Introduction to repolarising a Lucas dynamoRepolarising a dynamo (sometimes also referred to as ‘flashing’ or ‘remagnetising’) is a way to ‘wake up’ generators that may not have been used for a long time, been incorrectly wired up in the past or are not producing any volts for some other reason. This guide describes how a Lucas dynamo works, why and when you might need to repolarise it, and how to go about this. It is also (probably) applicable to other makes and models of dynamo, but it is based upon the Lucas type “E3NL” dynamo fitted to my Matchless motorbike.
This article covers the following topics:
- Introduction to repolarising a Lucas dynamo
- How a Lucas dynamo works
- When to repolarise a dynamo
- Getting ready to repolarise the dynamo
- Repolarising (or ‘flashing’) the dynamo
- Conclusions and your comments
How a Lucas dynamo works
A Lucas motorcycle dynamo can be thought of as a simple DC motor running in reverse. Rather than applying a DC current and getting out rotational movement as in a motor, you supply rotational movement from the engine and (all being well!) get out an electrical current. It should be noted that dynamos are DC (direct current) devices and are therefore very different to more modern alternators which, as their names suggests, are AC (alternating current) devices.
Let’s first give some thought to exactly how dynamos work as this will hopefully make what we are going to do later make much more sense. A basic dynamo (or motor) consists of two main components: a magnet and a coil. In a dynamo, the coil is rotated within the magnetic field of the magnet which induces a current to flow. In a motor, current is supplied to the coil which induces a magnetic field which is opposed by that from the magnet, causing the coil to move (rotate). This is why some people recommend testing a dynamo by “motoring” it – basically rewiring it to the battery as if it were a motor and seeing whether it spins. But that’s for another day…
The various components inside a Lucas E3 dynamo are shown in the exploded diagram below – click to view a larger version.
Where a Lucas dynamo differs from a simple DC motor is that there are actually two sources of magnetic field within the unit. The first is a soft-iron core which is what retains the magnetism when the engine is not running. However, this is not a particularly good magnet and the amount of magnetism it retains is relatively small and certainly not enough to generate the 45-60 Watts required by a classic motorbike. But it is just about enough to get a small current flowing in the coil when the engine is first started.
This is where the ‘field coil’ (a secondary coil within the dynamo) comes in to play. Some of the output from the generator coil is fed back into the dynamo through the field coil which then generates a secondary magnetic field (hence the name) of its own. This magnetic field is stronger than the residual field from the soft-iron core and therefore induces a bigger current to flow in the coil. More current is therefore supplied back to the field coil, giving an even greater field strength, more induced current in the coil, and so on until an equilibrium is reached for a given engine speed. Simple hey!
The following guide describes the relatively simple task of repolarising (sometimes referred to as “flashing”) a Lucas dynamo and is applicable to models such as the E3N, E3M, E3NL, E3L (etc) as fitted to many classic bikes. It may also be applicable to alternative makes and types of dynamos fitted to other classic bikes and cars too, but you’ll need to work out for yourself which are the correct terminals etc.
When to repolarise a dynamo
So why would we need to repolarise a dynamo? Well, there are two reasons for repolarising: Firstly the residual magnetic field in the soft-iron core may not be strong enough to get things started, or secondly it might be the wrong way around (i.e. the north and south poles are reversed) causing the induced current to flow the wrong way to the battery.
The first condition may occur if the motorbike hasn’t been used for some time as the residual magnetism will gradually fade over time. Alternatively the dynamo may have been knocked or damaged which can sometimes cause the magnetism to be instantaneously lost.
An incorrectly polarised dynamo may have been caused by incorrect wiring (e.g. connecting the battery the wrong way round) or the unit could have been previously fitted to another bike which required the opposite polarity. The original Lucas service data sheets specified that all replacement dynamos would be supplied pre-polarised for positive earth motorbikes, and that they would therefore need to be repolarised for negative earth machines. What we are about to do is therefore a very standard procedure and requires very little technical equipment or knowledge.
The main indication that a dynamo will need to be repolarised is when everything else seems to be in order with the charging system (see my General System Checks guide for more info) but the dynamo output is still very low, usually after not being used for a while. A negative dynamo output voltage (see my Testing a Lucas Dynamo guide) usually indicates an incorrectly polarised dynamo.
