Dynamo mystery

[roy.plowman]

Afternoon folks. The '29 Lucas Magdyno from my '28 Model C has just been away to Merv Kendal for a magneto checkover. Merv didn't touch the dynamo (E3 third-brush) because it was working fine before it went away.
While the Magdyno was out, I did a decoke on the bike but didn't disturb any wiring, apart from disconnecting the battery.
The thing is...with everything back and refitted, the ammeter is showing a discharge, increasing with revs, when the headlight switch is put to 'charge'. (The ammeter correctly shows a discharge when the lights are on)
I know that dynamos can change their polarity, so have 'flashed' the live battery terminal to the Field terminal on the dynamo - many times, with the leads disconnected...but nothing I do has given me a 'charge' reading on the ammeter.
All leads have been checked for continuity and correct attachment, and there's a good earth between headlight shell and frame.
Mystified...any ideas, suggestions, previous occurences or offers of £100 for it?

Thanks in advance,
Roy in Cheltenham

[john.nash]

Is this the sort of arrangement where the dynamo mounts to the top of the magneto, and is driven by a fibre gear on the end of the mag?

If so, then you are describing a problem that I had exactly (also having just had the mag back from merv)....

Start the bike. Ammeter balances
turn on sidelights, which are all LED, and ammeter balances the small load.
turn on mainlights, ammeter in discharge, which actually gets worse with revs.
disconnect loom from dynamo and check output with multimeter and it seems ok.

My problem was the fibre wheel inside the magneto was slipping
I spoke to Merv, who apologises and sent me a new star spring, which I torqued up to the value in the lucas manual

[roy.plowman]

Not really John, unfortunately. On the 1920s magdynos the small fibre dynamo gear is driven directly from the magneto gear, with no slipping clutch. And with the old 3rd brush arrangement there's no voltage regulator - the dynamo only charges the battery when the switch on the headlight is turned to 'charge' or 'lights' - and that's when the ammeter shows a discharge! Thanks anyway...

[nevhunter]

You have "flashed" the terminal assuring the polarity is right. Anytime I have had intermittent performance from a generator, I have checked the commutator for out of round and, trued it up. Use a fixed steady on the commutator end, and undercut the segments again. In theory you should check the armature for continuity and earthing internally at the same time. and check the play of the bearings. Sometimes carbon dust gets in the slots where the brushes move and they don't contact the com very well.Check brush length and spring tension too. Nev

[roy.plowman]

SOLVED IT! I remembered an article by Jacqueline Bickerstaff (PUB) in Real Classic issue 27 about 3-brush dynamos - looked it up and found the words "Flashing the F terminal on a 3-brush dynamo will have the opposite effect to what you want (because of the internal connections) The way to do it is to remove the battery, temporarily connect the 'wrong' battery terminal to earth, then flash the new 'live' battery terminal to the dynamo's 'F'." I did all that, put everything back the right way around, and Bingo! it's charging properly again. The cheer probably woke the neighbours, as if a 1920's 500 single thumping into life wouldn't have...
One for the knowledgebase?

[admin]

Done.

http://forum.arielownersmcc.com/viewforum.php?f=42

[roy.plowman]

Thanks John