Magdyno wiring

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Hello everyone! The restoration of my '49 ES2 is almost done (you can see and hear it here https://youtu.be/fIiFUx617TM) and while I'm waiting for the last bits to come back from painting it's electrics time. Three wires come out of the dynamo, a black one from the inside and two brown ones from the brushes, I don't know how they are supposed to go on the F and D poles on the front plate. Another issue is about the positive/negative earth: the wiring diagram that came with the brand new Lucas harness shows it's positive, some people here wrote it should be negative in that year, on the manuals I have there are both positive and negative diagrams... How should I check what my dynamo puts out to be sure I don't fry everything?

Dynamo Connections

There should be 4 wires: 2 field & 2 armature. One of the field wires and one of the armature (brush) wires are connected to an earth terminal on the dynamo chassis. The other field wire is connected to the F terminal and the other brush wire is connected to the D terminal.

Next you need to decide whether to go positive or negative earth. If you still have the old MCR electro-mechanical regulator you can choose either. Electronic regulators are supplied in positive earth or negative earth versions. If you have one of those check which version you have - that will determine how you should connect the battery.

Briefly connect your battery (with your choice of polarity) to the dynamo chassis and F connections. A few seconds is all that is needed to magnetise the field.

With the dynamo on the bench temporarily connect the F & D connections together and to the battery + (negative earth) or battery - (positive earth). Connect the other battery terminal to the dynamo chassis. The dynamo should rotate as a motor. Check the direction of rotation, it should be as marked on the dynamo body (assuming the dynamo is the correct one for your ES2). If it rotates in the opposite direction to the marking you should swap the 2 field wires inside the dynamo, then recheck the direction of rotation.

If you want to avoid frying anything make sure you have a fuse in the battery connection to protect against short circuits. I use 20A in my bikes.

Ian McD

PS

Lovely bike, and what a workshop!

Nearly right.

The majority of what you write Ian is correct. One point I would have said is that the dynamo should have just 2 'connections' coming from it ie F and D, the actual 2 wires from the Field and 2 wires from the brushes should be inside the dynamo. The polarity question is as you say either, the owner chooses, but if the bike was Negative 'at birth' and you might sell in on eventually then keep it Negative, but the dynamo can supply either at the 'flash' of a wire.

Now direction arrow, this might have been put on the dynamo at the factory and is for the specific bike it was sold for 72 years ago! In the mean time it could easily have been changed. The rules are this- the direction of rotation is decided by the 'relationship' between the Field winding and the brushes, it is set up to charge for the specific bike (direction of rotation) it is going to fit. So ignore the arrow (unless it is correct for your machine). Further details of this are on aoservices.co.uk

And as has been said above, mechanical regulators can do either polarity. If you want an Electronic regulator then you have to decided Polarity as you buy said item (I know I sell lots and people forget to tell me!)

Thanks

Ian you are right about the four wires, i missed one! I have no porblems working on engines, gearboxes or frames but electrics are a mistery to me. When I got the bike a lot of parts were missing or incorrect and I had to buy a new cover for the dynamo and make a new housing for it (luckily as you can see I work with lathes) but I didn't find any instructions. I got a MCR replica for the looks and I'll see how bad or good it is, if it's bad I can always hide a modern one inside the toolbox

Dynamo Testing

Hi Vito, if the dynamo runs as a motor that will be a good sign but is not a guarantee that it is completely healthy. It sounds like your dynamo history is unknown. In that case it might be advisable to have it tested & serviced by a dynamo specialist. There is a list of UK specialists under the "Services" tab at the top of the page on this site. However, if you are outside the UK (especially since Brexit) it might be better to find a local expert. I have a Lucas dynamo document, I will get in touch via PM and send you a copy.

Cheers, Ian