Getting ready to repolarise the dynamo
So that’s the ‘why’ taken care of, now comes the ‘how’ bit for repolarising your dynamo. All you’ll need is a long (say about 1m) length of wire – that’s about it! The wire needs to be reasonably thick as you’re going to use it to pass quite a big current (albeit it only momentarily) through the field winding of the dynamo. Something like multicore mains wire or battery cable is ideal. Anything too thin might get hot and melt if the current is too large for it.
I would advise reading right through this article first as this will take you through disconnecting the battery and dynamo, identifying the relevant connections and checking the existing output voltages. After ‘flashing’ the dynamo, you will also need to repeat the measurements described in Steps 3 to 5 of this guide so it’s worth becoming familiar with them anyway.
Repolarising (or ‘flashing’) the dynamo
The first thing to check is which way round your bike is wired, or in other words, which battery terminal is connected to the frame (earth) of your motorbike. If the positive terminal of the battery is wired to the frame then your bike is positive earth – the normal configuration for bikes up until around the 1950’s. Later bikes and nearly all modern vehicles are wired negative earth, that is to say that the negative terminal of the battery is connected to the frame of the motorbike. It is important to get this right as it will determine which way round you need to polarise the dynamo unit.
To try to make this clear, I am going to refer to the ‘Live’ and ‘Earth’ terminals of the battery. The ‘Live’ terminal is the one wired to the ammeter and the ‘Earth’ the one connected to the frame of the motorbike.
Ensure the Earth terminal of the battery is still connected to the bike (i.e. it is earthed) but disconnect the Live terminal from the bike. In it’s place connect the length of temporary jumper wire. Take care to not let the other end of this temporary jumper touch the bike frame or engine as this will create a short circuit and allow the full discharge current of the battery to flow through the wire which could potentially be quite dangerous to both you and the bike!
On my bike, I have a 15A fuse fitted in the earth connection between the battery and the frame (my bike is positive earth and the fuse connection I had was red, so this seemed a more logical way to connect it, although normally a fuse would be fitted to the live side of the battery). The high current drawn when flashing the dynamo will probably blow the fuse so I temporarily swapped it for a higher rated fuse (say 30 Amp). The alternative would have been to swap the fused connection for a direct earth connection, but this would have taken longer and anyway, I liked the reassurance of having a fuse there in case I did something really silly! If you have a fuse fitted to the live side of your battery, then just remove this to effectively disconnect the battery whilst leaving the earth connection in place.
Next, carefully identify the Field terminal on the dynamo unit. This should be the one on the left (click on the photo at the top of this article for a close-up view) and is normally marked with the letter “F”.
Carefully touch the Live end of the jumper wire to the Field terminal of the dynamo for just a second or so, no longer. You should see sparks as you touch the wire to the terminal, but this is ok (it just shows that current is flowing from the battery). If you don’t see any sparks, check that the earth terminal is still connected to the battery which will form the return path for the current via the dynamos metal case. Also make sure the battery is well charged. Touch the Live wire to the dynamo in this way a further couple of times to complete the ‘flashing’. Immediately disconnect the temporary jumper wire from the battery before it causes an accidental short.
That’s it – you have now repolarised your dynamo! Not at all difficult was it? Now go back and recheck the dynamo output using a multimeter as described in my other article and you should hopefully see a nice healthy positive output voltage. If not, please refer to back to the guide for further trouble-shooting ideas.
I hope you found this article useful. Please feel free to leave me a comment below to let me know how you get on, if you have any further tips or suggestions, or maybe if you have spotted a mistake.[sc:disclaimer]
Many thanks for info on polarising E3LN dynamo. Just rebuilt B31 from many parts including 2 dodgy dynamo’s and spent 3 month’s head scratching and re-adjusting cut out and regulator and couldn’t get a positive charge. Worked first time. 2 minute job Brilliant! -BSACLUELESS-Leics
Glad you found the repolarising info useful Mick 🙂
hi! absolutely superb explanation of dynamo/regulators. many many would be teachers could adopt your style an simplicity.
once again thanks,
thank you very much for this imformation as it made changing from 6v to 12v very easy.
all the best ron taylor.
