RM20 rotor suddenly lost its magnetism?

A couple of weeks ago I posted about a mysterious carb problem, where the spray tube had dropped down and was sitting around the jet holder.
After fixing that problem and changing to 3.5 slides, the bike seems to be running quite well again, but it's not charging.
I have a battery state monitor light fitted, that shows if the battery voltage is low, normal or high. I noticed during my test run that no matter what revs I was doing the battery monitor was showing a slightly low voltage. Normally, above 3000 or so the light goes out, indicating a normal voltage.

I checked the voltage across the battery today with the engine running, and it shows around 12.4 volts, again regardless of engine speed.
Then I checked the output from the alternator - zero volts (and yes, I did set the multimeter to AC!).
There is about 1-1.5 ohms between the stator leads, with no continuity to the motor or frame, which seems to indicate that the stator is ok.

That only leaves the rotor. I don't have the tools in the summerhouse to remove the rotor, so I tried with some light steel items to test for magnetism on the side of the rotor , but cannot feel any attraction (between the steel lump and the rotor of course...)

Any wise people with suggestions? The only thing I can think of right now is a demagnetised rotor, but that sounds unlikely.
Anybody?

I take it this is a Wassell Lucas "RM20" rotor as the correct rotor for a Commando is the RM21.
For some unknown reason Wassell seems to think their rotor is the RM20 which was a wider rotor fitted to the early BSA and Triumph triples.

L.A.B. said:

I take it this is a Wassell Lucas "RM20" rotor as the correct rotor for a Commando is the RM21.
For some unknown reason Wassell seems to think their rotor is the RM20 which was a wider rotor fitted to the early BSA and Triumph triples.

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Oops, sorry about that, I simply assumed it was a RM20. It's a genuine Lucas, probably from the 70's. Last time I had the primary drive off, I checked that it would hold its own weight. The bike it's self started life as an 850 MkIIa.

Holding something metal to its side will not tell you much, the magnet poles are not on the side.

You need better tools in your summerhouse.

kommando said:

You need better tools in your summerhouse.

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Or a Honda at the Summerhouse !

Fast Eddie said:

Or a Honda at the Summerhouse !

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Go and stand in the corner!!! Now!!!

Zero output from the alternator could be indicative of a broken stator wire.
It does happen.

B+Bogus said:

Zero output from the alternator could be indicative of a broken stator wire.
It does happen.

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Possible, but I checked continuity between the stator leads, about 1.2 ohms. No continuity 'tween them and the engine, so I suspect they are intact.

Update on the current situation:
Rechecking the stator output with a reliable contact using testbeds gives an output of around 20V AC, so the alternator is working.
Measuring over the battery shows that it's not charging - battery voltage remains at around 12.4 V regardless of RPM, ergo the regulator is taking a holiday.
It's a Boyer Powerbox, so not a big deal, but now I'm wondering what to replace it with.
I'm tempted to go with the one from http://www.aoservices.co.uk/products/a-reg.html, Al Osborne has a good reputation and his stuff works, but the landed cost will be about double of what's described as "Genuine Lucas 160W single phase rectifier / regulator" from a local parts pusher. He also has a 120W single phase Podtronics unit, but that's the most expensive option of these 3.
Has anyone actually tried one of these Lucas boxes?

They are not Lucas but Wassells, the 3 phase version I used started to drain the battery every 2 days due to current leakage and went in the bin. A cheap and cheerful Honda replica is more value and more likely to work. You can buy a 3 phase one and just connect any 2 phases ie the yellow wires and have a spare set of diodes if another set fails.

S’wot I’d get…

These cheap Chinese reg/rec boxes specced for CBR600 give me and some others very good service.