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MG TD TF 1500 - Q12 Coil

I have two NOS Lucas Q12 coils, one with a decal that reads "Positive earth Only", the other with "Negative Earth Only." Both coils have the same terminal markings, and read the same resistance.

I've never paid any attention to polarity markings, and have used the pencil test to see which way the spark jumps - if backwards, I just switch the two wires.

Any thoughts?

Tom Lange
MGT Repair
t lange

I think that the only time wrong polarity on a coil will do damage, is if you have electronic ignition (pertronix)....I'm pretty sure a points car will work either way....At least it will on a negative ground car, and no reason to think it would be different on a positive ground car....Especially since the tach is mechanical, and has no connection to the coil....
My understanding is that coil polarity has nothing to do with car polarity.
E.B. Wesson

Tom - The polarity of the coil is for efficiency ONLY. Hooking it up backward will reduce the voltage to the spark plug by 12 volts (or thereabouts) - not much considering the total voltage put out by a coil. It does not do any harm to the system, even with electronic triggering. Cheers - Dave
Daved DuBois

Thank you, gentlemen, for confirming what I thought - it just seemed odd to me thatr a coil would be specifically labelled, "Negative [or Positive] Earth Only. "

t lange

"it just seemed odd to me thatr a coil would be specifically labelled, "Negative [or Positive] Earth Only."

I suspect that the terminals on the coil in question are marked CB and SW. When marked this way, one doesn't know which terminal is positive and which is negative and would need to be marked as to which polarity is was meant to be used with for maximum efficiency. Cheers - Dave
Daved DuBois

What does CB and SW represent? On a positive ground car which connection should go to the distributer?

Dan H.
Dan Hanson

Switch and contact breaker I think.
George Butz

George is correct - Switch (ignition switch) and Contact Breaker (distributor points). In a positive ground vehicle, the CB goes to the distributor if the coil is marked as positive ground and SW if the coil is marked negative ground. If there are no markings on the coil, employ the test that Tom Lange details at the start of this thread. Cheers - Dave
Daved DuBois


Please elaaborate on the "pencil test".

Jim Neel TD28423

Hold a graphite pencil between the spark plug and the plug wire (insulate your hand...). If the spark feathers from the plug to the pencil point, then the polarity of the coil is right; if the spark goes from the wire to the pencil, the wires need to be switched at the coil to reverse the polarity.

Tom Lange
MGT Repair

t lange

This thread was discussed between 30/09/2012 and 02/10/2012

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