Certainly the best description of checking a dynamo I have ever come across. Really informative and easy to understand. Your guides helped me bring the dynamo on my B31 back to life. Thank you for putting this series of articles together.
Hello, I have a 6v Lucas E3H dynamo wih the direction arrow anti clockwise, but I need it to charge running in the clockwise direction. Is it possible to use it that way or modify it in some way to charge in a clockwise direction?
Hi Pete. Repolarising the dynamo (as described above) should allow it to operate in either direction. Fit it to the bike and check the output with the engine running. If the voltage is negative you will just need to repolarise it the other way round to get a positive voltage output, then it should work fine.
Thanks for very quick response, Pete
Excellent information,just rebuilt a dynamo for A10 replacing internals for 12v,struggled to get positive charge at dynamo,followed the flashing instructions,(still negative)not knowing what to do after about 6tries i i swopped the field coil wires,(still negative) ,flashed it again ‘bingo’ but I don’t understand why.Keep up the good work I know I will be referring to you pages again.
Cheers Brian, nice to hear from you. James 🙂
Hi. If you guide me how to build new dynamo and spare list for the same
Hi Allen. Have a look in the ‘Lucas Electrics’ section of the free downloads page and you might find some useful info, including the original Lucas service guide: https://matchlessclueless.com/resources/downloads/
Other than that, I can only recommend sending your dynamo to an expert for testing and rebuilding. There are some UK options listed here: https://matchlessclueless.com/resources/parts-servicing/
Just adding my thanks for a clear consise set of instructions. I’ve always been wary of DC systems, but having fixed the fault in 2 mins flat by “flashing” I’m now a happy teddy!
Many Thanks, Andy
I am also aware of the irony of miss spelling concise
Best Regards, Andy
Cheers Andy. Glad the instructions for repolarising your dynamo helped and you managed to get your charging system up and running again.
Thank you for posting your very helpful dynamo articles. May ask is it possible to flash the dynamo whilst it is detached from the bike and how one would go about it?
Hi Edward. Yes you can flash the dynamo with it off the bike – just find a way of connecting the other lead from the battery direct to the dynamo case on a bare bit of metal. The tricky bit is spinning the dynamo so you can test it’s output. Try connecting a battery drill to the dynamo shaft with a length of flexible hose to take up any misalignment and prevent damage to the shaft. Regards, James
well done, everything is clear and easy to understand, even for a non native english speaking person,
Thank you for your help,
Absolutely brill sorted my 1953 A.J.S. 16 ms(350) charging probs
WELL DONE THAT MAN.!!!
Cheers Keith 🙂
Great article, very helpful. One question, I’ve only got a 12V battery, can I use it to flash my 6v dynamo without damaging it?
Yes, it’s fine to flash a “6v” dynamo with a 12v battery. Actually the Lucas dynamos work fine at either 6 or 12v, so you can still use the same dynamo without modification even if you convert your bike to run at 12 volts. regards, James
Hello there, I am hoping to use this information to get the dynamo on my ’35 RE model “A” to work correctly. The one currently in use is marked for rotation in the opposite direction to which the bike runs. This bike is coil ignition with the points in the end of the dynamo and so far I have been unable to source one of the correct rotation. Given a charged battery the bike will run reasonably well (for an old two stroke) but it would be nice to get the charging working. I have asked many people and they all say “flash the dynamo across” but nobody could tell me exactly how!
I also run a’42 Matchless that I had to have the dynamo rebuilt for, I have fitted a modern regulator to this but got caught out coming home from a run when the battery gave up & I found out that the regulator fitted won’t work unless there is current in the battery.
A new battery, an LED tail light and a Halogen bulb have since been fitted to improve the lighting, the only thing is that the charge does not balance until 36 mph now!
Many thanks for the information. Andy.
