The BSA A7-A10 Forum

Technical => Lucas, Electrical, Ignition => Topic started by: mrshells on 17.04. 2009 23:30

Title: 6v electrics
Post by: mrshells on 17.04. 2009 23:30
Im not the brightest with electrics ( and neither is my bike)
if the dynamo is charging (just been rebuilt)
but the battery is not good ie F******
should the lights still work of the dynamo alone ? with the battery connected
I know my battery is U/S and have ordered another
it will charge but doesnt hold it for long
when the engine starts voltage goes up so I am assuming its charging OK
Title: Re: 6v electrics
Post by: RichardL on 18.04. 2009 01:17
My own experience has been that my good dynamo would light the light in proportion to revs when the battery was dead. Basically, this meant the light was off or dim-beyond-recognition at idle. Try disconnecting the battery hot lead and trying the same test. If still no light, it is not a certainty that the dynamo is bad; there are other possibilities, like bad wiring, bad lamp, etc. However, it will be a certainty that the problem is not a short within the battery (rare, I think) If you really question the dynamo, you could remove it and do the basic test of running it like a motor, as described in the Haynes manual.

All that said, your problem does have the ring of a bad or improperly adjusted regulator that allows a perfectly good dynamo to deliver too much current to the battery for too long, resulting in dead battery. If this is the case, your new battery is soon to be your next bad battery, and so on. I'm sure you've read here on the forum about the DVR2 regulator, love mine (stopped buying new batteries every 20 miles). It seems that everyone else here, who has one, love theres', as well.

There are those here who can give you more detailed values for dynamo performance, but I think I've touched on the main points. I've learned a lot from forum chums correcting me, so, if needed, I hope they do so.

Richard L.
Title: Re: 6v electrics
Post by: groily on 18.04. 2009 07:12
Richard's got it bang to rights - there should be progressively brighter lights with a dead battery or a disconnected battery, until the engine reaches the revs at which the regulator regulates, when the brightness factor should stay constant. The ammeter would probably show a very high charge with a low or nearly flat battery if it were connected, but with a totally-***** one, perhaps nothing at all.
Can only support the DVR2 comment, which I and others make constantly!
A new battery will probably sort you out for the time being though?
Title: Re: 6v electrics
Post by: trevinoz on 18.04. 2009 07:26
Richard, the "motor" test is not a very satisfactory method of testing the generator. A bad one can motor OK on 6V but will not generate OK. I have found over a lot of years and a lot of generators that 3-4V applied and generator motors, it is probably OK.
Much better to short the D and F terminals and drive the generator and see what output you get.
Title: Re: 6v electrics
Post by: a101960 on 18.04. 2009 08:26
I agree. A poor armature will allow the dynamo to motor due to inertia, The fact that a dynamo motors does not mean that it will produce a charge. This is another of those urban myths that have grown up over the years.
Title: Re: 6v electrics
Post by: a101960 on 18.04. 2009 09:12
I should have also explained that the reason a dynamo will motor if it does not charge is because inertia will allow it to motor on over an open circuit commutator segment. The thing to look for in this instance is a jerky motoring action. Also check the bearings before replacing an armature. worn bearings will allow the armature to run off centre and it will very soon fail. Before you do anything else wash the commutator and look for signs of scorching. If all is well it could quite possibly turn out to be that a dirty commutator is the cause of the dynamo not charging. Also bare in mind that the dynamo is a very willing animal and it will just go on producing a charge until it melts down if it is unregulated. I would be loathe to run a dynamo for to long with a shot battery. The dynamo will do all that it can to supply the demand but the high current that it will have to generate to meet that demand will eventually lead to failure.
Title: Re: 6v electrics
Post by: BSA_54A10 on 18.04. 2009 10:32
With a 6V system, cleansliness is godliness.
Bad connection , poor earth a little corrosion here & there and you end up with 2 or 3 Volts at the headlamp .
The brightness of the globe is proportional to the voltage it gets .
The M20 sat idle for a year while I was chasing up pistons & sleeves when I got it running again the headlamp was non existant and I thought that the rebuilt generator was cactus.
Not the case the sparky checked it 100% OK. regulator needed a clean on the points then we opened up the headlamp and measures the voltage at the globe 3V.
Now I have run seperate earths to the headlamp & tail lamp both are painted over with liquid electrical tape as is the battery terminals . After that with the engine running I now get a healthy 6.5 to 7 V at the headlamp and a very bright light indeed.
My generator & regulator are mismatched so if the battery is cactus the headlamp will flicker slowly as the regulator turns the power on & off ( to protect the generator) 
Title: Re: 6v electrics
Post by: mrshells on 18.04. 2009 11:53
I know the dynamo is good as its just been rebuilt
wiring harness is perfect as its just been rewired
battery is questionable (4.8 volt with the engine off)
regulator is also questionable as i dont think its a genuine Lucas item but it seems to check out ok
I turned the lights on this morning and every thing is working
last night when i came home from work the light got dimmer an dimmer untill the lights went out
that was hairy as i live in a village with very few street lights
Ill have to get the new battery on then go through the regulator settings
I have the PDF file from another post on the site
Title: Re: 6v electrics
Post by: RichardL on 18.04. 2009 12:27
Trev and A101960 (John, I think),

Thanks for straightening me out on the dynamo-as-motor issue. I now seem to recall that that might have been mentioned here before, probabaly by one or both of you. I guess one could say that if it didn't motor whatsoever it is ceratinly bad, or is that another falacy? Could one use the motoring test by attempting to start the motor from several different rotational posistions equal to the angle between commutator segments? Could there be a bigger waist of time when other testing methods are available?

Richard L.
Title: Re: 6v electrics
Post by: BSA_54A10 on 18.04. 2009 15:01
If you are going to use the bike very regularly then I might suggest that you consider fitting one of the spiral would sealed batteries.
Paul Groff in the UK sells a pre made set of 3 x 2 V cells that fit into the original hard rubber battery case and while not cheap, it will be the last battery you ever buy for your A 10 , virtually indestructable and rated at something like 20Ah.
Or you can buy the individual cells from Powersource  ( .
I switched over to spiral cell batteries years ago in the fleet of hire cars and have never looked back.
They can sit in a roller for over a year without being used and still fire it up without a problem where as the "extra super powerful marine grade conventional batteries" die if they are not taken out & recharges on a monthly basis .
Every thing that you could want to know about Lucas generators in on Bob Keiser's site  (

Bike Beesa
Title: Re: 6v electrics
Post by: RichardL on 18.04. 2009 16:06

But, I think it still fair to say "make dang sure that regualator is perfect" (maybe a DVR2) so that mrshells' new expensive battery is not soon mrshells' "I must dispose of this properly according to local ordinances" issue.

Richard L.
Title: Re: 6v electrics
Post by: trevinoz on 18.04. 2009 22:57
Richard, if it doesn't motor at all, it needs some serious attention. A good generator will always motor in the driven direction.
Title: Re: 6v electrics
Post by: olev on 18.04. 2009 23:40
This site has workshop instructions for Lucas and includes bike generators - magneto's too.
might be of interest.
Title: Re: 6v electrics
Post by: G/F DAVE on 19.04. 2009 11:37
Lucas dynamo,s can still motor with a open circut within the armature.The only way to bench test is to  spin them up and check output. Dave.