Electrical Mystery: New Alternator doesn't charge and drastic voltage drops

[Couper]

Hi Guys,

Hope you guys can help solve this mysterious electrical problem on my 1984 300TD that I've been trying to solve for over a week.

Thought the car just needed a new alternator as the Bosch Platinum Battery (1yr old) was not being charged. The following are results of the initial testing:

• Old alternator Failed AutoZone in car testing
• Yet, same alternator (old one) Passed AutoZone's Bench Test
• Battery Passed AutoZone's Bench Test
• New alternator that was installed also Failed AutoZone in car testing

Therefore, thought alternators are not faulty and it must be some type of wiring issue causing the battery not to charge. I inspected the following:

• Removed and Checked the main ground strap
• Remove and cleaned Negative Battery connector wire to frame
• Cleaned both battery posts and connectors
• "Charge Indicator Lamp" bulb installed(was missing & working now)
• Checked Alternator Connector: Key Off- Battery Voltage Blue wire slot no voltage when Key Off and slightly less than battery voltage when on

Still not charging at all or properly. With the car idling, the voltage at the connected plug on alternator measured only ~ 12 volts at both slots, but only around `11.8 volts across the battery. Doesn't seem to be charging even though though the voltage is higher at alternator than at the battery.

Why do you think the voltages are so low? Shouldn't the voltage readings at battery and alternator be at 13.0 to 13.5 volts? Does the lower voltage give a clue to what could be wrong with the charging system?

Now here's the mysterious or weird part. It's seems that my battery's voltage drastically drops after draw down of one start and idle a short while. A fully charged battery at 12.6V, drops to 11.8V after only 1 start and couple minutes of idling.

My Schumacher smart charger might be malfunctioning, but my multimeter confirms the voltage readings. After only two starts, the charger says that the battery is only at 10% charged at around 11.0V. It's taking about a hour to charge it back to 100%. The voltage while charging climbs slowly and only reaches over 12.5V only when over 80% charged. I suspect the charger's percent function might malfunctioning or acting up but not the voltage readings.

Does anybody have a idea of what's happening to the battery's voltage? How does this issue relate to my alternator not charging?