Alternator output ?
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Mountain Pass
Joined: Dec 2009
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Alternator output ?
I think my alternator is going out. This morning when I started the truck KO/EO 11.5v engine start it fell to 10.5v. The voltage when the motor was at operating temp fluctuated between 13.3 to 13.6 to 13.9 constantly up and down the range. My FICM which always remained steady at 48.5v started to fluctuate from 47.5 to 48.0 to 48.5. When the alt. volts dived down to 13.3 the FICM voltage would go to 47.5v. I hardly ever monitored the alt. voltage but it seems to me it used stay steady at 14.1v when the motor was running. I always monitor the FICM power and it was always a steady 48.5v. The batteries have 12.8 volts with my multimeter hooked to them and the motor off. I had Advance Auto run an alt. test and it showed 12.6v no load and 13.28v loaded (bright lights and AC on) at 1500 RPMs. The test also checked the batteries and it measured 1083 CCA and they are rated at 1400 CCA. The alt. test claimed no problem but the battery test failed. Is this the first signs of my alternator going out ?
Wiser by FTE
Joined: Jan 2010
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If your voltmeter is accurate, 12.8 volt indicate a charged battery. did you disconnect 1 & then check em? The voltage will vary with the load on the batteries. When hot the output of the alternator will go down 10-20% 13.28 is low for full output, should be 14 volts & not shure if 1500 rmp is enough for maximum output. If your batteries are weak, you need to replace both of them. CHeck/clean your ground wires, they are real important for proper charging system.
Last edited by Curt's05; Aug 22, 2012 at 09:22 PM. Reason: add line
Senior User
Joined: Apr 2012
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From: Orlando, FL
Be willing to bet the boys at Advance didn't realize the alternator is off for the first 2 minutes.... Sounds like you need two large good batteries... then check FICM and alternator voltage.... But if it's 13.5 or higher but less than 15 you are good to go.
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Mountain Pass
Joined: Dec 2009
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What is the expected life of our batteries ? I bought these batteries 3 years ago Motorcraft 750CCA - 36 month FULL WARRANTY - 100 MONTHS LIMITED. Its a daily driver but I let the truck sit for 2 weeks and I'm hoping the batteries just discharged a little from the hot weather. My commute is 26 miles round trip and the FICM seems to be a little more consistent at 48.5v the more I drive it. The voltage started hitting 14.1v a few times today (that has not happened all week) but still jumped up and down between 13.5v - 13.9v. Is 13.5v TO 13.9v the expected range for the OEM alternator ?
Posting Guru
Joined: Sep 2011
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From: Fraser Valley
What is the expected life of our batteries ? I bought these batteries 3 years ago Motorcraft 750CCA - 36 month FULL WARRANTY - 100 MONTHS LIMITED. Its a daily driver but I let the truck sit for 2 weeks and I'm hoping the batteries just discharged a little from the hot weather. My commute is 26 miles round trip and the FICM seems to be a little more consistent at 48.5v the more I drive it. The voltage started hitting 14.1v a few times today (that has not happened all week) but still jumped up and down between 13.5v - 13.9v. Is 13.5v TO 13.9v the expected range for the OEM alternator ?
if the alt. is in good condition, the batteries are never let to die and then recharge (just like all rechargeables auto batteries only have so many recharge cycles before they fail) electrolyte levels are taken care of, if applicable, and no major draws are regularly imposed then the batteries should last a long time.
they wouldnt warranty something for 3 years if it was common for them to fail in that time is how i see it and is generally effective. electronics can be funny though and fail for no reason at times.
Senior User
Joined: Apr 2012
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From: Orlando, FL
13.5 to 14.0 is what I see with my stock truck. I have a 04. But that is with stock equipment and stock alternator. IF you have a high output alternator you may see in the 14's. It also depends on the loads. If you have a custom audio system with 10s and 4 audio amps (ie loud).... your voltage with the radio on will be lower and depreciate battery life. Even the extra lights and construction or tow equipment can load down the alternator. From what I understand the truck needs about 100 amps of power to run and came stock with a 110 amp alternator. ..... the extras will tap into that 10 amp cushion real fast.
As far as your commute... how long? Do you do short trips. The batteries need time to recharge. Any trip that is less than 5 minutes long is murder on the batteries.... I mean after all the first 2 you can't guarantee the alternator is charging and then it will need 2 to 3 to put in what it took out....
As far as your commute... how long? Do you do short trips. The batteries need time to recharge. Any trip that is less than 5 minutes long is murder on the batteries.... I mean after all the first 2 you can't guarantee the alternator is charging and then it will need 2 to 3 to put in what it took out....
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