Cheers Andy, glad you found the site useful. Good luck with getting you RE’s dynamo working right. For dynamo regulators I would heartily recommend the ‘DVR2’ units as these work even without a battery fitted. Regards, James
I need to flash the dynamo or motor it to establish whether it has been set for + or – earth I intend to run it as a – earth, the dynamo was restored by the someone for the previous owner(bike was in bits when bought) so I am unsure which polarity it is. My question is if I flash it do I have to disconnect it from the AO regulator before flashing it, it’s all assembled on the bike and connected but it has not been run,I thought I should check polarity before running I just assumed it would be – earth !!
Hi Bob. Yes, you need to disconnect the dynamo from the regulator before flashing (also best to disconnect it before starting the engine if you are unsure which way it is polarised). If the dynamo hasn’t been used for a while then it is likely to have lost its residual magnetism anyway, irrespective of how it was previously polarised, so best to ‘flash’ it anyway to boost the magnetism and ensure it is correctly polarised for your negative earth bike. Good luck! James 🙂
Many thanks for the reply Here goes !!! 🙂
hello, having a few troubles, my dynomo 6v does not put out any charge , when disconnected and + touched to D it turns clockwise, but it needs to run anti. so i swapped wires inside off dynomo, now it turns anticlock wise, but still no charge output, even after cleaning it all inside and flashing it. any ideas..thanks
Hi c10. Have you gone through the various dynamo tests described on this page?: https://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/ Regards, James
hello james , dont have multi metre at the moment, but have just wired F and D together,and put small bulb between there and frame, wound it over with drill, and nothing, not producing any current, flashed it alot as well
Not sure, but you might find the drill doesn’t turn it over quick enough, especially if it is just a battery drill. Flashing is pretty much instant so there’s no advantage to flashing it a lot.
But it only spins as fast as my cam, because the dynomo is drivin from cam gear. can i use some other 6v generator.
A little faster with gearing but its not much faster.
Just as a matter of interest the E3L dynamo on my Viper has an extended (D)pole that protrudes through the end cap. Is there a function for this ?
Hi Bob. Not exactly sure what you mean I’m afraid. Can you elaborate a little or maybe send a photo? James
Do i need to buy a multimetre to test voltage? even if its not lighting a bulb when dynamo is drivin, ..its been a 15 year project, and this is only thing holding me up from ridding it, any help would be much appreiated . i will go and buy a metre now.
A multimeter will be a pretty good idea to try and see what is or isn’t working. It will allow you to test the voltage output as well as coil resistances (see the testing a Lucas dynamo page link in my earlier reply). Hopefully this will allow you to narrow down the problem, but unless its something simple, you will probably be best to get the dynamo professionally rebuilt. Good luck, James
Ony getting a small 0.44v out of one wire,when put two together its less, about 0.17v ,when both wires are together and touched with + motor spins fast,. it use to be touch one and motor turned slow, and if i touched the other wire it would just short. somthing has changed… at least im getting some kind of voltage, but hardly enough. wont even light a bulb..
Have a read through the other page I mentioned earlier as that gives some possible reasons for various output voltages: https://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/
In particular: “If you get a low voltage reading of about half a volt then the service manual suggests that the problem is probably with the field winding.” To be honest, everything I know is pretty much explained on these two pages, so if you are unable to diagnose or fix the fault from this info, then I’d recommend sending it to be reconditioned.
hey mate,sorry to bother you again, i put dynamo into shop and they said it was producing 14v and charged me 100$ they put multimetre red onto both wires and black to dynamo case,(normal) but i think they were useing wrong settings or somthing, as when i got home and bench tested with same speeds, it was same as when i took it in to shop, ((what i need to know is,if i put small bulb from joined D and F, to dynamo case should bulb light up if spun)), im spinning it way faster then bike could and still nothing, but they say its producing 14v. im getting 0.47v
Sounds like you’re having real problems there C10! Just because the multimeter reads 14v doesn’t necessarily mean that the dynamo is any good. The multimeter doesn’t impose any sort of load on the dynamo (i.e. it doesn’t draw any current) so is not by itself a perfect test. Your bulb test is better as it imposes a real load – use a 6v 25-35W headlight bulb and it should glow at reasonable revs (and blow through too many volts if you spin the dynamo too fast!). You’re doing it right by connecting the bulb between the connected D and F terminals and the ground of the dynamo body. Sounds like you need a trip back to the shop! Good luck and let us know how you get on. James 🙂
yes driving me nuts,Took it to the shop. Put in vice,multimetre grounded to vise,red from multimetre went to 12v bulb to D and F ,spun half the speed i spin it. and it lit the bulb, i try it at home with small bulbs/12v/6v but dont get anything, case is spotless/good ground area, i carnt figure it out
Hi c10. So how did you get on? Any luck finding a solution?
