Introduction to rewiring the dynamo and regulator
The Lucas dynamo-regulator charging system; simple and yet also strangely complicated for anyone more familiar with modern vehicles electrics (or not familiar with vehicle electrics at all!). The main point of this article is to provide a simple and easy to follow wiring diagram should you need to repair or rewire your classic motorbike. I am basing this article on the Lucas E3NL dynamo and MCR2 voltage regulator units fitted to my 1951 Matchless G3LS, but the same holds true for the majority of makes and models from the same era.
This article covers the following topics :
- The dynamo connections
- The regulator connections
- The ammeter connections
- The battery connections
- Conclusions and your comments
The dynamo connections
There are only two connections here (well actually 3 if you count the earth, but that is through the dynamo’s metal case which is bolted to the engine so we don’t need to worry about that!). The two terminals are for the Field Coil (F) and Dynamo Coil (D). Both of these terminals connect directly to the voltage regulator unit, as described in the next paragraph.
The regulator connections
This is slightly more complicated as there are four terminals to connect on the voltage regulator. I will assume that you are still using the original Lucas unit, but even if you have a modern solid-state regulating device fitted, the connections should still be the same. The four terminals are for the Field Coil (F) of the dynamo, the Ammeter (A), the Dynamo Coil (D), and Earth (E). They form the nice little mnemonic “F-A-D-E” which is easy enough to remember.
So we have two terminals that are connected directly to the dynamo – F on the regulator goes to F on the dynamo and similarly D on the regulator goes to D on the dynamo). The Earth terminal (E) is connected to a good solid earth connection on the bikes frame, somewhere under the seat. This forms the return circuits to both the dynamo and the battery. The ammeter connection (A) goes directly to the ammeter as you would expect, as described in the next paragraph.
The ammeter connections
There are only two connections on the ammeter and the worst that will happen if you connect them the wrong way round is that you ammeter will indicate a discharge (negative reading) when it should be showing a charge (positive reading). On my bike, when looking at the ammeter from it’s top readout side, the left connection goes to the battery and the right connection to the regulator. This might be different on your bike, so try it and if the ammeter reading moves to the right (showing a charge) when you put the headlights on without the engine running, you’ll just need to swap them over.
The battery connections
Only two connections on the battery, but it’s really important to get them the correct way round depending upon whether your bike is to be wired negative or positive earth. Positive earth was standard up until around the late 1950’s when later bikes swapped to using negative earth instead. That’s not to say how your bike is setup now though as this can easily be changed! A positive earth bike has the positive terminal of the battery connected to the frame, and for a negative earth bike it’s the negative battery terminal.
The other battery terminal is the ‘live’ connection (i.e. the one in which you should really fit a fuse) and this is connected directly to the ammeter as described in the previous paragraph. The connection must be on the opposite side of the ammeter to the regulator and light (etc) connections. (The one exception to this is the horn which is wired directly from the battery side of the ammeter).
So as you can hopefully now see, the wiring connections for the Lucas dynamo charging system and really quite simple. It is also helpful to have a copy of the original manufacturers wiring diagram to hand when working on your bike’s electrics and this can usually be found in the owners manual. A selection of Matchless and AJS owners manuals can be found in the Free downloads section of this website, and there are also a selection of documents relating to the Lucas electrical system and components that you might found useful. Be aware that the colour coding of wirings may not be the same on your bike as they are in the manuals though, although there aren’t exactly too many wires to get confused between![sc:disclaimer]
Spotted a good idea on a dynamo recently,instead of push-in connectors in the bakelite end cap the holes were threaded and the wires were fitted with ring ends and retained with pan head screws.when i got home tried mine and found the sockets are the right size to tap 6mm. This gives a perfect connection unlike the original which is hit and miss.
Useful tip and interesting idea, thanks Chris. Do you have a photo available at all we could post up? Thanks, James
To all that were helped by Chris. He passed on 4 July 2019 and a GoFundMe site has been setup for him. Please help Julie if you can.
I have a picture but don’t know how to put it on, will seek advice.
Your information on dynamos and 1950s 6v charging systems is really helpful to me, at this moment in time.
Im having trouble with the charging rate, on my 55 ajs 350 at the moment,i tested the dynamo under load with a headlamp bulb of 45w/12v, and its full brightness on medium to fast idle. But when connected to the wiring, its an unstable voltage, i think i need to take a closer look at the wiring & connections, the cable looks a bit thin for 6 volts & lighting switch looks like a cheap pattern, maybe these are all part of the problem !
I’ll let you know how i got on with it.
MANY THANKS. DEREK
Hi Derek. Good luck with your fault finding and glad you have found some useful info on the site. James 🙂
Does anybody know when the change from rubber wiring insulation to plastic took place?
Thanks in anticipation. Dave.
Hi Dave. This is all the info I have taken from a 1966 Lucas technical response:
“Pre-war insulating material was usually basically wood, rubber, ebonite, glass or later bakelite….. Since the war, plastic insulation has been used extensively.”
So I guess the change took place just after the war (mid-1940s).
Found article very useful Thanks
I’m currently converting my 1954 Matchless Twin to 12v, just wondering about the connectors, with rubber insulating sleeve. Do these cause unecessary resistance? Would you recommend changing them to modern equivalents?
Great advice concerning converting to 12v!
I have had the dynamo completely overhauled, new armature etc. I have checked the hell out of the wireing looking for a possible short. What is happening is that the positive wire from the battery which runs to earth gets hot very quickly and will no doubt melt if the fuse was not in place. in your opinion, have I missed something obvious, Safe riding, John.
Hi, I have lost the details of the wiring on my E3L dynamo, there are 4 wires the 2 which come off the brushes and 2 more which look like they are earth wires, these are the wires im not sure about.
I have fitted a 6v wassell regulator and have lost instuctions how to change dynamos field wires for a negative earth, does one field wire go to earth or does it go to d.
Hi Geoff. I have just emailed you electronic copies of the Wassell regs, just make sure you read the one of positive or negative earth regs as appropriate as the connections will be different. Regards, James
Hi james Thanks for that info just got it all connected up went to test it but no petrol in, so will tell you how i get on later,thanks a lot.
Hi There, thanks heaps for all your help this past year, I decided to bite the bullet and get my dynamo restored, its just like new now, and I took your advice on the after market state of the art control box`s and have installed one, all is now like bliss in the electrical department, of course, I am touching wood as I say this. I hope you have a great Christmas and a wonderful New year, thanks again for a great web site, Safe Riding, John.
Cheers John and thanks for your kind words. Wishing you a great Christmas and many happy miles of safe riding in the new year. James 🙂
[email protected] excellent article. Thank you it helped me to wire in an ammeter. Can you tell me if getting the “F” and “D” connections reversed on the dynamo will cause a problem? How can I determine which way is correct. The markings on the dynamo are long gone, and I have removed the third brush regulator and am using a Wassell solid state regulator.