Hi BDA,
Whenever I've measured voltage at the battery to check the charge rate it's always been rock steady, maybe fluctuating by 0.1 or so but that could be down to me rather than the electrics. I wouldn't expect it to change markedly from say 1,500 rpm upwards unless you have a very large pulley on the alternator. A smaller pulley on the alternator generally makes it go round faster/give higher output at tickover so I don't think that would solve the problem. If you want to reduce the speed of the alternator it's either a smaller pulley on the cam or larger one on the alternator. (think racing bicycle gears
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14V is about right I think but again I'd expect it to be constant rather than moving about.
Hmm, what next ? My checklist would start something like this;
1. Does the alternator need a separate earth lead (like the original on mine) or does it earth through the alternator body itself ? If the former, check that lead or make up a new temporary earth lead to tick that box.
2. Battery. I can't recall if you've had a new one but earlier you mentioned you'd had to put a lot of water in. I've no idea if previous overcharging could damage the battery but it would be on my list. I would take it off and charge it on the bench. After charging let it settle & measure the voltage. Leave it on the bench & repeat after several hours or even next day, it should be the same. If it's dropping steadily by 0.1v as you measure, it's bust and that might be why your alternator is trying so hard to charge it up, it even might explain momentary 15V readings.
3. I'd check the leads/connections on the charging circuit, just in case although I doubt that's the cause. Either clean all connections or with the engine running/charging, check the voltage between the +ve battery terminal and the heavy charge lead at the B+ connection at the back of the alternator. Ideally it should show nothing for perfect connections but I'd go with 0.1/0.2V. I'd be very surprised if this is a problem having seen your car, but it's a box ticked.
4. If those check out then despite the lack of trust I'd be tempted to change the VR just to see what happens with another unit.
Actually, if I'm honest I wouldn't change the VR again, I'm so lazy that I'd just go out and buy another alternator ! Yeah, I know, that's not proper engineering but that A127 I fitted to mine has been a really neat solution.... and it was silly cheap as well.
Brian