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2017 F450 with factory dual alternators. Started getting the above warning a while ago but was so sporadic that was hard to diagnose. Various tests including battery voltage, charging amps, etc., finally indicated main 220 amp alternator was not charging. Replaced with OEM unit but the warning and battery light still illuminated. Returned to dealer where parts man suggested could be chaffed control wire between PCM and main alternator. He stated this was an issue on some of the '16s and earlier models but had no further details. Does anyone have wiring diagram/color codes that I can use to further diagnose this problem?
 

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P0620 is an indication there is a concern with the Generator A sense circuit used by the regulator to measure battery voltage. Significant variation between it and the B+ circuit flags the code. This is typically the most common charging system fault I see. The three terminal connector on the generator - disconnect it - locate the yellow and red wire at pin 3. Test it for proper voltage, high resistance and shorts to power and ground. It is connected to fuse 98 in the battery junction box. The harness runs down from the BJB onto the engine block in a plastic support where it crosses the crank pulley and heads up the left front of the engine to the generator. My observation is that the wires are poorly taped. There are some potential chafe points including the wires themselves as the can vibrate, collect dirt which is abrasive and rub through the insulation. If the wire is broken or soon to be broken you can sometimes just find an exposed portion of the yellow and red wire and give it a tug. If the wire is broken or weak from corrosion it will come out.
 
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