Introduction to replacing a Lucas ammeter
The only real clue to how well the charging system is working on an old Matchless bike (other than the judging by the brightness, or lack therefore, of the headlights at night!) is that little ammeter neatly tucked away in the headlamp cowl. So it’s quite useful to have one that works and gives a reasonably accurate idea of how much charge is flowing into or out of the battery.
Once you’re used to normal behaviour of your motorbike’s charging system, you can actually gauge quite a bit of information (if you’ll forgive the pun) from a quick peer at the dial. In most situations, the ammeter needle should hopefully stay fairly close to the middle. This indicates that the electrical system is balanced with what power is going out (mainly through the lights) is being replenished by the dynamo in order to keep the battery topped up.
A large positive charge into the battery whilst riding along probably means that there is something wrong with the battery. It might just be flat and hence the positive reading on the ammeter shows it’s being charged, but if this persists then the battery might not being taking or holding charge.
When I bought my bike back in India, it had a local ‘Minda’ brand reproduction ammeter fitted. This worked in a fashion, but was not very accurate and prone to jumping around a bit – a problem which didn’t help when trying to diagnose some charging problems I was having at the time!
And so I decided to fit an original Lucas ammeter I bought from back in the UK instead. This guide shows how I went about swapping the old Minda ammeter for a shiny new Lucas type 36084 (+/-8 Amp) unit.
This guide covers the following topics:
- Introduction to replacing a Lucas ammeter
- Removing the old ammeter
- Fitting the new ammeter
- Testing the new ammeter
- Conclusions and your comments
Removing the old ammeter
Before doing anything else, the first thing to do is to disconnect the battery. The terminals on the back of the ammeter are live and the headlamp shell is earthed, so one false move and you have a short circuit leading to a few sparks at best, or a melted wiring loom at best. You can disconnect the battery by removing one or other of the wires, or else by pulling out the main fuse if you have one fitted (if you don’t have one fitted, do this now before even considering any other electrical work!).
Once the battery is isolated, you then need to remove the glass and reflector from the from of the headlamp unit in order to get inside. Loosen the screw next to the outer retaining rim at the top of the unit (you don’t need to remove it completely) and the glass should lean forward away from the headlamp cowl.
When it’s leaning outwards enough, you should easily be able to unhook the tab which keeps the bottom of the glass in place. Remove the bulb holder (or disconnect the headlamp bulb wires if possible) so that you can completely remove the headlamp glass and reflector unit from the bike and put it somewhere safe. You then have free access to the nuts and wiring on the back of the ammeter mounting panel.
I found it easier to disconnect the wiring on the back of the meter first before pushing it through and out as there is probably not enough slack in the various wires to do otherwise. Make sure you keep the two bundles of wires separate so you put them back on the new ammeter the correct way round.
You can see the Minda ammeter shown in the photos actually has a bulb in the back to illuminate the dial at night. Whilst this is quite useful for seeing the charging / discharging status at night, the Lucas units don’t have such a back-light bulb and so these extra connections also needed to be removed.
Fitting the new ammeter
With the old ammeter removed, the new unit should just slot straight into the hole in its place. However, when I was replacing the Minda ammeter unit on my Matchless I found that this was slightly bigger diameter than the original Lucas unit (only a millimetre or two) and that the hole in the headlamp panel had been opened out slightly to accommodate it. This meant that the new Lucas ammeter was too lose a fit and so it was necessary to wrap a couple of layers of insulation tape around the meter before installation to ensure a tight fit. The ammeter is only held in place by this tight fit alone and so it’s important to get it nice and tight to prevent it popping out or rotating as you ride along.
Reconnect the various wires to the two terminals on the back of the meter making sure that you get them the right way round and in the right bundles. A quick check that you haven’t missed any wires off and that the terminal nuts are nice and tight (you don’t want them working lose!) then refit the headlamp lens and reconnect the battery.
Testing the new ammeter
The first test is to turn on the headlight (no need to start the bike though). Does the headlamp light up? – great! The ammeter should be showing a significant discharge to the left of the dial. The exact reading will depend upon whether your bike is running with 6 or 12 volt electrics and what wattage headlight bulb you have fitted. Divide the wattage of the headlamp bulb (plus an extra 5 Watts or so for the tail lamp) by the voltage of the electrical system on the bike.
On my G3LS I have a 35 Watt headlamp bulb fitted with a 5 Watt tail-light bulb, giving a total of 40 Watts being used up. The bike’s electrics have been converted to 12 volts (see myguide for more info) so the ammeter should show 40/12 = 3.3 Amps. This is just under half of the full-scale reading on a 8 Amp meter. On a 6 volt bike with a 25 Watt bulb (plus 5W tail-light), the current draw should show approximately 30/6 = 5 Amps. Don’t expect the meter to be exact, but it should be there or thereabouts.
If the needle is moving to the right with the lights on, this indicates that you probably have one or more of the wires on the back of the ammeter the wring way round. Current is flowing through the meter backwards, hence it is giving a positive reading when it should be showing a negative one. It’s simple enough to swap them over though.
With the lights all off the meter should return to its zero position, although they do seem to sometimes prefer to settle just either side of the centre. Next start the engine and increase the revs to a fast tickover. The needle should move to the right indicating that charge is going into the battery. How far it goes to the right will depend upon which dynamo your bike has fitted and the state of charge of the battery.
If the battery is fully charged, the ammeter might only read around +0.5 Amps – i.e. the needle is just to the right of centre. However if the battery is flat, it will accept a much bigger current from the dynamo. We can work out the maximum charging current to expect in a similar way that we did for the discharge current due to teh headlights. Lucas dynamos come in either short 48 Watt (e.g. type E3N) or long 60 Watt (e.g. type E3L) varieties, although these figures are really more a guide than a true maximum output power. But that’s fine for our current purposes.
