Introduction to the Lucas charging system
The Lucas charging system found on many old bikes and cars differs from those fitted to modern vehicles in that it uses a DC dynamo (as opposed to an AC alternator) to generate an electrical current from the rotation of the engine. It is a reasonably straightforward system consisting of only three main components; a dynamo to produce the current, a regulator to control it and battery to store it for later use. Throw in an ammeter to show the rider what is going on and some simple wiring, and that’s about all there is to it!
The Lucas dynamo and voltage regulator system is normally very reliable if maintained correctly, although with the bikes which have these systems now well over half a century old, problems do of course occur. But another big difference to the charging systems found on modern vehicles is that the Lucas system can be readily maintained, serviced and (in most cases) repaired by the home-mechanic. There are no computer chips or high-tech electronics to worry about, and no special tools required. So what are you waiting for?
This article is a simple step-by-step guide to checking over the Lucas charging system on your old motorbike to ensure that it is operating roughly as it should. It is written and illustrated with respect to my 1951 Matchless G3LS motorcycle, but is equally applicable to other motorbikes and cars that use a similar dynamo charging system. The purpose of this guide is to give an overview to help you locate and identify any possible faults. The majority of the tests described are very straightforward, but where the testing process is a bit more involved there are links to separate articles I have written which go into the procedure in much more detail (see also the links to the right). You can either work through the checks in the order I have suggested, or use the contents list below to jump straight to a specific section of interest.
This guide consists of the following sections:
- Introduction to the Lucas charging system
- Step 1 – Checking the headlight brightness
- Step 2 – What does the ammeter say?
- Step 3 – Testing the battery
- Step 5 – Checking the regulator
- Step 5 – Testing the dynamo’s voltage output
- Step 6 – Checking the wiring
- Conclusions and your comments
Step 1 – Checking the headlight brightness
So let’s start at the very beginning and try to work out whether the charging system is actually giving any output. The easiest way to check this is to start the bike, switch on the headlight and give it some throttle to bring the engine up to a fast tickover. If the charging system is working, the headlamp should glow noticeably brighter when the bike is revving compared to when it is just ticking over. The electrical output from the dynamo is proportional to engine speed so that the faster the engine is running, the more electricity the dynamo is generating. That said, there should be no need to rev the engine too highly to generate output since the system is designed to give ample power to run the headlights and/or charge the battery when the bike is cruising along at around 30 to 40mph in top gear.
However if it is a bright day (and especially if your bike is still running on 6 volt electrics), the difference between the light output from the headlamp at tick-over and when revving might be relatively small. I certainly couldn’t see the lamp getting brighter when I did this test during daylight, but then it was a rather sunny day back in India at the time! This test is therefore best performed at night (or inside a dark garage etc) so that any changes in headlamp brightness are easily observed.
If you are able to see the headlamp getting brighter when you rev the engine, then the chances are that the dynamo charging system is giving at least some output and things are (more or less) working as they should. If the headlight brightness doesn’t appear to change then you might have a problem, or you might just have a fully charged battery such that the bulb is at maximum brightness even with the engine not running. This test is certainly not a foolproof one, but it is a useful daily (or rather nightly!) check to make sure things are ok since it requires no tools or set-up. To get a clearer idea of what exactly is happening though we can look to the ammeter display.
Step 2 – What does the ammeter say?
The purpose of the ammeter is to give the rider a pretty good idea of how much current is flowing into or out of the battery at any given moment. If everything is wired correctly then the needle should move to the right (+Amps) when the battery is being charged, and to the left (-Amps) when it is discharging. When the engine and all electrical items are off the needle should be approximately in the middle, although these meters aren’t always the most accurate and can often have an offset to one side or other of the zero mark.
As a first check to make sure the ammeter is working and wired in correctly, turn on the headlight without the engine running. The needle should read between about 3 and 6 Amps to the left (depending upon the wattage of the bulbs and whether the bike is running 6 or 12 volt electrics). That should be around half-way across on the scale of a normal 8 Amp ammeter display. If the needle moves to the right then it’s connected the wrong way around, in which case you need to remove the headlight and swap the spade terminal connections around on the back of the meter as shown in the picture on the right.
If the needle doesn’t move at all when the headlight is on then the ammeter most likely either faulty or not connected correctly.
