Jumper from alternator positive is in place in parallel with the original wiring. I presume there will always be pulses, although low ones. I'll check what range we used on the oscilloscope. Thought I would see irregular pulses if a diode was broken. I'll check the diodes with a multimeter. I...

Earth strap OK. I've tested with a jump lead but no improvement. Also attached an oscilloscope and only stable continuous pulses detected. I'll do some more testing over the weekend, to be continued.

Yes, I've been using different types of quality meters like Fluke etc.

Half a year ago I fitted a DELCO REMY DRA3673 alternator due to charge voltage variations (from too low to 16V). The new alternator worked well for half a year but then started to charge with up to 18V, also at low rpm. The cabling between the alternator and battery is upgraded. The voltage...