Again you also need to take into account the voltage that your bikes electrics are running at. For my 1951 Matchless with a Lucas E3NL dynamo running at 12 volts, the maximum charging current I would expect would be 48/12 = 4 Amps. If my bike was still running 6 volt electrics this would be doubled to 8 Amps (48/6 = 8). For a bike with a long 60 Watt dynamo running at 6 volts it could be as much as 60/6 = 10 Amps!
The final test is to see what happened when the engine is running and the lights are turned on. With a balanced charging system, the current drain by the bulbs should be about matched by the charging current from the dynamo and so the needle should be approximately in the middle (around 0 Amps) when you’re riding along. Stopped in traffic the meter will probably show a discharge (needle moves to the left) as the dynamo will not be spinning fast enough to generate enough current to balance the load. But at normal cruising speeds the needle should be near zero, or maybe even slightly to the right showing a slight trickle charge going into the battery.
Conclusions
And that’s about all there is to it! If you have replaced the ammeter and the charging system still doesn’t seem to be functioning as it should, then some further diagnostics are in order. The basic checks are fairly straight-forward enough that anyone with a basic understanding of electrics (i.e. able to use a multimeter!) can do them. I’ve written a ‘how to’ article explaining thewhich has lots of photos to guide you through step by step. I hope you find it useful.
Finally, just to draw your attention to theof this website and reiterate that you are entirely responsible for all damage you do to yourself or your bike. But you can also take full credit when you fix a problem yourself too!
If you have any comments, feedback or suggestions about this article I’d love to hear from you. Please feel free to leave me a comment at the bottom of the page.
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Hi there, Great website!
I’ve a question for you. Matchless 1956 G11 . E3L dynamo converted to 12 v (Field coil and armature wound for 12v), Pazon surefire ignition, brand new Battery 9ah AGM sealed. 45w headlight globe, DVR2 reg putting out 13.5v max.
My ammeter acts like a tacho needle when the motor is reved. The needle will go to 6-8 amps with no load, 5 amps with lights on at 50-60mph. The dynamo has spat the solder out of the commutator once and I’ve cooked two batteries. Brand new battery today and no difference so it isn’t a low or faulty battery causing this.
I’ve run out of ideas, suicide is the next option!
Mark
Hi Mark,
Hmmm, sounds like you’re having fun! Suicide seems a bit drastic, have you thought about a match in the tank instead? 🙂
But I’m sure it won’t come to that. My ammeter worked like a tacho too back in India; actually I used it as a sort of speedo before I got my chronometric refurbished. It seemed to always show a positive charge, but I later discovered that this was because the battery was dodgy (despite seeming to hold a good charge) hence the regulator (my DVR2) was always trying, but never succeeding, to fully charge it. A new battery solved the issue and now the ammeter needle sits about in the middle once the battery is topped up. But it sounds like you have already exhausted the battery idea on your bike. Have you tried fully charging the battery off the bike first to be 100% completely sure that it is good and fully charged before fitting it to the bike?
So it sounds like maybe a regulation issue doesn’t it? You should have lots of volts coming out of that long 60W dynamo if you have 12v coils fitted. One thing to bear in mind though is that the 12v coils have twice as many turns of wire as the original 6v ones, which also means that the wire has to be much thinner to fit in the same space. Therefore the maximum output current is limited before things start to get a bit warm inside the dynamo and melt. The original 6v coils are very giving as they have lots of spare current capacity, but 12v ones may not be and so you need to make sure that your electrics are sucking too much wattage from the dynamo for too long. Otherwise it could get messy and expensive!
The only things I could suggest therefore are to double check all of the wiring to make sure nothing is wrongly configured with the DVR2 regulator, and then if nothing is found send it back to Dynamo Regulators for testing. They are supposed to be very resilient devices to short circuits, over-loads, etc but maybe something has gone wrong internally? I’m assuming you have the correct polarity regulator for your bike (neg. vs pos. earth) and that the 6 or 12v selection wire is properly configured, right?
Good luck and do let us know what the problem turns out to be, when you eventually track it down.
Regards, James 🙂
Hello James, Thanks for the excellent website.
I’m having electrical problems with my ’59 G12. The ammeter works as it should for the headlights i.e. Negative with engine not running and about zero when riding. When I depress the rear brake peddle whilst riding the ammeter shows a huge positive charge at about 8 to 12 amps. I’ve replaced the battery but it’s still happening.
The bike is 6v, DVR2 reg. +ve earth. E3l dynamo.
Your advice would be greatly appreciated.
Many thanks, James Heffernan.
HiJames, I have exactly the same symptoms, my bike is a 52 AJS model 16 6V pos earth DVR2 reg.I have also noticed the same effect when the horn is sounded.
Can any experts advise?
Hi I have a triton motorcycle it has a headlight with the ammeter in it I decided to change it for new but forgot which way the wires go there’s 3 wires one comes from the light on and off switch does that one go to the left hand terminal of the ammeter as you looking at the lamp and the other 2 which are brown and blue and brown go to the right side thank you
Hi,
I reside in South Africa and have recently rebuilt a 1953 BSA which uses a Lucas magdyno.
Your articles are very informative but all your references are “whilst” the bike is running.
My problem is that I cannot produce any spark what so ever when attempting to use the kick starter.
Any suggestions of where to start to look for the problem.
Additional facts
* The bike has not run for the past 40 years
* Stripped the magdyno and replaces /lubed all bearings and checked the physical condition and circuitry seems to be complete and intact
* the ammeter may be faulty the outer casing seems to be squashed and straightened
* new Lucas wiring harness has been fitted
* no battery has been installed