Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
Charging issues !


Apologies for the daft question but ive managed to confuse myself (which isnt difficult !)

The dynamo on my 1940 M20 is playing up. I have used the guide on Matchless clueless and found the following:

Voltage reading approx 1.9v, field resistance 3.2 Ohms but dynamo resistance 100 Ohmns.

Brushes look clean and reasonable so if i understand it the armature winding is at fault.

The problem is i dont fully understand if i have a long or short dyno. I believe it should be a short 48w ? I cant see any markings on the dyno to identify it other than 'made in england'. Is there anything i can do to salvage my armature and if not please can someone direct me to a suitable replacement.

Your help would be much appreciated.



email (option):

Re: Charging issues !

Hello Ant,
The first thing to do is to see if your dynamo is producing some electricity, Make a lead of suitable wire that connects to the F and D of your dynamo, connect them together with a volt meter or spare headlamp bulb, the other side to earth, 12 volt is a good choice as you will easy better 6 volts if its working. Run your engine and if you get a decent reading 7-10 volts your dynamo is fine, if not your problem is the AVC or wiring. I hope this helps

email (option):

Re: Charging issues !

Hi Ant.

As for the model of dynamo, it should be marked E3L or E3M or a like, somewhere on the body,
But basically, on all of them there are 1 or 2 screws protruding from the outer body shell
Into the dynamo to attach the core of the field coil to the body from the inside.

The "Short" 40W dynamo has 1 screw, and the "Long" 60W dynamo has 2 screws.


Re: Charging issues !

Hi Richard / Noam

Thanks for the replys. i checked teh voltage and i only get 1.9v. I think is the armature that's the problem as the resistance is 100 Ohms. way over the allowable.

I was wondering if there was any thing else i should check before buying a new armature?

I thought the attached was what i need....

...but its only 36 watts and ive read on this forum it needs to be 48 Watts.

Can anyone advise where i can buy the correct armature ?



Re: Charging issues !

As I am incredibley tight and have time on my hands now I'm retired, I would spend an age cleaning everything up and checking all connections.

Pay attention to any black deposits on the commutator and give the grooves a good clean (there is a "right way" and "wrong way" of doing this - I have never paid any attention to this).

I would flash/polarise the dynamo again to what ever polarity you have wired the bike to, cross my fingers and give it one last try.

If this still doesn't work, then when the bike is running (carefully expose the commutator), I would give the commutator a little spray of WD40. Sometimes, I know not why, this can get an old dynamo to start charging again.

If the dynamo is off the bike, I would see if it can be made to motor and motor in the correct direction. Having said this, I have had dynamos motor, but not put out a charge.

If all fails, bite the bullet and get the whole thing reconditioned, field coil, bearings and all. That way you get a guarantee.

If you don't ride the bike in the dark, then any good battery (a deep discharge one if you can get a good fit) fitted to an LED stop/tail light will last for ages.

email (option):

Re: Charging issues !

Hello Ant.

Of course it would be a good idea to check what Patrick suggested.
I would just want to add that the brushes should be free to move and the springs should have a sufficient tension,
And it would be a good idea to replace the brushes, if you have another set.

Regarding the armature, the best indication of it being in a good working order
Would be to test it with a glower, if you know an "Old school" vehicle electrician
Who still has one and knows how to use it...

following the below may also help...[2].pdf


Best regards,

Re: Charging issues !


Many thanks, i stripped the whole thing down and cleaned (commutator grooves etc), replaced the bushes and reflashed.
My Dvr2 is now showing 7.2-7.4 at the ammeter terminal so it seems to be working again.

Will see how long it lasts but thanks again for the helpful advice.



email (option):

Re: Charging issues !

:+1: :clap:

Nieuwe pagina 1