I had a ripple problem in an car alternator once - fixed by replacing the rectifying bridge in the alternator. It would also cause the battery to go flat at times.

If you trust the shop that did the test, then it's a bad diode. You either need to replace the diode bridge or the whole alternator. Replacing the VR won't do anything for you. Nor would a capacitor mask the problem.

BTW...there are NINE diodes in the bridge of an internally regulated alternator....let's see who knows why.

The R320CDI has the main battery under the front passenger seat.

The battery was diagnosed as bad. I got the new main battery today, it will be installed by another shop tomorrow. Then I'll take it back to AutoZone for another electrical test.

I stopped by the alt shop today and told them what was going on. If AutoZone shows ripple again, I'll bring the car to the alt shop and they said they will take it off the car and test it again, replace diodes if needed.

The old voltage regulator was certainly bad. It was outputing 16.5V.

The remaining 3 diodes are for the excitation/ charge indicator lamp.

Electrical Engineering student here, I'll explain in laymens how a reservoir capacitor works to those who are curious without getting too deeply into theory.

In simple terms; When you connect a power source to a load, you'll have a sine wave or loop or bell curve. Sort of like climbing a mountain and then falling off the cliff.

When you put a capacitor in parallel with a load, since current in a capacitor leads the voltage by 90 degrees.. the result is the waveform becoming "smooth", which is to say instead of falling off that cliff.. you're zip lining from the peak to the bottom. (Different capacitance values will yield different effects obviously)

From ∩ to more of like a ࿐

Because of that, you won't receive a sudden shift in voltage because the capacitor is now supplying it instead of the alternator or battery. So if the output is rippled or "shakey" (jumping between voltages), then a capacitor will prohibit that ripple, provided you have a correct capacitance. Internal voltage regulators in alternators most likely have caps.

In car audio systems it gives the amperage the amplifier needs to blow your eardrums out without dimming the car headlights or undervolting other areas of the car's electrical system. If you've seen those guys who use caps instead of car batteries, they get away with it because caps charge up very quickly and they can start a car with ease because capacitors LOVE providing tons of amperage in an instant.

This is why you see capacitors on things like air conditioners and refrigerators. In many cases because of unavoidable inductance's (voltage leads the current within inductors naturally) capacitors help to correct that by balancing out the waveform (voltage peak to peak matching the current peak to peak; one doesn't lead the other), otherwise you'll end up using more power than you actually need and wasting money. This is known as power factor correction.

Lets talk about that some..
by excitation do you mean ' make glow by supplying voltage ' ?

or are you saying two different functions ?

The excitation being needed as the input voltage to run power the alternator ?

and another function being lighting the charge indicator light ?

I am thinking about the fact that you can not push start a gas engine with an alternator and totally dead battery... where you could with a generator... isn't that ' excitation ' what is missing if the battery voltage is missing for an alternator ?

So my question has to do with fixing the charge indicator light ....
What are the failure modes for the charge indicator light circuit itself.... as compared to there being a charging problem... ?

It seems like some of them glow when they are not supposed to ????

Not exactly, but you're on the right track.

Then why don't you explain ' exactly' ?

Too much fun this way. Here's a hint.

The three extra diodes (referred to as the trio) are only found on internally regulated alternators. (which includes all Bosch products since the 70's). The peculiar wiring of the indicator light is also typical of internally regulated alternators. Both features have to do with excitation, which is to say, supplying field current to the rotor.

The only failure mode of the charge indicator circuit is the lamp not lighting at all. If the lamp is on, the alternator isn't charging correctly. If the lamp never comes on, it's usually a bad bulb.

A capacitor can't correct a bad diode, which would be the source of "excessive ripple". That assumes the problem is real, as we're relying on the clerk in a parts store for diagnosis.

I agree. The focus should be on fixing the source of the problem, not trying to patch around it. Unless MB specified for capacitors and they are bad or missing or he is running a significant stereo I wouldn't go adding anything.

Just how much AC ripple is coming off the alt? Did I miss the post?

Can we divide this into more than one potential problem and solution ?

Can we assume that a ' ripple ' which causes a battery to go bad... is not just a DC ripple....but actually an alternator or voltage regulator putting out Some AC current..?
Ignore over charging as a cause of battery failure...

That a ' problem ' ripple involves a computer module ... if all the electricity is DC.. ?