Erratic voltmeter readings that vary from 12.1v to 13.7v

wli98122, When testing the voltages for battery/alternator test directly on clean battery terminals when possible. 13.7 V is OK when engine running and alternator putting out charge. You say the battery is good and charged, if you measure voltage at the battery(cold) it should be 12.4-12.6+ to be good(maybe have load tested). A battery that is on it's way out(faulty cell) or bad connection could cause the fluctuations you see. I have also seen new alternators out of the box not up to snuff. Faulty ELD(Electronic Load Detector) may display this symptom also and there may actually be a TSB on it for your vehicle as there was for a lot of Honda's. If able check all the connections to ECM as well, unplug check for corrosion reseat plugs/wires/grounds. Below are some interesting links that trouble shoot in depth charging systems in depth, with possible causes/repairs. Some of the auto parts suppliers offer free battery/ alternator/starter testing and also code reading like Pepboys, Nappa, Advanced auto parts,etc and some of the guys are very knowledgeable and may have seen the issue often and put you on the right track. Some time it takes better equipment(scanner) as found in dealership to retrieve all codes, which the places above will normally let you know if the case with your vehicle. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.

http://www.underhoodservice.com/alternat...

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