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!