Sounds like you might have a failed diode in the alternator after all. Ideally, you could look at the alternator waveform with an oscilliscope and the voltage ripple would be obvious. You can still check with a multimeter, though. Set the meter to measure AC voltage and Range to about 2 volts. Measure with the positive lead at the + side of the battery, and the - lead to negative. The lower the AC voltage, the better. Anything over about 0.3 vAC spells trouble.

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1989 300 SEL that mostly works, but needs TLC