Alternator not charging battery - Mercedes Benz SLK Forum

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#1 Old 03-12-2014
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F1 alternator fuse blown

Just installed new alternator and battery. Alternator is putting out 20v on the large terminal but the battery is not charging. Checked f1 fuse and it is blown. Jumped it and battery is now charging at 14.2V. Any idea what would cause that to blow?
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#2 Old 03-14-2014
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Higher than expected (20) voltage will increase the current, causing the fusing to blow out.

Try Check

1) Bad voltage regulator, leading to too high a voltage, causing the
fuses to blow out.
2) Wrong fuses.
3) There is a short circuit in the circuits controlled by that fuse.

It should be easy to test for [1] with a voltmeter and [2] by checking the
fuse ratings against the ones in the owner's manual. If it is [3], You need to go and figure out where the short(s) is.

14.2-14.6 Volts is normal but not 20 volts..

Good Luck
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#3 Old 03-15-2014
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It might be a little simpler than that. If the battery was left connected while changing out the alternator, the wire could have shorted to ground, blowing the fuse. That is the more likely scenario.

Just replace the blown fuse and you'll be good to go. BTW, 20 volts is not out of the question if the alternator is not connected to the battery.
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#4 Old 03-15-2014
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You may be right but I know off NO mechanic that would change out the alternator without disconneting the battery. Also as the battery was changed at the same time I don't think that will have happened. BUT you never know. But that's covered under the Number #3.



Let us know what you find out..
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#5 Old 03-16-2014
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Battery was not connected during alternator install. Actually, I had purchased a battery and a new alternator because the PO had mentioned that the battery wasn't charging. It turns out that the original alternator was fine and the blown 125 f1 fuse was the problem. I'm wondering if someone reversed their jumper cable leads and blew the fuse. Curiously, the 50 amp fan fuse next to the 125a was blown, as well. With the fuses replaced everything is running great. Guess I may never know what caused it.
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