fuse issue.... - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 01-18-2015
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Alternator or mega fuse issue....

My truck lost all power while driving on the highway yesterday. I was stranded away from home for 3 hours

I charged my battery last night and today, I tested everything except the mega fuse. I can't find the damn thing!!!!

I think it's the alternator giving me issues, but based on everything I've found on this site, the mega fuse MIGHT be the culprit as well.

My truck is an '04 4.0L V6. Can anyone post a picture of where the mega fuse is? It's the 175A fuse that prevents power from going from the alternator to the rest of the truck.

Thanks.
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Old 01-18-2015
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Never heard of a mega fuse.. but if I had to guess. it'd be under the hood somewhere. Got a manual? Test the alternator?
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Old 01-18-2015
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Get a Volt meter, set it for 20vDC

key off, engine off
Touch Black probe to "-"(Negative) on battery
Touch red probe to "B+" on the back of the alternator(it is the big terminal with the big wire and nut)
If it shows battery voltage(12volts) the "mega fuse" is fine.

Follow the B+ wire it goes directly to the mega fuse, usually located next to the Power Box in the engine bay.

Vehicle can continue to run just on battery power, with headlights on about 20 minutes, headlights off 40 minutes.
And it won't just shut down, it will start to run rough as spark voltage drops and then die as spark and fuel pump stop.

So there would be no sudden shutdown.

Battery voltage should be 12.3 to 12.7 volts
With engine running battery voltage will show 13.5 to 14.8 volts, the extra voltage is the alternator working.
4 to 5 minutes after startup battery voltage should be down to 13.5 to 13.7 volts, that's the maintenance charge to keep battery charged but not boiling, continuous voltage above 13.9 volts will ruin battery over time.
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Old 01-19-2015
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Bad alternator...

I had already tested the voltage yesterday morning (forgot to mention that). It only registered 11-12 volts with the engine running, but I had read that if the mega fuse is bad, it would prevent the battery from charging too...

...but then again, it would also prevent ANY power reaching the vehicle's other systems.

5 minutes to remove the alternator
1.5 hours to go had the alternator tested and buy a new one
5 minutes to install the new alternator

lmao
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Old 01-19-2015
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Good work
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