Lucas E3L dynamo not charging

[Rick Rybicki]

I've already posted this on BSAOC forum in UK but had had no definite results so far. There was a suggestion that there's a guy called "CheeserBeezer" out there who is a wizard at this sort of thing. Hope you see this.

I'm running a quite non-standard '59 A10 and it's stopped charging the battery. What's happened so far;
Was working fine but while I was sorting out a problem with rear light I found that the battery wasn't charging from the dynamo. It charges fine from a charger.
Checked the output of E3L dynamo and it was showing about 1.6v.
Checked resistances, F---3 ohm, D---0.6 ohm. OK.
As the bike hadn't really been used for a few months I decided to "flash" the dynamo.
After the "flash" I tested the output again and now it's showing a NEGATIVE output of about -1.6v.
The dynamo was professionally rebuilt some years ago by Dave Lindsley but has only done about 3k miles in the last 7 years.
It's set up through a 6 - 12v conversion regulator, negative earth and driven by a "Hawker" geared up belt drive.
Help!!! How do I fix it???? Please give any answers in the simplest of terms!

[Triton Thrasher]

Are you joining F & D together, to test voltage between them and the case?

And revving to midrange?

[CheeserBeezer]

Thanks for the mention. There's a dynamo test page on my website. I suggest you use that s a starting point
https://sites.google.com/site/priorymagnetos/dynamos
You need to make sure you're spinning the dynamo fast enough to achieve the output you're looking for. A 6v dynamo produces about 9v @1500 r.p.m.

[Rick Rybicki]

Are you joining F & D together, to test voltage between them and the case?

And revving to midrange?

Yes.

[KiwiGF]

Are you joining F & D together, to test voltage between them and the case?

And revving to midrange?

Yes.

Try cleaning the commutator, brushes can have too high a resistance and this prevents “self starting” of the dynamo, even when armature resistance measures ok.

At the risk of confusing, if you (alternatively to the F+D join test) put a current through the field coil and measure the armature voltage (or better still put a headlight bulb across the armature) that can give a better indication of the health of a dynamo.