I had one of those little Denso jobies, brand new, seemed to charge ok but the light wouldn't go out. It might have been self inflicted as I was trying to be clever, inserting a relay as the load in order to operate an led. But it wouldn't work with a regular bulb either( relay removed). I gave up on the led idea, got a two wire bulb holder that would fit into the lovely light fitting I had (the use of which was driving the whole arrangement!) and with another, similar but not identical, new alternator, everything worked fine. I've got a wiring diagram I need to find, but they're commonly available. Perhaps my initial issue was me thinking I'd be clever and use a low revs alternator, more suitable to a narrow boat, with the idea being a strong charge at idle because the relatively low amperage alternator might need the best chance it could have to maintain the battery! Its replacement has thus far worked flawlessly.

Might running the wrong way round have a bearing on the problem? Doubt it but you never know, fickle things these electiceries.

Chris