I have a 2001 Honda Civic where I am driving at night with the headlights on, the headlights dim when I accelerate and get bright when I leave off the accelerator. I checked with a multi meter and scanner and both showing it is charging between 14.5 and 14.9 volts. I recently replaced the engine wiring harness with an OEM one due to throwing codes for the transmission shift solenoids even after replacing them. I also replaced the ELD and the alternator and still getting the same problem. I also replaced the the negative battery cable and ground cable and cleaned the connecting points. Battery tests fine and it's only 2 years old. Any input on this would help.
I also replaced the ELD
Sorry but what's an ELD?
Something is loading down the system the faster you go. I would look at the ignition components (coils, plugs, wires etc.)
Also check the condition of your accessory belt.
I replaced all the coils about 3 years ago with OEM coils and spark plugs. ELD is the Electronic Load Detector. The new wiring harness clips into the coils. I am worried about the high voltage coming out of the alternator.
charging voltage looks normal. I would be more worried about the dimming headlights.