Magneto winding resistance question

[JimF]

I have a 250 with the AC (alternating current) electrical system.

It has three wires coming off the Dynamo/magneto/whatever. Two wires are at opposite ends of the same coil.

The third wire is at one end of a second coil, The other end of the second coil is attached to the engine/frame of the bike.

To make a long story short my wires were shot and I had a couple of firends over one evening and one of them proposed we change out the wires. So in a flurry of activity we took the engine cover off, the clutch out, and the magneto.

I quickly changed the wires and the whole thing went back in just as fast.

Today I measured the reistance of the wires and as expected the two wires that go across the same coil have continutity to each other and none to ground.

Here now is where I am second guessing myself..

I measure the resistance of the third wire and as expected it does not have any continuity to the other two wires. That's good.

I then measured its resistance to the engine case and I expected to see some resistance to ground, that is I expected to measure the DC resistance of the coil winding. Arguably it is a short circuit (straight piece of wire) but it is a long wire. I know in the ignition coil you generally see several ohms owning the length of the coil winding and diameter of the wire (very small magnet wire.)

I see no resistance whatsoever on a digital multimenter. It's zero ohms.

So either I have a pinched wire or the DC resistance of the coil winding is about zero ohms which I suppose it could be given that it's a larger diameter (wire guage) than the ignition coil windings and probably far shoter too.

Anyway, I guess I am looking for assurances that it's probably OK to read zero ohms across that coil.

I could try an old analog meter if the nature of the beast might reveal something about whether there is a large inductor (the coil) in the path.

Has anyone ever measured this resistance? Or does anyone have a magneto sitting in a drawer that they could measre?

In retrospect I should have checked for a shorts when the whole magneto thing was on the work table after soldering the new wires on, and then I should have measured the resistance of both coils so as to know what to expect to see later after it was remounted.


Thanks,

Jim

[Jordan]

I measured the resistance of a spare Ducati magneto coil. I'm not sure what model it is from, or whether it's in serviceable condition. It is just on 1 ohm.
The wire diameter is 1mm.

Jordan

[wcorey]

I've recently set up my 'magneto' alt from the R/T to spin it up on the test rig, had to use 4 different meters before finding one that would read more than 00.0. I have a brand new aftermarket replacement coil and that one reads the same.
They do seem to be very low resistance but also seem to work regardless...

Bill

[JimF]

Thanks guys...

I will proceed then as though all is well. Only by firing it up now will I know for sure.

I feel much better knowing that the resistance is very low, almost immeasurable.

Jim

[DewCatTea-Bob]

By: Jim...
" It has three wires coming off the Dynamo/magneto/whatever. "

____ Those three wires are connected to the 'alternator stator-coils' (to use American-terms,, "Dynamo" is the Italian-term, and "magneto" has become 'Ducati-slang' for referring to the alternator on the non-battery type Duke-models).


" Two wires are at opposite ends of the same coil. "

____ Those two wires should be a Yellow (intended for the ign.coil), and a White (intended for the [later concocted] brake-light circuit).


" The third wire is at one end of a second coil, The other end of the second coil is attached to the engine/frame of the bike. "

____ That lone 3rd.wire (connected to the main-lighting power-coil) is the Red-wire, and it's opposite coil-end is normally/originally (internally)- 'grounded' to the alt.stator-plate.
__ As I've made point-of a fair number of times before, when replacing the original alt.stator/cable wiring, it's advantageous to avoid such (internal type) alt.stator ground-connections, and instead add extra alt.stator lead-wiring, (so as to move any intended ground-connections to an external location, [in this type case] ).


" I then measured its resistance to the engine case and I expected to see some resistance to ground, that is I expected to measure the DC resistance of the coil winding.
I see no resistance whatsoever on a digital multimenter. It's zero ohms. "

____ Whenever doing such ohm-reading, ya need to make sure that your ohm-meter is properly set to read resistances which are less than 1-ohm !


" Anyway, I guess I am looking for assurances that it's probably OK to read zero ohms across that coil. "

____ Not actually, (if you're using the proper meter-scale) !


" Has anyone ever measured this resistance? "

____ It seems a pity Jim,, that either you haven't kept-up with reading our related threads on the w.site, or your memory is lacking. _ As of course this stuff has already been covered (at least once before) ! ...
__ As I recall, the main-lighting alt.power-coil resistance has been read as being .4 (+/- .1) ohms, and the reading for the alt.power-coil of the ign.circuit is .7 (+/- .1) ohms.
____ I believe I recall that Capt.Paul last provided such resistance-readings from a good/usable magneto-ign.setup type of Ducati alt.stator (which he had a few models of).
__ I haven't seen any posts by him for a (uncharacteristically) long time. _ So I'm hoping he's still with us !


Dukaddy-DUKEs,
-Bob