Rectifier/Regultor is week
The voltage rectifier/regulator (R/R) seems to be a week point to me.
About 13 years ago on a 25 000 mile overland trip from UK to Aus my 3AJ charging system died, and a year ago on a 30 000 mile trip from Alaska to Patagonia on another 3AJ the same thing happened. I think that on our bikes the R/R is gets too hot and over time this kills it, and when it dies it can take out alternator windings with it. I guess the faulty R/R can pull too much current through the windings and burn one or more of them out.
It is interesting to see that on the last XT600Es the R/R has fins added to help with cooling, but there isn’t space to fit one of these in the original location on the 3AJ. Also, its cramped location on the battery box of the 3AJ doesn’t give much cooling air flow. In fact any air it gets will have been warmed by the engine!
My solution has been to fit a physically larger, and finned R/R under the tail piece behind the saddle (where they are fitted on the XT600E) and extend the wiring to reach it. If you use a late XT600E R/R I think you need to change the position of the wires in the socket that connects to it. Anything from a medium sized bike with a permanent magnet alternator should work, but some Hondas have a remote sense line that needs wiring in too (A good thing, but a little more work). My current 3AJ has one off a Super Tenere fitted.
If you decide to go to this trouble it might be worth looking into MOSFET R/Rs, which are said to be more reliable and long lived than the regular (thyristor) type.
My take on it is that a thyrister on will probably be fine for 20 to 30 thousand miles though.
One other thing, riding with the headlight on lightens the load on the R/R as that’s 55 watts that it doesn’t have to dissipate as heat most of the time.
Oh well, at least we don’t need a charged battery to produce sparks to run the engine. I retro-fitted a kick start so I don’t need the battery to start it via the starter motor either. My attempts to bump start it have always failed.
If you would like more info, just ask
All the best,