Mr. George

After driving for a long time, and at RPM > 3000, such as when the engine downshifts to maintain cruising speed on steep inclines, my lights suddenly dim noticeably and voltmeter drops to slightly to just above battery voltage for 3 to 5 seconds, then returns to normal. On a long grade where high RPM is maintained, this may repeat in 30 to 45 seconds, or the next time RPM goes up with a downshift. The alternator light does not come on when this happens, and whether the AC and with all loads are on or not does not appear to make a difference. It does not happen below about 3000 RPM, nor have I been able to replicate it by driving in a low gear to force the RPM over 3000 for any significant time. There is no indication of a short circuit, or burned wire smell anywhere in the car or under the hood and everything works normally, and the battery stays charged. I'm suspicious of the voltage regulator, but everything is built into the alternator these days and is not user serviceable. I just don't want to be stranded in the middle of nowhere.

tech2

Hard to say without knowing the model year, engine as builds can differ. depending on the build the vehicle may have 10 charging modes...some bring charging voltage to 12.6volt...hardly charging at all.

it can be difficult to diagnose a problem such as yours.
I had a 04 grand prix with symptoms of flickering headlights. the digital display of the instrument cluster battery voltage was in the correct range but was constantly up and down where as it used to be steady. No codes. Testing, everything passed. pcm on off signal, voltage drop checks, alternator output. I went to the junk yard and got a $30 used alternator and all was fixed. If all the testing does not show a direction...sometimes you gotta throw a part at it.

Mr. George

2001 Tahoe LT, 5.3 V8; sorry forgot to post that