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2003 Buick Park Avenue 3.8 with a P0620. Voltage light comes on dash after running 30 seconds. I have replaced the alternator and the PCM. Alternator does charge, digital cluster displays 13.6v, multimeter directly on battery shows 14.7v.

I followed the Factory Service Manual steps for this DTC:
1. Did you perform the Engine Electrical Diagnostic System Check? = Yes, Go to Step 2
2. Install a scan tool. Start the engine. With a scan tool, observe the Generator PWM parameter in the PCM data list. Does the scan tool indicate that the Generator PWM parameter is within the specified range? 5-95% = Yes, Go to Step 3
3. With the scan tool command the generator OFF. Does the Generator PWM equal the specified value? 0% = No, 7-14%, Go to Step 4
4. Turn OFF the ignition. Disconnect the generator harness connector. Start the engine. Measure the voltage between the generator turn on signal circuit and a known good ground. With the scan tool command the generator ON and OFF. Is the voltage equal to the first value with the generator OFF and equal to the second value with the generator ON? 0.0v / 5.0v = No, 0.1v / 5.11v, Go to Step 6
6. Test the generator turn on signal circuit for a short or open.

Based on this, I investigated the red and gray wires going from the PCM to the alternator for continuity and all was good. I even went as far as running new red and gray alternator wires directly from the PCM (red=C2 61 to pin B on alt & gray=C2 29 to pin C on alt). All terminals at PCM and alternator looked perfect. I'm at a loss.

This sounds somewhat similar to an issue I had a few years ago after driving through deep puddles and burning a hole in my alternator. The replacement alternator would charge and operate. Manually checking the voltage would show good operation and proper 13-14+ volts. But the voltage gauge on the dash would drop to 0 after a few seconds of running and throw warnings. I do not remember if it through a code however. In my case, I had a blown fuse in the rear block for the dash integration module, #38

I had the exact same scenario last week. My fuse was OK, however going through a ton of water cooked my alternator.

My car exhibited that exact same conditions you describe regarding the gauge.

I figured this out. Assumptions were made and they bit me. Turns out Bonneville and Park Avenue alternators are not the same. I was using Bonneville parts that I had on the shelf because I already had them and I assumed they were the same. They physically look identical, they must just have a different voltage regulator. I replaced the alternator again with the correct part, no issues at all.

I made this exact same mistake. Put an alternator from a Bonneville in my park Ave