I wouldn't be so sure it is the alternator from the above information.
From the above reports and testing the alternator’s plastic connector on my normally operating 91 car, I think you FIRST need to correct the +12V that is present on one of the two upper most wires going from your car to the alternator via the three-wire connector. Since 86-91 cars can use the same alternator, the wire positions in this connector must be the same between the car years. You reported that two of the three wires on your connector have +12V on them when the car is at rest and the connector is disconnected from the alternator. Only the bottom most wire on this connector (thicker red wire on my 91) has +12V on it.
Perhaps others can disconnect their alternator connectors and verify the at rest voltages on the three wires. Report the voltages by wire position in the connector, not by the wire color, which may vary from year to year. It is very easy to measure on an L98 motor.
The fact that your alternator is producing over 14.5 volts at reasonable engine rpms probably indicates that its power diodes are okay, too. A diode that is leaking current from the battery to ground will typically cause a reduced output voltage from the alternator.
You can further verify that the alternator’s power diodes are not leaking battery voltage to ground by measuring current flow from the battery through the alternator with your ammeter connected between the negative battery cable and the negative battery terminal (use 10 amp scale and connections on your ammeter). This measurement should be made with the three wire plastic connector DISCONNECTED from the alternator. Only the large 10 or 12 gauge wire should be connected from the car to the alternator’output power insulated post. A minute or two after first connecting your ammeter in the circuit, the current flow should settle down to around 20 milliamps (.020 amps). Anything higher than about 50 milliamps will drain your battery pretty quickly. As you have seen, a flow of 500 milliamps will drain your battery overnight.
While I think your problem is originating within the car (+12V on a second wire in the three wire alternator connector is suspect to me), if you decide to get another alternator, I would strongly recommend something other than stock and I would not use a parts store rebuilt unit. The OEM Corvette alternator is undersized and prone to overheating and premature burnout/death. Years ago, I went through one new AC Delco alternator and about 4 of the parts store alternators in 70K miles on my 86 Corvette. Since I had a "lifetime" guarantee from a national chain, I only had to pay for one of the parts store alternators, but the pain factor of replacing a couple of these failures out on the road was considerable.
Re-look at this recent thread and try these larger./reworked alternators that have been recommended by other forum members:
Recent alternator thread:
http://corvetteactioncenter.com/for...ht=alternator and heavy and duty&pagenumber=1
Forum members/suggested non-stock C4 alternator sources:
rrubel
email -
TK4Sports@aol.com
Tom3
Email -
Info@corvetteclinic.com
www.corvetteclinicinc.com (click on C4 solutions)