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[Mark B]

Thanks Ric for that excellently clear analogy. I'll have to look into the bank volts to see if I can find any extra electricity...

[Frank]

12.3v across a freshly charged battery is not exactly brilliant. 12.5/12.6 would be ideal. A 13.0v charge isn't good either, 14.2-14.5v would be better. Looks to me as if you may have a weak charging system and a weak battery.

[Gwilly]

Got to go with Franks figures, sounds like the battery is not taking a charge if your home charger is not doing the trick.

Did you measure the AC volts across the yellow wires motor running?

Quick test, If you point the headlight at a wall does it get brighter when you rev the engine?

[Mark B]

Update: with the engine running I'm getting between 0 and 3 volts across the two yellow wires from the powerbox. The black earth seems OK. The battery is still holding 12.3v so the headlamp's got a strong beam, but it doesn't brighten with revs. This is what they call a learning experience!

[Gwilly]

Mark just to be clear i meant disconnect the yellow wires from where they join into the alternator wires and insert the red and black probes from your multimeter into the two wires coming from the alternator.

Multimeter dial set to 200 volts AC, start engine and read results at different engine rpms, i.e. tickover to medium high revs..

If this is what you've done and only getting 0-3volts AC (not DC) then remove primary and inspect alternator stator coils for signs of damage/scorching.

Could also be rotor demagnetised/damaged, possible you need a new alternator complete..

Stator coils could be ohms resistance checked if no obvious problems found..

[Mark B]

Thanks Gwilly; those are the two wires that I put the probes on, at the connections with the yellow wires, but without disconnecting the yellow ones first. Time to take the primary case off... I think I owe you a pint!

[Revband]

Mark you will get a false reading you must disconnect the input to the powerbox, if still low volts it's the alternator if you then get around 40v AC you probably have a faulty powerbox.

[Mark B]

Thanks for that Revband. I disconnected the yellow wires and put the probes on the green and white wires coming from the alternator. Essentially no reading at various revs. So it looks like the alternator....

[ric]

If the output from the alternator wires is unequal check for 360* stator/rotor clearance.

[Mark B]

Is that just a matter of sliding a fag paper in the gaps or is there a proper engineer's method?! I've taken the stator off and there are no obvious breaks in the wires, although the output leads are just twisted(seemingly securely) round the copper wire rather than being soldered.