Shame on me...
Car also wouldn't crank with disconnected alt....
Charged the battery, altough it was reading 13.5 volts, and vrooooem..
Gonna change battery annyway and then back to the overcharging issue.
To be continued..

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Got the time to work on the overcharging issue today. Did all the tests mentioned above and think everyting is ok, except still overcharging, 15 to 17 at idle, running up to 18 volts with increasing rpm !!
Went back in my thoughts when this all started. A few months back I pulled out the alternator to connect a extra wire directly to the starterrelay.
One of the bolts broke and had to drill it out of the alt and make a new thread in it. I noticed then that I slightly hit the windings of the alt while drilling out the broken bolt. Now the windings are a bit damaged, so could thát be the reason of overcharging ?? that the windings are internally short now? Just guessing..

Luigi

Yes change out the alt. DO NOT run it until you do as that high of voltage will wreak havoc in short order

I will not run it for the time being.

Disconnected VR while engine running : 14.5 volt at battery
Disconnected green wire from alt : 14.5 volt at battery

Still a bad alternator?

Googled around about this and read something that a bad battery can also cause overcharging. I haven't changed the bat yet because I first want to solve the overcharging issue...

yes a bad/sulphated batt can force it to overcharge cuz it wont take a charge so the reg responds by upping the voltage to try & make it charge but if it is being charged to that level (14.5) its gonna stay there for awhile I'm thinking when you disconnect the reg. yes if you can borrow a known good fully charged batt, sub it in for a quick test then you will know before you shotgun money for another alt. (but I'm still thinking alt). EDIT All your connections are clean and tight (including the ground paths)?

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Changed the voltage-regulator.....
Solved the problem, engine is running fantástic !! with the right values on the voltmeter..
As often, things are much easier to repair then we mostly think..

Luigi

(1) good deal (2) over here we call that the K.I.S.S. principle (Keep it simple Sam)