Next turn the headlight off and start the engine. At tickover the needle on the ammeter should sit around the zero mark; at low revs the dynamo won’t be producing enough voltage output to charge the battery. Raise the revs slightly and the reading on the meter should also increase to the right indicating that charge is going into the battery. It should reach between around 5 to 8 Amps at moderate engine speeds (depending upon which dynamo type is fitted and whether the bike is 6 or 12 volt) although the meter may not be 100% accurate. As mentioned earlier, the Lucas dynamos are designed to give their full output at above approximately 2000 rpm so there should be no need to race the engine.
One thing to watch for though is that the ammeter may not show a positive charge going into the battery if the battery is already fully charged, irrespective of the engine speed. Usually the ammeter will show a few amps charge when the motorbike is first started, but this will drop down to only a fraction of an amp after a few minutes of riding once the battery has been replenished. This small trickle-charge should continue until the battery becomes partially discharged, for example by use of the lights or from being left idle for a few days.
The next thing to try is turning on the headlamp again with the engine running. At tickover the needle will again show around 3 to 6 Amps discharge (to the left) but as the engine speed is increased, the needle should rise to around zero or perhaps even an amp or two on the positive (right) side. The exact meter readings will depend upon what dynamo is fitted to the bike (they come in 5 and 8 Amp maximum output types) and what wattage of bulbs you have fitted to your lights. Ideally the dynamo output should balance the lighting load such that the ammeter shows zero amps (i.e. no charge going into or out of the battery) when riding along at normal speeds. If the ammeter only ever shows a discharge when riding with headlights on then the battery will constantly be depleted over time and may run out completely on a long run leaving you with no lights at all! My other articles on Improving your headlamps and Installing LED lighting give some suggestions of what you can do if this is the case.
The video clip below shows how the ammeter needle moves back and forth with the engine running on my Matchless G3LS in order to give you an idea of what range you might expect to see. This bike has a short Lucas E3NL dynamo fitted giving around 5 Amps maximum output and the ammeter has a range of -8 to +8 Amps. Apologies that the clip is rather wobbly, but I didn’t have enough hands to keep the camera steady whilst operating the lighting switch and twisting the throttle! Hopefully it gives a good idea of what sort of readings you should expect to see on your bike though.
If you do find that you have a faulty ammeter then replacement gauges are readily available from all the usual classic motorcycle suppliers (some are listed here). My guide Replacing the ammeter will also take you through the process of removing the old ammeter, installing the new one, checking the connections and making sure that it is giving a sensible readout.
Step 3 – Testing the battery
A simple check to verify that your battery is in good condition is to measure its open circuit-voltage using a multimeter. The open-circuit voltage is the voltage measured across the terminals when there is no load connected to the battery, so it is first necessary to disconnect it from the bike by removing the fuse. If you don’t have a fuse fitted then have a read of this article, Installing a fuse, to see why you really should do, but for the meantime just disconnect the live battery terminal. Whilst this isn’t necessarily a foolproof test, it should give a good idea as to whether the battery is any good or whether it may not be taking or holding a charge.
Set your multimeter on the voltage measuring mode (10 or 20 volt range) and connect it across the two terminals of the battery as shown in the photograph on the right. Lead-acid batteries are made up from a series of individual ‘cells’ which each have an open-circuit voltage of about 2.1 volts. A 6 volt battery has three such cells connected together in series so that the total open-circuit of the battery is actually around 3 x 2.1v = 6.3 volts when the battery is fully charged. Similarly 12 volt batteries have 6 of these cells connected in series giving 6 x 2.1v = 12.6v output when the battery is fully charged.
As batteries become discharged so their open-circuit voltage slowly falls such that we can use the voltage we measure in order to estimate how well charged a given battery is. For a 6 volt battery, open-circuit readings of between around 6.1 to 6.2 volts indicates that the battery is in good condition but just slightly discharged. Anything below 6 volts indicates a likely problem; either the battery is very flat (maybe the headlights have been left on) or is faulty. In this case it is necessary to remove it from the bike and connect it to a mains charger to see whether it can be recovered. Many high-end battery chargers are digitally controlled and have special functions which apply specific charging voltages and currents to a tired battery in an attempt to bring new life back into it. So put the battery on charge overnight and see whether it takes and hold a charge. If not, you know the battery is at fault and should be replaced, but if it takes charge from the mains charger but not from the bike then you know there is something wrong with the dynamo charging system.
If the battery is a unsealed lead-acid type (i.e. one where you can remove the caps on top to reveal the plates inside) it will also be worth checking the fluid levels inside the cells to make sure these have not run low. Unscrew the caps and have a look inside; the water level should be visible and just covering the plates inside the battery. Top up any cells that are low using distilled water (not tap water) then try charging the battery. Of course many modern batteries are now sealed units so it is not necessary to check or top-up the fluid inside.
