MGB & GT Forum

I would like to test the coil on my + ground 67 B. The Moss video says to disconnect the wire to the points and attach a jumper to that terminal. Then, position the coil wire near ground and scrape the jumper on a good ground. A good coil will then spark. Which terminal (the + or the - )goes to the points? The white wire goes to the - terminal and the other to the +.
Gerald

Earth the point side (+ve) in your case. in truth it makes no difference which way the coil is connected AFAIK
You actually wrote that anyway "The Moss video says to disconnect the wire to the points and attach a jumper to that terminal"

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Bernie

"discere mutari est"

Wiring Diagrams: http://www.advanceautowire.com/mgb.pdf
Paul Hunts very useful site: http://www.mgb-stuff.org.uk
Manual for download: http://www.geomatique-liege.be/MGJP/DocumentsPDF/MGB_Workshop_Manual.pdf
UK MoT guide: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/518634/mot-inspection-manual-for-class-3-4-5-and-7-vehicles.pdf
Some basic guides: http://www.howacarworks.com/

1970 Roadster (First Reg July 10 1970). Std exhaust (Bell SS) AFAIK rest is bog standard

The function of your points is to manipulate the coil's ground, so the ground side is always the points side of the coil.

Well, almost always. If you were to convert to negative ground and not change coils, there is controversy on which way the coil should be wired. Generally speaking, all modern coils are set up for negative ground, positive ground coils were wound the same but labeled opposite. You have a small primary winding and a 100x larger secondary winding. If you hook it up backward, they do not produce as much output. Positive ground coils generally were labeled CB (contact breaker) and SW (ignition SWitch). In your casse, you may have a modern coil hooked to an older system.

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jeff@advanceddistributors.com

Jeff, I believe that positive ground and negative ground coils are wound differently otherwise there would be no problem reversing the leads. I believe you are right in that it matters depending on the coil which side of the coil the circuit is interrupted. That might be a way to compensate for a coil of differing polarity, but not without re-designing the ignition system layout. Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
'You can't shake hands with a snake'

Fred, how would they be wired differently? It two simple windings and a resistor. You may be over thinking this one. Primary and secondary windings, labeled externally in a way that's easy to hook up. My understanding is that the ignition wire always feeds the primary coil, no matter the polarity. Correct? I admit this is not my area of expertise, and I may be over simplifying it, but that is the theory behind the best ignition systems.

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jeff@advanceddistributors.com



Edited 1 time(s). Last edit at 2016-12-09 09:08 AM by B-racer.

Jeff, Look up autotranformers, and then don't think of them as transformers anymore, but rather as ignition coils. The two windings (no resistor that I am aware of--resistance being the diameter of the wires and length, not that there couldn't be a resistor in some cases) have a common connection and switching the coil at the common connection (I believe) is the usual configuration. You can see that creates a different circuit than if the coil was switched at the positive connection (the connection that is not common) . With a typical transformer that has the windings isolated from each other, simply reversing the connections would work quite well, but with the autotransformer design the configuration matters, and the direction of both windings would be reversed for a positive ground coil. I'm afraid I haven't ripped many coils apart to find all of the differences, although some of this can be measured externally. I didn't think the connections made much difference either until a Morgan fellow told me that an engineer friend told him to never reverse the connections and that doing so would reduce spark energy. I tested this and his friend was right. Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
'You can't shake hands with a snake'

Fred is right. When wired correctly the back EMF of the primary adds to the output spark. Wired "backwards" that extra energy is lost. Whether it makes much difference to the firing of the plugs...

Herb
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A polarizing subject, to say the least......

What about the old pencil test?

Where-in a wooden pencil "lead" is held between spark plug and disconnected wire end. If the "lead" from the pencil vaporizes toward the plug, the polarity is correct. If the "lead" vaporizes toward the wire end, it is backwards and needs to be reversed at the coil.

Peter c