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Old 07-07-2014, 07:27 PM
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Low Voltage Issue

I have had this happen a few times now and usually putting a new alternator on will cure it but this time its not. The truck has two newer motocraft 850 CCA Batterys with brand new alternator. When i try to crank over it turns really slow. If i can get it to start by jumping it the voltage only shows 12.5 at idle and 13.2 while driving. The Ficm is healthy and is running between 47.5 and 48. I checked for draw and i am only seeing between 16 and 21 mA. I have had the new alternator load tested and it passed. I have also dropped the batterys off at Oriellys and the claim both batterys are good but I took them to the dealer to double check and they confirmed both are good. Anyone have any ideas to try next i have done everything i can think of and my next step will be dropping it off at a local diesel shop to just get it working because this is boating season and I NEED MY TRUCK!!
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Old 07-07-2014, 07:28 PM
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Starter?

Do you have a multimeter?
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Old 07-07-2014, 07:33 PM
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Yes I have a Fluke
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Old 07-07-2014, 07:42 PM
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Test the batteries independently from each other and see what they say. If you have a battery charger you can also unhook one battery at a time and charge it. If they're both at 100% and the start is still slow then I would look at the starter.

Edit: if it starts up quickly with charged batteries and in a week or so the starts slow down I would check out my alternator.

Last edited by SargentoPistola; 07-07-2014 at 07:46 PM.
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Old 07-07-2014, 07:47 PM
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But i still dont understand why the alternator tests out ok but the truck will not charge fully when running.....
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Old 07-07-2014, 07:50 PM
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Because those machines are stupid. I wish I had a more scientific smart sounding answer but those machines are just plain stupid.
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Old 07-07-2014, 07:59 PM
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Also. The stock alternator of a single alt system is 110 amps. That's the stock amperage of my 99 F150. I upgraded to a 130 amp on that just for piece of mind. Do you really think a 110 amp is going to charge two batteries in parallel if it's a little unhealthy? I tend to think they may test good on those machines because they aren't taking into account the power requirements of a big truck running two batteries.
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