Step 5 – Checking the regulator
Now this section isn’t going to be one of the most comprehensive I’m afraid for one very good reason. When I first bought my bike back in India a few years back now it had an intermittent charging fault whereby it would sometime work, sometimes not and sometimes just flicker in the very best Lucas tradition. After some investigation I found that this was due to some of the soldered connections in the Lucas MCR2 have melted and broken lose. At the time in India I didn’t have access to appropriate spares or tools to repair my regulator, so instead I opted to replace the iriginal mechanical voltage regulator with a modern sold-state electronic version which I had shipped across from the UK. This also allowed me to convert my bike to running 12 volt electronics as described in my articles Replacing the regulator unit and Converting to 12v; you might find these pages useful if you find yourself in a similar situation with a fault regulator!
The Lucas MCR2 voltage regulator is an electro-mechanical device which controls the connection between the dynamo and battery. What this means is that it uses good old-fashioned coils, springs and moving contacts to automatically open and close switches as the output from the dynamo and state-of-charge of the battery vary. So unlike with a modern solid-state device we can actually see the device operating and the contacts moving back and forth. This allows us to get an indication at least of whether the regulator is functioning correctly.
Start by removing the cover from the regulator unit and peering inside. You should see a couple of coils of wire and some spring-loaded contacts which are pulled back and forth by the magnetic field generated by the coils. Start up the engine and observe these contacts as you gradually increase and decrease the engine speed with the bike on the stand (it would probably be useful to have a helper at this point!). As I explained previously, there should be no need to rev the engine highly for something to happen; a fast tick-over should me more than enough.
Rather than trying to explain what you should see, I have instead made the following short video of my own Lucas regulator for you to have a look at. You can clearly see the two coils of reddy-coloured wire with what looks like a silver coin on the end (right side) as each one. As I vary the engine speed, you can hopefully see that the coins move back and forth towards the coils making and braking the connection between the battery and the dynamo. Well you can at least see the bottom one moving. As I explained above, my regulator was (and still is!) broken as one of the connections to the coils had melted and come lose. This is why I suspect the top contacts are not operating and also why the coils themselves are also vibrating back and forth. Hopefully your regulator will look to be operating more effectively than mine though, although my mechanical regulator is only there for show now as it has been replaced with a modern electronic regulator hidden round the back.
Step 5 – Testing the dynamo’s voltage output
This is the bit where things are going to get a little more technical, therefore I have written a separate article on Testing a Lucas dynamo which can be found here.
This separate guide allows me to go into a lot more details on how to test the dynamo’s output voltage including how to connect up the multimeter, what readings you should hopefully expect and what other readings may be an indication of. As an overview though, it is possible to test most aspects of the dynamo without needing to remove it from the bike. The first thing is to disconnect it from the bike electronically by removing the wires which go to the regulator, and we then connect the two coils inside the dynamo together by inserting a link wire between the two terminals. What this allows us to do is to measure the number of volts generated at different engine speeds with the dynamo disconnected from the regulator, battery and the rest of the bikes electrical systems.
The output voltages measured can be used to verify that the dynamo is most probably, although not definitely, in good working condition. Alternatively they may point to the possible source of a problem with the unit needing repolarising (see the article Repolarising a Lucas dynamo for more information), servicing or replacement windings.
Step 6 – Checking the wiring
If all of the above checks and tests have drawn a blank and you still have an elusive problem with the dynamo charging system, then the chances are that there is a problem with the wiring or connections somewhere along the line. Wiring problems are amongst the easiest to fix, once you have found where the actual problem lies of course! And that’s the hard bit as wires and soldered connections may look physically fine, but may be broken or corroded electrically speaking so that current cannot flow. Intermittent faults that come and go as you ride along, day to day, are a good indication of some faulty wiring or a dodgy connection somewhere.
The first thing to do is to carefully clean and examine all of the connections between the dynamo, regulator, ammeter and battery. Rather than running two lengths of cable for each electrical component, one there one back, the bike’s electrical system use the metal engine and frame assembly as the earth. This reduces the amount of wiring substantially, but it does make it critical to ensure that all of the earth points have a good clean connection to bare metal somewhere on the frame, gearbox or engine. This can be especially tricky if the frame has been powder-coated as the plastic coating is an insulator and stops the current flowing from a connection to the metal of the frame underneath. Dirty, rusty or oily connections will have a similar effect and so it is important that they are all clean and shiny (rusty terminals can be cleaned up with some fine sandpaper or wire brush).
