Wednesday, 3 October 2012

Oily Dynamo

We need to get moving on the engine and we don't have the parts to finish the refurbishment of the dynamo, so we will put the on the list for Drags & move on. Before we reassemble though it makes sense to check everything out.
So, the service manual tells us that the field coil resistance, across both field coils, is in the range 2.6 - 2.8 Ohms:
So we set the meter to the 200 Ohm range and short the test cables together, to reveal a resistance in the measuring equipment of 1.3 Ohms.

Now let's connect the field coils into the circuit. This reveals a total resistance of 4.0 Ohms, giving a value of 2.7 Ohms for the field coils alone. Spot on, correct to specification!

Next test - field coils to earth. Should be open circuit, hopefully...

Hey presto, no reading on the meter. Field coils good! Next job, clean the armature & the commutator. Easily done with some solvent, mildly abrasive paper and a razor saw. We'll not attempt to test this since the brushes are in very poor shape and as we said, we don't have the parts to rebuild this beast yet. Continuity tests & earth resistance tests look good though.

Here is a view of the brush assembly about to go back in. Clean enough for now, but not clean enough to use. We will finish that when we have some new oil seals & an idea of how this unit left the factory. Black gloss paint maybe? There is no evidence of any paint except for on the end cap:

And here is the drive end. I've re-tapped the hole for the distributor retaining bolt. Fortunately, whoever tried to put a BSF screw in there didn't try very hard and the thread recut nicely. Spare 1/4" BSW screw inserted for now, to be replaced with a stainless one when I can find one. Also, an appropriately sized (but incorrect thread) nut serves to retain the sprocket. Losing it is not an option!

Lastly, tired & oily brushes. Very worn, mangled insulating sleeves, fit for patterns only.


  1. Very good explanation thank you

    1. I'm glad you found it useful Keith.