I bought an electronic regulator “podtronic”
I conected it according to the instuctions except the repolarizing step.
What happend is that my voltmeter showed charge but after about 10 miles
I had the fuse blow out (conected to “live” between regulator and positive of battery)
Changing the fuse did not change the situation and it kept blowing out.
I have light on my bike but directly from reg and not from the battery that by now is empty.
Not having a healthy 6V battery around and wanting to check the 6V dynamo on my Norton 16H positive earth Lucas system (which indeed shows a minus 7-8V reading on fast tick-over), I was wondering if I could use a 12V battery to repolarise the dynamo? After having the dynamo disconnected from the regulator and the rest of the electrics, of course.
Thanks for any answer!
Hi Tangopeter. Yes, no problem at all in using a 12 volt battery to repolarise your dynamo. You should only need to touch the wires for a second or so though. Good luck! James
Hi just rebuilding a 1952c 11 that’s been stood since 1970s dynamo strip down and serviced repolarisied and bingo working fine thanks for the knowledge great stuff Jim
Many thanks for a superb set of articles. I knew about reploarising dynamos, but have never done this and I will now attempt to jump-start my E3L after its 20 year rest!
james-Hi c10. So how did you get on? Any luck finding a solution?
Reply- it was cracked dyno brush base plate, didnt allow enough pressure on one of the brushes,,brush cradles needed good clean, . still no charging, ethier wireing or reg stuffed. can i check voltage from reg
neg earth set up.
reg wires–black to tank/earth.
–red to ignition/back to battery
–green to D
–yellow to F on dynamo. dynamo producing around 2-4v slowish/4-7v fast. but nothing showing on volt gauge , if light on goes to -4 and stays,
Hi c10. Try the tests here first to check the output from the dynamo (you need to connect the F and D terminals together): https://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/
Do you have a mechanical reg? If so, disconnect the A terminal to remove the battery (or place a piece of paper between the cut-out contacts in the reg) and measure the voltage at the D terminal on the reg (after reconnecting the dynamo as normal). Should see regulated output voltage (i.e. around 7v if your bike is 6-volt) at medium revs. If not, there’s probably a problem with your regulator. James 🙂
Does it matter which way round an E3L field coil wiring is connected and if so, how do you distinguish which side of the field coil should go to earth?
By the way – thanks for an excellent & informative web site.
Hi Andy. Yes, the way the field oil is connected is important as it sets the direction of rotation for the dynamo. If it rotates backwards for your bike then it won’t generate. Are you familiar with the motoring test (connect F and D terminals together and drive the dynamo as a motor from a battery, assuming it is removed from the bike of course)? If the field oil is connected backwards then the dynamo will motor in reverse, so then either swap the field oil connections or the brush connections (but not both) and it should then go the right way. The direction it motors is the direction it will generate. Hope this helps, regards, James
James – many thks. I quickly reversed the field coil connections this morning and re-polarised for good measure – and bingo shes charging again with a vengence. I have to admit that previously I had mended something that wasn’t broken (putting in new brushes!) and am paying the price for my incompetence – but thks again for a super informative web site.
No problem, glad you got it working! 🙂
I bought an unrestored (bodged) 1956 G3LS a couple of monts ago. It starts/runs nicely. The battery was no good and the wiring was suprising in places. Wires connected by twisting the bare ends together and covering with sticky tape (now cured). I replaced the battery with a Cyclon sealed 6v 5ah unit in a battery box. The dynamo appears to charge nicely with the engine running, showing a positive charge on the ammeter. However, when the lights are switched on the ammeter shows an even greater charge rather than the expected lower charge.
Can you suggest what may be wrong?
