78 SC - Slow fail of alternator?

[lespaul]

While driving my 78 SC, the alternator light came on and the voltage suddenly dropped from 13.2 V to 11.8. Because I was away from home, I needed to keep driving. When I turned on the headlights, voltage dropped to 11.4. With headlights off, voltage returned to 11.8. Then, over the next 80 miles or so, the voltage slowly dropped to 11.7 and eventually to 11.6. Shut off the engine and turned key halfway and without the engine running, the voltage was 12.0. Belt tension seems fine. Some questions:

Can an alternator slowly die (brushes failing) or does it go all at once?

Would a bad ground account for this slow fail over time?

What steps should I pursue to ID and isolate the cause of this issue?

TX!

[wazzz]

What you describe sounds like a dead alternator. No slow fail here. Voltage dropped slowly because car ignition had to run on the battery instead of alternator. Once you stopped the engine battery voltage climbed back up to 12 as battery was no longer drained by ignition. But if you keep using your car this way you'll soon run out of battery energy.

[dkirk]

I had a similar experience with my '88 911. Charge light would glow dimly at idle and get brighter when headlights were turned on. At 120,000 miles, I removed the Valeo alternator and had it rebuilt by a local shop. Brushes were worn, slip rings grooved, and bearings were making noise. After rebuild, alternator works perfectly. Cost was a little over $200.

[PeteKz]

Also could be voltage regulator. Check that too.