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Checking a Lucas E3H dynamo off the 3HW


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Any Lucas E3H dynamo experts here, I am trying to find out if my field coil or armature has gone open circuit?

The dynamo is partially dismantled and by way of a simple test I tried to see if there was continuity from the field coil cable that runs to the terminal in the end cap and the body of the dynamo, which there is. So my questions are, does this indicate failed field coil insulation, or if the test method is not valid, could some kind soul tell me how to test the field coil please.

I also tried testing the armature with a simple test that I found on the web of seeing if there is continuity between the copper segments and the steel body of the armature, which there is not, does this imply the the armature is Ok?

I was told by a local car mechanic that the dynamo was u/s, but he didn't do repairs and I am very reluctant to trust the unit to the 'gentle' care of either a courier or the postal service, If I can find what is duff, I will replace the parts myself.


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There should be continuity between the copper armature segments, with everything disconnect, ie the brushes lifted, there should not be continuity between the armature shaft and copper segments.  follow the field windings and see if there's continuity of the field coils,

Common failures are sticking brushes  i.e not making contact with the armature and loss of residual magnetism.

A mechanic who knows what they are doing should be a bit more precise than "it's U/S"



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Apart from the field coil still being within the dynamo body the rest is dismantled.  On the armature I have continuity between the segments themselves, but not between any segment and the armature body.  Turning to the field coil, the insulation wrapping looks good, there is continuity between both leads off the the coil.  I have just realised that my first test to see if the coil is shorting to the body was invalid, because I had not disconnect the other coil lead that is connected to the dynamo body at the point where the brush lead is also connect.  Having disconnected that lead there is no apparent short.

As the bike last ran in 1947, I wonder if the magnetism has faded?

My mechanic has  a good reputation for classic car charging systems, but was slightly unsure on classic bike, he did diagnose correctly that the magneto wasn't working, but we have a guy here on the Island that is very good with those, I just can't find a dynamo man


Edited by T8Hants
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Update, I have now checked the disconnected field coil leads.  Setting my multi meter to 200 ohms, I get a reading of 4.2 ohms, which I think is slightly high am I right in thinking this is not a good thing? I have also checked the stator, which I think should show a reading of 0.5 - 0.8 ohms, which is what I am seeing.


Edited by T8Hants
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