Thanks and kind regards, Paul
Hi Paul. Sounds interesting! My guess is that the ammeter is wired backwards so that is showing a charge when it is actually discharging, and vice versa. Hence the positive charge with lights on is actually possibly a small discharge (or just zero error on the meter) and the high charge with the lights on is actually a high discharge. Does that sound possible? The easiest way to check is to stop the engine and turn the headlight on. Which way does the ammeter go? Swap the leads around until it shows a discharge (to the left usually). Hope this helps, good luck getting it sorted, James 🙂
Many thanks for your reply received this morning. Yesterday I had another good look at the wiring on the G3LS and tried the headlamp as suggsted in your reply. The ammeter needle showed Positive. So I had another, this time Careful, look at the wiring and saw the wire labelled by the previous owner with a +ive label was the one going to the ammeter. The book says that a 1956 G3LS has a positive earth so the label was misleading. I removed it and connected the that wire to the Negative battery terminal and the earth to the Positive terminal. Of course, everything then worked as it should. A lesson learned here. DON’T trust dodgy labelling and LOOK carefully.
With the engine running teh ammeter shows only a small charge probably as the battery is new and fully charged, However with the headlights on the dynamo keeps up with the discharge so it is working ok. Just on case the wrong wiring had caused the dynamo to need ‘flashing’ I did that following your very clear directions. Unnecessary as it turns out.
May I say that, as an ex-teacher, I admire the clarity of your writings.
With thanks, aul
Thanks Paul. Glad you have found MatchlessClueless to be useful and have solved your wiring problem. Happy riding! James 🙂
I have a 1933 Rolls Royce, it has a dynamo and i need to polarize,wiring consist of A B C A shoes – side field, B shoes + side field, and C is + output armature. can any one direct me how to polarize?, Thank You , GT .
Hi GT. The principle will be the same as described above, but you will need to determine which lead is which first, so you’d be much better asking someone more familiar with your Rolls Royce!
Came across your website by ‘accident’, I have just bought a 1956 Norton model 50 that has stood for 25/30 years for a restoration project,the info on here is really informative and will be referred to when I get started on it. Very well done!
Many thanks, John G.
Cheers John, good luck with your Norton project – sure it will give you many hours of fun! 🙂
Clear and useful. Thanks very much for sharing your knowledge.
Hi James, just wanted to say thanks!You’ve saved me a lot of time and trouble with your excellent instructions!
The old bikes charging a treat now.
Keep up the good work.
Cheers Steve. If the MatchlessClueless website has been useful, then a copy of my new “Classic Motorcycle Electrics Manual” book would be well worth a read! It’s available on Amazon UK here.
Regards, James 🙂
The brush holder on my Dynamo has been detached from the stationary coil inside the body. Where do I reconnect the two wires from the coil? Thanks for excellent articles.
hi from john.le 200cc 1953 mk2.positive earth.plugs blackening .i have just pulled the millers 6v brass mag of. the spark blue .i have used a spark tester.passiss red white as you you try to go to green. it tells there a fault accuring ?
i bought a new carb.4 carbs used.still blackening the plugs.vaults passing through the windings.6.56v .running .6.98v voltmeter not moving only when points open. .as i was just about to send the brass mag of to someone in the club.
or could i do it my self.
hi james, a great article. Is it ok to repolarise with my dvr2 already in place, thanks mick.
The best explanation I have read on dynamo electrics. For once I was cumfterble working on a friend’s 1950 triumph. Top man. Thank you.
Your articles are very clearly written and illustrated. I note that the magnetism of a dynamo can be lost over time due to inactivity. Does the same happen to a magneto? I have a Lucas K2F magneto which when I bough circa 20 years ago and had a spark when turned by hand then. The magneto has been on a shelf since and now won’t spark when turned by hand. I didn’t check it since buying it. Any ideas?
Hi Micheal. Yes, I guess the same loss of magnetism can also occur with magnetos too if they have been ‘sat on the shelf’ for years. Probably worth getting it looked at by an expert who I’m sure will soon be able to breathe life back into it! Regards, James 🙂
Thanks James, Happy new year!
