I may be off tonight but it looks more like straight noise to me rather than ac ripple. I noticed a strange noise component when I was playing around with glow plugs and a scope.

It was almost as if the car wiring was acting as a noise receptor. Even just across decent ground cables. I only eliminated it by grounding directly to the engine block when looking for any form of abnormality riding on the glow plugs. That got rid of it.

So you should be reading from the engine block to the alternator output to just check. I never bothered to locate or examine where the noise was originating from. Your peak to peak values are not that strange to me. Allow though that I might be getting old as they do not look like any rms type waveform I remember.

Your 300d posting looks more rms to me. Probably some action in the voltage regulator as well. I have no ideal where the extrainious noise was originated in my case. Brush arc in the alternator perhaps? There will always be a little. The amount indicated perhaps more than likely by whatever wiring is acting as an antenna for it.

For example if you take your meter on a very low range and place a probe on the engine and the negative terminal you might read something. This reading to me has to be radiated noise. I guess a scope would be better for the test. As you might see the meter reading any voltage drop across the cable with the engine running. Although there should be no ac component to read across it.

All I know for sure is I ran across some strange waveforms during my short period of use of my scope on the electrical simple 1984 300d. Your most interesting pattern was to me taken at the radio connector. I would have to think about it a little but suspect some action in the voltage regulator. The lack of general noise there is noticeable as well. As I said I would have to think about it for awhile.