Battery / Alternator Problem ? - Ford Powerstroke Diesel Forum
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Old 10-06-2008, 01:39 PM Rookie

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Battery / Alternator Problem ?

Three weeks ago, truck would not start so removed both batteries and had tested. Both were bad so I replaced both of them. Truck ran fine for about 3 days, and then battery light came on. Tested it and alternator was putting off 12.7 volts, lower than normal but still enough to crank completely fine. Went to airport and parked truck for 4 days. Got back to it and it wouldn't start so jumped it off and it ran for about 10 mins and cut off. With help of very nice mechanic for Parking Spot in Atlanta, he charged both batteries and tested alternator which was then putting off 7.0 volts, but he charged batteries enough so that I could make it on just battery power to grandmothers house (2 miles away). parked and left it for 2 weeks until I could get back to it with a new alternator. Unplugged batteries and replaced alternator. Charged batteries with small battery charger and tried to crank after about 15 minutes of charging. Would only click when I turned it over and then gauges went crazy flashing and flipping both ways and making all kinds of buzzing and vibrating sounds and would never do anything but clock when I tried to turn it over. Got neighbor to try and jump it off but still would only click when I turned it over. I have heard that a bad alternator can fry a battery. Could this be my problem and the battery's are bad and cannot even hold a charge. I am going to try to get my Dad to test the batteries with his voltimeter but does anyone know anything else or may have had a similar problem? Thanks,
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Old 10-06-2008, 01:54 PM

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Take the batteries to get tested under load. A bad alternator will kill batteries. Driving it around like you have will kill the batt's/Alt. When the batteries weren't fully charged when you replaced the alternator, you might have fried another alternator. They are NOT designed to charge the batteries like that.

If the alternator was putting out 12.7, it is below tolerance and would have tripped the battery light.

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Old 10-08-2008, 06:25 AM Rookie

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Thanks Dirk,

I checked the batteries and both were killed by the bad alternator. They were under 2 year free replacement and after that, truck runs fine.
Thanks again.
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