Many thanks for publishing this. It worked a treat today to persuade my dynamo to start generating again.
Simpler way is to take the cover off the regulator and push the coil armature down for a second or so. If you need to reverse the polarity for some reason then reverse the battery connections first.
Thanks for a brilliant article on magneto. Having trouble with my Norton ES2. Completed your first check and getting very low voltage across terminals F and D. Thought I would try to flash terminal F to see if it would improve and found that it would not flash on terminal. Checked all terminals in place still no joy, so looks to be terminal. Thanks again
12 Church Road
I have a problem with my charge system. When I start the engine I can’t get any voltaje but if with a cable I put together in the control box F and D the sistem starts working but without regulate voltage of course.
If I disconect the two dynamo wires and put the multimeter between D and frame y can reed 1.2 v.
Do you have any tip for me?
Thanks for all.
Hey James, I am rebuilding a Lucas E3L dynamo, and I cant work out which way to connect the field coil connectors – Any guide on this? Thanks so much for this article by the way – super helpful.
Hi Martin. Sorry, I can’t remember of the top of my head, but I’m pretty sure you will find the answer in my ‘Classic motorcycle Electrics’ book if you can get a copy (see Amazon here for example: http://amzn.eu/iW19RY2). There’s a whole chapter covering Lucas dynamos, their connections, repolarising, testing, etc, etc. Sure you would find it a useful read!
Thank you for your articles discussing the Lucas dyno. Your diagnostic steps are spot on! I followed the steps exactly and was able to determine why the E3L on my 1954 Triumph T110 was not charging. Ended up repolarising it and bingo, it started putting out 8v+ at a fast idle. Never had to pull it off the cycle either. I have bookmarked your website and will no doubt be using these treatises again. Learned a lot. Fascinated with knowing about the option to convert to 12v. Wanting. Now to add turn signals to the bike just for safety while riding. At 72, I like keeping both hands on the bars at all times! Any suggestions?
hi I have a bsa c11 I have had my dynamo off to replace the drive chain my ammeter shows a samall discharge when the ignition is turned on as normal when the gngine is running the discharge increases the batteay is being charged checked with a meter whats going on ? any help pleas.
So I went and bought another regulator because I connected backwards (+ive earth grrr) and thought I’d blown it. That didn’t work, but guess what? 3 quick connections and I’ve got an astonishingly good charging system. Thank you so much, one more job off the list! BSA B33
Many thanks for this excellent article. Not being able to get my BSA B31 to charge its battery I used it to remind myself of the best way to check a dynamo output, and to “flash” the field winding. Worked perfectly and now the bike is running as it should…… sadly for its sale in the next week or so, after over 40 years of ownership.
Hi, I have read this article with great interest as I am doing some work on a friends 1955 AJS 350 & the Dynamo is kicking out 0.7v even after striping it down & cleaning all the gunge & surface rust off. I then “flashed” it as per the instructions but still only 0.7v !!! Any ideas would be greatly appreciated ?
Hi, and thanks for all the excellent articles.
I have recently (inadvertently!) proven your statement from a few years ago that if the dynamo motors in the reverse direction, it will not function as a generator. I had this situation, and as soon as I reversed the brush connections, the unit motored in the correct direction – and when driven produced a healthy voltage.
My question is why would the dynamo not generate at all with reversed connections, and why would it not just produced a current with a reversed polarity? (I assume that the act of motoring would be sufficient to flash the field coils).
I have an older mechanic friend who built a test rig for Harley generators. If they don’t charge he will ground the field terminal for a couple seconds while the generator is turning to put residual magnetism into the magnets. Would this work on a Lucas dynamo?
This was super easy and my 12 volt conversion is complete. I am now running 12 volt negative ground with an LED headlamp and tail lamp. All good.
1960 BSA Super Rocket
the field coil in is parallell to the dynamo rotor, that means it gets the 6 Volt from the Dynamo, when it is normal operation.
How the hell shall a external 6V Batterie then drive more than the same current through the field coil?
I got a Lucas E3 M-1 TYPE L-0 (with a cut off relais integrated). I cannot reproduce this “High current pulse” in just connecting the field coil to a battery.