Whenever I come to a stop light, the voltage readout drops to 12.. 11... 10.5 etc. It is NOT underdriven.

Am I correct that this is a tell-tale sign of the internal voltage regulator failing?

 

i don't think so, wouldn't a voltage regulator failing cause overcharging? in excess of 13.5?
i seem to recall on my 85 that it would dip down like that with all the accessories running - A/C, fan, aux fan, radio, etc.

it's been a while since i had to trouble shoot this - and have since sold my vettes, so i could be wrong - you might want to swap out the alternator as a test

what does it read while "cruising"?
if it reads 13-13.5 then it's probably just accessory overload

 

No radio, no AC
But it DOES have heavy duty cooling

 

It could be a sign of a failing regulator but not always. I had the same symptoms once and changed regulators and that fixed it.

 

May want to double check voltage with a meter. Should pull around 14 volts. Alternator is easy removal, if voltage is low, run it in to auto parts store, they usually will check for free. Good luck Rich

 

The voltage regulator attempts to maintain 14.7 volts across the battery all the time. When the alternator warms up this voltage drops to 13.7 volts. The dash voltmeter reads about 0.3 volts low due to the voltage drop across the ignition switch contacts, so, cold: 14.4v, hot: 13.4 volts. During idle the alternator is not turning fast enough to generate its rated output current and under a heavy electrical load the voltage can sag. My 87 is stock and I have never seen the voltage drop at stop lights.
Your voltage readings sound like the alternator has no output at idle and if this is true the red battery symbol light on the dash will light telling you the alternator has no output.
It also sounds like your battery is near the end of its life or it is not fully charged as the battery voltage should hold up above 12.0 volts when it is called upon to supply power to your electrical system. You can tell the state of charge by measuring the battery terminal voltage after it has sat unused for several hours. 12.0 volts and lower, discharged. 12.9 volts and higher, fully charged. The dash voltmeter in my experience is not accurate when it is being powered by only the battery. I would recommend that you charge your battery for 24 hours with a battery charger and see if your idle voltage still falls below 12.0 volts. You may have a battery that is at the end of its life and cannot store many amp hours.

 

The red warning light has not come on yet, but I think the voltage has to drop down to 10 for it to come on. The battery is relatively new, and has been tested. It's in perfect condition.

The alternator has not been tested.

 

Your battery cannot be in perfect condition if its terminal voltage falls below 12.0 volts , your battery is badly discharged and should be charged back up. NEVER use the alternator to charge you battery up because discharged batteries draw large current until they start building up a charge and this heats the alternator and shortens its life.
The battery light comes on when the alternator has no output current and NOT when the voltage sags to a certain level! You also should measure the voltage on the alternator output terminal with a voltmeter and compare that reading with the battery terminal voltage while the engine is running. They should read the same and if the alternator is significantly (more than 1/2 volt) higher, you have either a poor connection from the alternator to the battery or the fusible link in the alternator output wire has been damaged and should be replaced. It also wouldn't hurt for you to remove your battery cables (neg first) and clean the cable lugs and the battery posts and replace the cables (neg last).

 

The battery IS in perfect condition since it was charged and tested over at napa. However, I have not checked the voltage with a voltmeter. I will tomorrow. Thanks for that advice. Is any voltmeter suitable?

 

Yes, any dc voltmeter that can measure 12v will do. Modern digital voltmeters are more accurate than analog meters (meters with a swinging needle across a calibrated scale), but either in good condition will tell you if your battery is still fully charged and if the alternator output is the same as the voltage across the battery with the engine running.