hey all hopefully someone here can help me out with this issue searched the forum and couldnt really find anything. so i have a 2013 f350 dual alternator set up. so the last month i have had an intermittent battery light an p0620 code. checked batteries and they are 12.5v with truck off. so in the beginning the light would only come on while in drive and when im stopped at a light then voltage would bounce between 13.8 and 14 start moving light would go out and voltage would go back up to 14.2-14.3 so i figured alternator issue so i tired to find diagnostic process to figure out which alternator didn't come up with much. I checked connections at the battery all were clean, i checked connections at alternators and top passenger alt had a little corrosion on the pin nothing crazy cleaned it and reconnected. drove around some days no light some days light would come on. then it wouldnt come on when stopped but switched to it would flash on when accelerating moderately. i load tested my batteries and one was ok at 100% charge and the other was marginal at 100% charge, i had thought about changing the batteries but i would hate to spend the money and have it be something else like one of the alternators being bad hence why im trying to make sure its 100% the batteries. so then no light for a while seemed to be fine. this morning started my truck to drive to training for work and battery light was on and stayed on the whole 15 to 20 min ride and said battery voltage was 14.3-14.4. so im kinda scratching my head without just throwing parts at it. i about broke down and went to ford to get 2 new batteries and get the upgraded 850 cca ones. if anyone has any insight that would be awesome besides throwing parts at it or bringing it to ford and hoping they find it quick before racking up a large bill. im currently 3 hrs from home for a week so no tools but if its something that i can get by for the week till i can get home to my tools that would be awesome. thanks in advance and sorry for the long post.
Battery light p0620 Dual alternator
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CATDiezel
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How were the batteries load tested?
What device. It sounds to me like a low fluid level in one of the batteries. However. If you replace one please replace both.
I've also seen the plates in batteries deteriorate and break causing intermittent issues.
12.5v raises a small concern to me. A charged battery should be 12.6 floating volts. 2.1v per cell.
If your up for the test. Pull the caps on the batteries and with a good voltage meter put the ground lead on ground and use an extension (so your not dipping the meter lead in battery acid directly) and check each cell on the battery. 2.1v is what you want to see.
The regulators on the alternators are going to try and give you 14ish volts even with 2 bad batteries.
If the batteries were tested with a digital battery tester that also measures internal resistance and continuity between plates then your good. If it was a toaster tester then not so accurate. A battery store should have the proper tester.
Hope that helps some.
What device. It sounds to me like a low fluid level in one of the batteries. However. If you replace one please replace both.
I've also seen the plates in batteries deteriorate and break causing intermittent issues.
12.5v raises a small concern to me. A charged battery should be 12.6 floating volts. 2.1v per cell.
If your up for the test. Pull the caps on the batteries and with a good voltage meter put the ground lead on ground and use an extension (so your not dipping the meter lead in battery acid directly) and check each cell on the battery. 2.1v is what you want to see.
The regulators on the alternators are going to try and give you 14ish volts even with 2 bad batteries.
If the batteries were tested with a digital battery tester that also measures internal resistance and continuity between plates then your good. If it was a toaster tester then not so accurate. A battery store should have the proper tester.
Hope that helps some.
Last edited:
Hey I used the load tester at my work I believe it is an auto meter i selected the proper digital settings and it came back both batteries at 12.6 100% charge with 550cca for the lowest Value. So far last couple of days no battery light and voltage has stayed at 14.3-14.5 so not sure can't really test it with the system working properly and not able to duplicate issue but I guess I'll just keep running it and if it happens again either see if I can check it or just replace battieres. Not sure what would cause voltage to drop to 13.8 besides something like a regulator which means replacing an alternator.
OK so just an update i broke down and took it to the ford dealer. They ran a test on my charging system and said my batteries were good, and alternators were good. so than they did the wiring pin point test and found the return wire from the L/s alternator to the PCM that tells the PCM what the charging system is producing was bad. Wire has high resistance. Unfortunately there wasn't enough time to fix the issue before the dealership closed and i wasn't about to ask the tech to stay and fix it on a friday. So made a follow up appointment and service writer said there are two options i can do to fix the issue. either A run a new wire from PCM across the cowl over the motor and down the L/S of the truck to the alternator pig tail, or i can have them spend the time to cut the harness apart and try and find the exact spot and cut and splice in new wire. so i'm going to have them run a new wire that why i know it wont happen again and it will be a solid wire and i don't have to worry about the connection going bad between the wires. So issue should be fixed later this week i will post gain with a follow up if this truly is the fix.
allski
New member
Hi I have the same issue with my 2017 f350 6.7 . When you say L/s do you mean the drivers side or passengers side alternator. My drivers side plug seems to have continuity on all three wires . I cannot test the passenger plugs because I am not sure where they go to? if the PCM Im not sure which position on the plug? It's hard to find that information. I too have replaced Batteries and the drivers side alternator . Any help is appreciated.
