175A megafuse keeps blowing out - Ranger-Forums - The Ultimate Ford Ranger Resource


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Old 09-01-2013
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175A megafuse keeps blowing out

Alrighty I've read up a lil and it's saying that usually there is a short of some kind on the charging circuit to cause this. The fuse isn't melted or anything like that it just keeps blowing out. I've put a couple of them in there so far. Is there anything else besides a short that could cause this? I'm pretty sure there isn't a short anywhere. I did leave the ground strap from the body to the hood undone which i just corrected but I don't think that has anything to do with this. Could it be a bad alt?
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Old 09-01-2013
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I would have your alternator checked at autozone or something, its free and they can tell you if it is over charging. What does the volt meter say while it is idling?
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Old 09-01-2013
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I've seen an alt ground itself internally once. Like Zabeard said have 'em test it.
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Old 09-02-2013
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Does the fuse blow when the key it turned on or when the engine is just running ?

After checking the alternator, check all the wiring for the alternator ?
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Old 09-02-2013
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What numbers should I be looking for at which rpms? I'm gonna have it tested pretty soon. It blows intermittently sometimes it's a couple days or even sometimes a week or so. It's only blows engine running then the battery light comes on. So it just stops charging the battery.
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Old 09-02-2013
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Quote:
Originally Posted by Alex98 View Post
What numbers should I be looking for at which rpms? I'm gonna have it tested pretty soon. It blows intermittently sometimes it's a couple days or even sometimes a week or so. It's only blows engine running then the battery light comes on. So it just stops charging the battery.
The best idea is to start at either a garage with an alternator/battery tester to test these components under load or at an auto parts store but I think you will have to take the alternator out.
Once you have determined that your alternator and the battery are good, no bad cells in the battery, then you can proceed to checking out the wiring.

BUT since it shorts over time not right away, as the others have stated it is probably a back alternator, partially shorted or just getting dirty inside and will need to be replaced. Get it tested first and then decide.

Don’t waste your time trying to determine what amperage at what rpm, just take it there and get it done.
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Old 09-02-2013
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Quote:
Originally Posted by Scrambler82 View Post
Once you have determined that your alternator and the battery are good, no bad cells in the battery, then you can proceed to checking out the wiring.
x2

But with the "amazing build quality" of alts these days I'd start with the alt. I remember I replaced one under warranty at the Ford dealer only to have the new one not even putting out 7v. If you do get a new one, do yourself a favor and have them test the new one before you leave.
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Old 09-02-2013
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Most auto parts stores can check them in or out of the car.

You should see close to 14v at idle and it really shouldnt change if you rev it up
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Old 09-08-2013
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yeah it checks out fine, did the test and everything. Putting out 14.3 V or something like that. if it had an internal short that intermittently caused problems don't you think the tester wouldn't be able to find the issue unless it was doing it at the time?
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  #10  
Old 09-08-2013
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I have run into the same problem. The alt tested fine at idle. Replaced the fuse and it was fine for a couple of weeks before the same problem occurred. Replaced the alt and never had the problem again.
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Old 09-25-2013
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Well I currently have a couple other issues to work out but its stopped blowing the fuses and started simply to cause the battery light to come on and have the alt not charge the battery. When disconnected it seems to reset and everything seems to work fine again for about another 250 miles and does the same thing over again. Seems pretty strange.
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Old 09-25-2013
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You didn't say if you replaced the alt. If you did have it checked for output. Chances you got a faulty one. Happens all the time. Your truck was charging before, therefore it should be charging with new alt.
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Old 09-26-2013
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I have not replaced the alt yet, sry lol.
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  #14  
Old 09-26-2013
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The reason it quit blowing the fuse is because your alt quit working. Very simple have it checked. You have to try what we suggest.
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