I copied the voltage regulator as detailed in the following link for use on a Wisconsin VH4D in a MF skidsteer. After experimental testing I ended up using a 14 Volt Zener diode but all other electrical components were identical. I used a slightly different layout because I made my own box and then potted it to resist vibration. Works great!

https://forum.allaboutcircuits.com/threads/bmw-d12-marine-diesel-rectifier-regulator-project.71103/?cb=1719456505" rel="nofollow - https://forum.allaboutcircuits.com/threads/bmw-d12-marine-diesel-rectifier-regulator-project.71103/?cb=1719456505

Those older type Wisconsin 'permanent-magnet alternators' had an unusual regulating scheme wherein there is a third 'regulating' winding that is tightly wound around the two main charging windings. Since the field isn't controllable, you have to somehow control the output directly; when this low-amperage third winding is shorted to ground, it effectively kills the output of the main windings. The regulator accomplishes this by using a Zener diode to sense the voltage and trigger an SCR (Silicon Controlled Rectifier) which shorts out the regulating winding.

I dissected the original (dead) regulator, which had tiny components, and was well on my way of designing a copy when I stumbled upon this BMW copy on the "All About Circuits" forum and said "AHA!". So I un-ashamedly copied it. This design uses much heavier components so it should be pretty robust; it also incorporates the rectifier into the same package as the regulator, unlike the original Wisconsin style that used separate modules.

Most of the inexpensive permanent-magnet type alternators found in motorcycles, etc. use a simple 'shunt' regulator where the excess energy is simply turned into heat via some type of Zener diode arrangement. The Wisconsin third-winding type doesn't generate near as much heat (in theory) but is certainly more expensive to manufacture, which is probably why they discontinued them.

A friend of mine owns this skidsteer and mentioned that the regulators were no longer available, so I opened my big mouth and said I could probably build one for it...