• 1987 300E
• 1-year-old battery
• 3rd re-man alternator + 4th new voltage regulator
• Battery voltage at full charge - 12.4 V
• Voltage indicated at the battery terminals with engine on but no electrical load - 13.6 V
• Voltage indicated at the battery terminals with engine on with all accessories on - 11.6 V and steadily drops until the battery loses all charge
• Bench test of all alternators shows normal alternator, diode and voltage regulator function;
• Diagnostic tester with alternator installed and engine operating without an electrical load shows normal alternator, diode and voltage regulator functions;
• Diagnostic tester with alternator installed and engine operating with all accessories engaged shows normal alternator and diode but a failed voltage regulator function (4th new voltage regulator).

Any ideas?