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Old 11-07-2011, 03:04 PM
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Voltage Problems

I've got a problem with charging system. When first started, check battery light is on, voltage is 11.7 volts...revving engine goes to 12.3...drive for approx. 8 miles and the voltage jumps to 13.7-13.9 volts...replaced factory alternator with NAPA...same problems...replaced with NAPA 140 amp alternator...same results. Checked grounds, all wiring between batteries, everything I can think of. Ford says install Motorcraft alternator to fix problem. At times, while running 65mph voltage would drop to 12.2 and light would come on, then jump back up to 13.6-13.7 volts. Any suggestions?
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Old 11-07-2011, 03:21 PM
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have you had your batteries tested? you need to check them separatly.
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Old 11-07-2011, 03:25 PM
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Replaced both 3 days ago.
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Old 11-07-2011, 03:33 PM
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The alternator will not come on until the glow plugs are turned off.
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Old 11-07-2011, 03:45 PM
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I've owned the truck [2004 F250 King Ranch] for 3 years...never had this problem show up before...it started back in the summer [August]. On the way to get batteries installed, the voltage was all over the map...11.7-13.5-14.3 with light coming on as voltage dropped and going off as voltage spiked..
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Old 11-07-2011, 04:18 PM
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it should be around 11.7-12.0 while the glo plugs are on which is the first minute or so then it should be around 13.5-14.0 when at idle or driving.

the only thing that is concerning is where you say it drops to 12.2 while driving.

I have had batteries go bad after 3yrs...
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Old 11-07-2011, 04:21 PM
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Quote:
Originally Posted by bismic View Post
The alternator will not come on until the glow plugs are turned off.
?
The alternator is always on?
?

Like others said, the GPs do draw so much that you might show low voltage for the first couple minutes it's on. You could try disconnecting the GPR to see if it's coming on and off.
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Old 11-07-2011, 04:40 PM
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When I start truck in morning, the check battery light is on...voltage is 11.7v...drive truck for approx. 15 minutes at 40-60mph...during this time voltage is 12.3v. Then voltage goes to 13.7-13.8 and light goes out...Question is, light has not come on in the 3 years before now. Also, will glow plugs activate for 10-15 minutes in 80 degree weather before going off and allowing alternator to then go to 13.5-13.6 volts?
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Old 11-07-2011, 04:54 PM
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I would go to the local parts store and have the batteries and alternator checked.

What does it do when you turn the lights on and the a/c?
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Old 11-07-2011, 05:05 PM
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No, GPs won't go that long.
I have heard problems of these alternators having the wrong voltage regulator on them.
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