Soldered connections can become ‘dry’ over time which means that whilst they might remain physically solid, the electrical connection through the soldered joint has broken down so that current cannot flow. You can sometimes spot a dry joint from its colour and appearance as they are generally dark and dull, whereas a new solder joint is bright and shiny. Such connections can easily be renewed using a soldering iron and some fresh solder; soldering flux or paste will also help the solder to flow better if it is old or grimy, although you should attempt to clean the joint as much as possible before starting.
It is not uncommon for wires to break or corrode mid-way along their length. The outer plastic insulation may appear fine, but the wire running inside may be broken, hidden away from view. It is therefore very hard to spot this type of electrical fault by eye, although signs of damage to the outer insulation can sometimes be a good indicator of the quality of the wiring inside. Such faults are most common where this is physical stress put on the wires such as where they flex between the headlamp and frame, or are squashed-up next to connectors. This is a problem I experienced myself on my first motorbike, a Honda NSR125, many years back. It had an intermittent charging fault which I just couldn’t track down until one day I wobbled one of the wires in a multi-way connector back and forth and found that this made the lights flicker. After stripping back the wires insulation I found a section an inch or two from the terminal which had pretty much corroded away to just white dust. Like I said at the start of this section, it was a very easy fix once I had found it!
The best way to go about checking the wiring in a more logical way is to disconnect each length of wiring one by one and use a multimeter to check the continuity along its length. This can be done by setting the multimeter to measure resistance and connecting it across the length of the wire (note that the wire must be fully disconnected from the bike both ends, otherwise you won’t be measuring just the wires resistance). The resistance should be very low, almost zero and certainly not more than a few Ohms. If it is high or if the reading changes fluctuate as you move flex or move the wiring harness, then you may well have found a problem. But the chances are that if one wire is faulty then others are too, so it might be time to replace the whole wiring harness.
Hopefully this overview guide has provided a good overview of the things to consider and check if you’re experiencing problems with the electrical systems on your classic motorbike. Electrical gremlins are notoriously hard to track down so this may require some patience, but the key is to be logical and work through things in a sensible order, eliminating possibilities one by one.
Please also have a read of my other articles and guides regarding the Lucas electrical system that I have linked throughout this page and which are also shown in the sidebar down the right of this page. If you have any thoughts, comments or suggestions then I would love to hear from you so do please make good use of the comments box below.[sc:disclaimer]
Hi, Having read lots of great info I still have not answered my ‘opportunity’
Having adopted a bike from my father, BSA A7 the battery has never charged. I have had the Dynamo re-built and want to check that I connect it the right way into the bike. I can log which is positive and negative output from the Dynamo, I can see ‘F’ and ‘D’ on the regulator but on the Dynamo. This bike is Positive Earth, should F or D be Positive or Negative output from he Dynamo ?
I Love your ammeter video, hence I will know if I got it right, but do not want to damage anything.
Hi Ian. Have a read through the article linked below, this should hopefully help you to identify the correct connections on the dynamo (in particular section 8): https://matchlessclueless.com/electrical/lucas/testing-lucas-dynamo/
The ‘F’ and ‘D’ terminals are the Field winding and Dynamo winding connections respectively; not positive and earth as such. You can polarise the dynamo either way depending upon how your bike is wired (i.e. whether it is positive or negative earth). More details here: https://matchlessclueless.com/electrical/lucas/repolarising-lucas-dynamo/
Hope this helps. Regards, James
Hi. Running several old bikes I have found it useful to run a dedicated earth wire to my head and tail lights, my 11.50 Brough only has a 40w lucas dynamo
but by using thicker wires and L.E.D tail and clock lights I have a really good headlight even though i’m still on 6v, another useful thing I found on a bike I bought from a engineer friend was,he had tapped the inner end of one of the 3/8 headlamp bolts 5mm and used this to provide a secure earth connector.
Hi Chris, some useful tips, thanks. The dedicated earth wire from the headlight is definitely a good idea, especially at 6v, as otherwise the electricity has to find a return path through the rusty and oily head bearings. Interesting idea with the headlight mounting bolts, but I think your dedicated earth wire is a better idea to bypass the head bearings. Cheers, James 🙂
The 5mm bolt is used as a termination for the earth wire from the main chassis earth terminal to give an earth connector in the headlamp so it still bypasses the bearings etc.As it’s a vincent with twin 5″ clocks with two lights each,a side light and the main headlamp bulb it would be a rats nest of wires in the headlamp if say a connector strip was used to couple the earths.
