I'll pass on my observations and experience with my own 916 charging system failures.
The stator windings and leads are just lengths of insulated wire. If the insulation fails, the wires short together and the alternating current output of the stator is diminished - often to zero. The voltage differential between stator wires increases as engine rpm increases, so shorting due to damaged insulation is more likely as voltage increases.
The stator wires carry a substantial current, so any point of electrical resistance is a point of heat generation. The connector between the regulator/rectifier (RR) and the stator is THE point of increasing resistance over time due to corrosion — and because it has a higher resistance than the attached wires. Consequently, the connector gets hot — really hot — and because copper wire is a good conductor of heat, the adjacent wires get hot for a substantial distance from the connector. Immediately adjacent to the connector the insulation becomes discolored from internal heat, but more importantly, it looses the ability to insulate its wire from adjacent wires and components.
On Ducatis, the stator wires are run inside a sheath that provides additional insulation from adjacent grounded components, but this causes a problem — inside the insulating sheath, the stator wires are held close together so they touch.
So what happens is — the connector heats the stator wires, the hot wires damage their insulation, the wires short together where they touch inside the sheath (at higher voltages produced at higher engine rpm).
You can diagnose damage to the insulation to the stator winding by doing a continuity and resistance check using a low-voltage ohmmeter because the windings have a different type of insulation than the lead wires. A diagnosis of damaged and shorting lead wires will display normal battery charging voltages on an on-board voltmeter, but at higher rpm the voltmeter will drop to sub-12 volt values indicating that you’re not charging.
When the stator output is shorted out it can damage the internal electrical components of the RR. Often, a new RR will fail soon after replacement because the damaged wiring was replaced only where it showed discoloring adjacent to the connector.
On my 916 I had four RR failures before replacing the stator lead wires. The insulation on the stator wires where it entered the engines side cover was so soft from heat that I could peal it back with my fingernail. Removing the inline connector and soldering the wires directly solved the problem with over heating.
Finally, you should also be aware that an RR will often fail from overheating because it is located inside a fairing and adjacent to exhaust pipe heat. A hot environment combined with internal heat generated by the RR is a well known problem. Relocating the RR to a location on the bike where it can receive better cooling air flow is a wise modification.
