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Lucas E3L dynamo woes


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It may come as a shock, but I am having some issues with a lucas dynamo.

Wondering if anyone on here could shed some light on my situation.

Its a Lucas E3L dynamo from 1949. At some point in its life its been uprated to 12v with a new field coil and a new armature. Both look pretty ham. Bike is -ve Earth.

The problem i'm having is that it only makes around 1.5v at idle (and only climbs as high as 2 volts as revs increase). This is measuring voltage between D and ground.
If i connect the F (field) and D (Dynamo output) together and measure the voltage between these and ground its around 0.5v.

If I put 12v on the F terminal, I get 20v output. This is to be expected as its got a good magnetic charge to generate against.

So it looks like it's not making enough juice to self excite. Connecting F and D is meant to feed output into the field coil making more magnetic field thus increasing output. So this is how you test them.

What i've checked:

  • Stripped and cleaned. Sanded commutator, tidied up brushes etc.
  • Runs as a DC motor when connected to battery F & D to 12v. Ground to body. (runs in correct direction too - clockwise)
  • Field coil is about 5.5 ohms. So that seems fine.
  • Putting 12v on field coil makes a real strong electro magnet!
  • Armature seems to be between 26 and 160 odd ohms across the windings (each pair on the commutator). This seems high compared to internet literature, i should be seeing about 0.6ohm. Nothing is open and nothing is direct short... so it seems all good asides from high resistance values.
  • Seems to be about about 26 ohms between ANY sections of the commutator (are these meant to be isolated from eachother except for the opposite side??)


So yeah, not sure whats going on. Could be the armature? Has anyone had any dabblings with dynamos/generators.?

INB4 its lucas so replace it with modern stuff etc. I'd like to keep it OG as she's 70 years old this year!




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Yeah. Reploarised. Seems to hold enough magnetic magic to make 1.5 to 2 volts.

But wont climb more than that by itself.

12v on field coil and it goes hard. But still wont keep itself going if i take the boltage off the field coil

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Chair my dudes. High resistance cant be good tbh.

That matchless site is really good. Thats what ive been following to get to this point. But yeah, everything there points to armature being chooched. Might just fork out for one of those. To start with anyway....


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I've found a 12v armature for 100gbp shipped. So will get that on the way. 99.9% sure the field coil is fine. Which it should be as the bike hasn't been used much since the conversion (apparently). Plus I'm scared to take the old field coil out as previous butcher mechanic had really given the retaining screws a hiding.

Need a whole heap of other small bits n bobs too, but can't find a single place that has everything which is a pain. So have to double up on 25quid shipping. :(
Will get a new brush holder plate and brushes at the same time. Hopefully make it a legit lightning machine.

Will report back once I've acquired and installed new gubbins for thread closure.


Thank-you my bros

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  • 3 weeks later...


So, i ordered a new 12v armature. It turned up and i put it in today.
It makes voltage! However, it only makes between 6 and 8 volts at fast idle. Goes up to 9v if you give it a rev. Its like its geared for 6v. Could they have sent the wrong armature?

It should be making around 20v according to the internet. Anyone have any suggestions? The field coil wouldn't effect the voltage output would it? Like it it still had a 6v field coil, would that half its output?

Its a bummer dealing with some previous owners messing around. Can't take their word for whats been done.
I'm all out of ideas now. A bit disheartening. 

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I think the wrong field coil would have an affect, something something less lines of flux being cut etc 

I assume there's some kind of external regulator? Normally there's a few adjustments to be made there

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That the guide i've been using. Seems super useful. Although now my output readings are not in any of his lists. Looks like there are 3 options. No voltage output, 1-2voltage output, or working fine. Mine was 1-2volts first which pointed to armature. So now with new armature it somewhere between option 2 and 3. Its making voltage, but not the required output. So now i've got no idea what's wrong with it.

After I put it all together, I popped 12v on it and it seemed to motor fine, in the correct direction too!
I did the resistance checks, and i'm getting about 6ohm on the field and about 6 ohms through the output. So the resistances seem to check out fine...

I'm just running unregulated for testing just to see the max voltage it can produce. If I connect it to the regulator I get about 2volts out of the regulator. Its one of these modern solid state regulators for dynamos.
Wondering if this is poked also. Maybe it will work if I connect it all up how its meant to run etc. Will try that tonight.

Im gonna rage quit on it for a bit now. I was really hoping this would fix it. Its not like it will be ridden at night. And now its got a mag, it doesn't need any charge to run. So perfect for sunday rides. I just really wanted everything on it to be all working properly.... maybe need to look for another dynamo for testing purposes.

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That usually required if there is zero voltage output and no shorts internally.

I gave it a flash anyway, same diff. I flashed it the other way and it makes a negative voltage. So magnet is working correctly. Just output it low.

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  • Goat changed the title to Lucas E3L dynamo woes

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