Ah, that makes more sense. Nice idea. 🙂
Whenever possible I run a dedicated earth wire to the tail light as well, I usually use a piece of 3 core black flex, pond pump cable is very good, especially as it’s made for use underwater and is very flexible and is “rubbery” not “plasticy”.
I have just finished putting a coat of lacquer on the outer case of the Lucas E3 dynamo to help prevent further corrosion, and it has now crossed my mind that it might not have been a good idea.
So the question is this: Does the body of the dynamo need to be earthed to the frame of the bike by means of the clamp? I have never seen any mention of this being a possible source of problems, so I am hoping that there is no need for it, but would like to be certain.
Hi Temp. Good question! I was wondering the same as the battery clamp of the G3L I’m in the process of rebuilding has been heavily painted by the previous owner. The ground connection has to be via the clamp through the paint on both it and the dynamo, there’s no other way. But whether a layer of paint is too much and will insulate the dynamo, well I’m not yet sure. I guess it depends a lot on what type of paint has been used.
As you say, no one ever seems to mention this as a source of problems, but you would have thought that it could be. So do let us know how you get on and what you find out!!!
That was a quick reply! This is how I have got on:-
I went all over the generator with a multi tester, and the metal casing is definitely earthed as per the wiring diagram. So is the armature. On my bike, the genny is mounted by being clamped hard up against the engine crankcase, so if the bare metal is clean there can be no problems.
However, now I have lacquered it, I should in theory at least have insulated it from the rest of the bike. Anyway, I replaced the genny and clamped it down hard. This was enough to make a contact to the rest of the bike, so all was well – I hope! The lacquer was a good quality model aircraft fuel proofer, so it should not have permitted contact, and I was all prepared to scrape a bit of the lacquer and put a piece of stranded wire on the bare patch to make contact with the crankcase, but it has not been necessary.
As a further possibility, now I have put back the chain drive to the genny, there might be a contact of a sort through the armature, although how reliable that would be is anybody’s guess. But it is belt and braces!
It has not run yet. I am a bit concerned as well because the bike has an electronic voltage controller (AO services) and the instructions say ‘use a good battery of not less than 5 ah.’ I have been a misguided skinflint and bought a much smaller capacity battery, on the grounds that all I wanted it for was to power the stop light, and I am unsure whether this will throw the electronics out, or possible cook the battery.
We shall see. Cheers, Temp.
Great, not sure how it is earthing through the lacquer, but so long as it does then that’s fine! It might be checking that the resistance is low though, just to be sure you’re not wasting any power.
Are you running with 6 or 12 volt electronics? 5Ah is pretty small already if you’re at 6v, but fine at 12v (current halves as voltage doubles). Exactly how small is the battery you’ve bought? It would probably be wise to stick with their recommendations.
Good luck! James 🙂
Yes, I was surprised to find that the dynamo was earthed to the engine, so I took it off and put it back again several times, and each time the earth was OK (.1 ohm on the 20 ohm setting of the multi meter).
I have now run the engine, and the charging system shows the right figures on the headlamp ammeter, so I have to assume that everything is working.
This is a 6 volt system, and the battery is 2.5 ah. If it does get cooked by the charging system being too enthusiastic, there is nothing much at stake.
I still can’t get it on the road. As you will know, I had to take the primary chaincase off to get the sprocket off the dynamo, and now I have put it back it looks as if it might be leaking. The book says ‘half a pint of oil,’ but this is in fact far too much, and there is now oil all over the floor. It was still dripping three days later, but whether this is just overflow, or a failure of the chaincase seal, I’m not sure. There was so much oil on the floor that I thought that the chaincase would be empty, but when I looked through the filler hole the oil level was just right. So I’ll hold on for a few days to see if it stays at that level.
One of the remaining troubles is that the footrest pegs are very tight on the square bar through the frame, and to get the chaincase off again to improve the seal I have to take the left footrest off. I am very reluctant to fight this battle again unless I really have to. I’m still hoping that the new improved chaincase seal will have done the job.
Why am I bothering? I have a perfectly good Honda 250 in daily use!
In response to the query by Temp, ‘how does the dynamo earth? ‘ One way is through the drive chain and sprocket-the armature shaft-through the ball bearings to the dynamo casing.
(The magneto is different as the ball race outer track is insulated.)
MY QUESTION! What is the resistance of the ‘warning light’also the voltage of the Warning light globe. To me this is critical as the circuit to the field windings supply current to the dynamo fields on start-up.