91 GL1500 charging issue.

[wn33]

I have a 91 GL1500 with a charging problem. I have fitted a new Yuasa HD battery and a new Honda alternator but still the issue persists. Works perfect on start up charging around 13.9 whilst on choke then around 11.9 at idle @ 850. Once the bike starts warming up the charge rate drops. Struggling to get 12 with revs and at idle it drops like a stone till the bike cuts out and then the battery is flat. Dog bone and battery connections all good. Obviously a heat related issue. Both fans work as they should. Ive checked the earth by the thermo switch and all ok. Any ideas much appreciated. Thanks.

[Rambozo]

Have you measured charging voltage right at the alternator output terminal?