Originally Posted by Gregory

The voltage ripple is caused by a semi-bad, or fully bad diode in the rectifier circuit of the alternator. The alt originally makes 3-phase alternating current. The bridge rectifier (composed of diodes) converts the AC to DC current. Then the DC current goes through the voltage regulator attached to the back of the alternator.

Yes, if the alternator is making ripple voltage in excess of spec, the alternator should be removed and repaired, again. Or be replaced with the $700-$1000 new.

The test showed about 460 mV of ripple with the bad battery in the circuit. Spec is no more than 50-90 mV. The car is in the shop today getting both batteries replaced. Main battery will be a AGM sealed Johnson Controls from NAPA. The Aux battery will be an Interstate 12V 1.3 amp hour sealed.

After the new batteries are in, I will have it re-tested to see what's up with the ripple voltage.

Since the electronics in the R320CDI are SO sensitive and SO expensive, it seems prudent to install a large electrolytic capacitor on the alternator to prevent ~$5,000 of damage to the car's finicky electronics. Or sell the stupid car and let some other fool deal with the problems inherent to this screwed up car.