Sorry for the delayed answer, but I am on the digest by choice.
The late '77 and up MG's had a ballast resistor in the wiring harness.
Since many cars were modified:
First look for a ballast resistor on or around the coil. It is usually
cream color and a 1/2" squage about 2" long. One side should have a
white wire, the other should connect to the + side of the coil. If you
don't have a discreet resistor, then see the next paragraph.
You can tell how yours is set up by the color of the wire going to the +
side of the coil. If it is white, it is tied directly to the ignition
circuit without a ballast resistor wire. If it is white with a green
stripe, it is tied to the wiring harness ballast. The standard
configuration is a white wire with a green tracer going to the + side of
the coil.
The original purpose of the ballast resistor as I remember it is that
the coils on the first twelve volt cars were still six volt coils. The
resistor was used to drop the voltage from 12 to six. Then to give more
spark during cranking, the resistor was shorted out.
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