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.

What was wrong with the battery that made it bad? A bad cell? Weak?

What was used to measure the ripple and who did it? I do not have much faith in Autozone for these things. I'd want to measure and look at the ripple wave forms with an oscilloscope to make sure it is coming from the alternator and not elsewhere. At $700 to $1000, if it indeed has bad didoes, I'd take the alternator apart and replace the diode(s) myself.

I'm not exactly sure what the problem is with the battery. It is the original, 7 years old. Both the Indy and AutoZone tested it and said it was bad. The car starts just fine with the old battery.

The Aux battery is being replaced because its not that expensive, $25. Just to be sure there is no issue with it.

AutoZone used their hand-tool to check the car's electrical system and found the ripple, after the alternator shop "rebuilt" the alternator. They also said the battery is bad.

To retest it, it has to be removed from the car, a real bear of course, and bench tested. The alt shop has the bench tester for that. They said they did that test and it passed. They are an old school shop, so their idea of OK ripple may be significantly more mV than the Mercedes spec for the R320.

The bad battery may just have needed distilled water. Running with the bad regulator and 16.5 volts and perhaps more at rpms may have boiled it out somewhat.. I wonder if many even bother to look anymore. Most batteries still have a way to access the cells for electrolyte level checks.

It concerned me when visiting a large alternator rebuild place. That even though their testers had provision to hook up a scope to examine in comparison the amplitude of each of the three generated phases they had none.

For this application a cheap 20.00 used scope would have been more than adequate.

I would not try a decent size capacitor for several reasons. Basically if the battery cannot handle the designed ripple component enough. There is too much ripple being generated. Or maybe the battery is really, really bad.

This car has two batteries? I wonder if the bad battery is not serving it's function as a ripple filter? A two battery design also makes me wonder if they had set the car up electrically with one battery servicing the starting and high load devices and another serving the more sensitive areas?

I was never a lover of european electronics and system designs in my now long ago non automotive service days. I also have not examined any mercedes setup after about the 1985 models from an electrical/electronic perspective.

To further post my ignorance. I noticed in posts that some of the newer Mercedes had two batteries and wondered why at the time. Whatever the reason was I felt the need to know was not there and likely would not be.

The main battery is an AGM sealed battery, never add water.

The 2nd battery is tiny. It is there to make the brakes function in the event of main battery failure while driving.

What has to be removed for re test? I would go to a different store like Advance auto for another test. Make sure all electrical accessories are off (blowers, radio etc) during the ripple test. The battery is a very good filter but if a diode is bad in the alt, it will not filter it out, nor will a big electrolytic cap. What size cap are you thinking of putting in? If it does not come from the factory with an electrolytic, I'd suggest not adding one yourself.

Instead of buying a new main battery, can you temporarily hook up one from your other car for a ripple test?

The ripple test is supposed to be done at 3000 idle with the low beams on.

The alt is best tested at the back of the alt, not at the battery.

The main battery is under the front passenger seat. By the time you rip everything out of the way to get to the stupid battery location you may as well install the new battery.

Since two shops tested the battery and its bad, replacing the battery is the way to go. Also the new battery has been bought, dropped off with the car at the shop to install it, its a done deal today.

So this evening when I get the car back (assuming no gremlin delay) I will then test the alt for ripple.

Is it the battery that has to be removed to be bench tested then? :confused:

It will be interesting what the ripple test results will be after a new battery.

The shop got the main battery installed, but ran out of time to install the Aux battery. I'll get the car tomorrow and then test the alternator again.

They said that the main battery had been replaced. There was an Interstate battery, the date was not marked on it, so no telling its age. I'll look through Dad's pile of receipts and log book to see if he ever changed it and when. Maybe there is some warranty remaining on that battery.

A super-capacitor might help. You can see youtube videos of people starting their car with an array of them. Best to put the capacitor as far from the alternator and as close to the sensitive device as you can. That way the resistance (R) in the path will form a better "RC filter" with the capacitor. No need to try filtering the large current flow from alternator to battery.

I am surprised M-B didn't design a proper filter capacitor into their electronics. Most car makers do. Most alternators put some ripple onto the 12 V lines, despite the filtering of the battery. I have a Black & Decker smart charger that has an "alternator test" function. All it does is look for a slight ripple to determine there is a working alternator.

When I read your effort and high costs, I have to compare with the $400 I paid for my 1984 300D and driven ~30,000 mostly trouble-free miles. I am sure your mom's car is much purtier though. A co-worker has a 2002 Kompressor and complains of $400 to replace a bad neutral safety switch (plus towing several times), high-priced batteries, ...