MG T-ABC

The Instruction Manual that came with TC0820 was printed in May 1946.
Wiring Diagram on page 87 is titled "Illustration No. 50.--Diagram of electrical wiring system." (Attached)
The system is Positive Grounded and shows the CB terminals of the ignition coil going to the [Ignition] Switch and SW terminals going to the [Distributor] Contact Breakers.
This diagram shows the Lucas RF91 voltage regulator.
(The same wiring diagram is repeated in the "Export" section as Illustration No. 53 and also shows the
terminals switched and also shows the Lucas RF91 voltage regulator.)
Illustration No. 50 is not repeated in later Instruction Manual's wiring diagrams because a year later the voltage regulators were switched to the R95 models (but retaining the Positive grounding of the system).
Later TC instruction manuals, show the ignition coil terminals reversed (corrected?) for the same Positive grounded systems. These show the voltage regulator as the Lucas RF95 model -- and the wiring of the ignition coil shows the terminals switched to appear "correctly" wired for positive grounded systems.
At some point in its history, TC0820 had its Lucas voltage regulator switched from the RF91 model to the RF95 model. The electrical system also was changed to Negative grounding.
Now wanting to show "original" voltage regulator of RF91 outward appearance (but with electronic interior and keeping Negative grounding for accessories), and not wanting to degrade the performance of the ignition coil, how should the system be wired and the Ignition coil terminals be connected?
Very important to get this right because wrong system polarity is said to fry the electronic voltage regulator.
Any input will be appreciated.
Robert Griffey

There are a few documented errors in the factory wiring diagram. I don't think the regulator will be affected by which way the coil is wired since polarity of the the system is determined by which way the battery cables are connected to the battery. Having the coil wired opposite of system polarity will simply reduce its efficiency.

Thank you for the reply and great information to know about digging more into the factory diagram(s) of the electrical system.

So the real question is whether Illustration No. 50 shows the "backward" terminal connections on the Ignition Coil as being related to some peculiarity of the RF91 voltage regulator? -- or is it a documented mistake?

Basically, are the connections to the Ignition coil really "backward" (and the illustration a verifiable mistake) --

Or is there some feature of the RF91 Voltage Regulator that requires the "backward" connection in a Positive grounded system?

Octagonally yours,
Robert Griffey Kansas City

Since coils come in both positive and negative ground, anything is possible. Ultimately the thing to do would be to figure out which polarity coil you have and just wire it to suit. There is a test that I can't remember the particulars of, but I'm sure it's on the web somewhere.

Also, I just remembered Doug P was researching this at one point and has some info HERE.

Thank you for the alert that Ignition coils might come with various polarities.
So will try some tests specific to the coil.
That Doug Pelton technical note, that you reference, features known mistakes about the wiring color coding and number key -- with the same Illustrations (No's 50 & 53) that are mentioned above.
The technical note does not mention anything about the coil terminal connections with possible "reverse polarity" that are shown in those same illustrations.
Will send this comment to Doug Pelton at FTFU for a look.
Octagonally yours,
Robert Griffey

Various sources talk about reducing the performance of the ignition by use of "backward" coil polarity. Keep in mind that is a loss of "potential" performance that very few people are ever going to experience using the original coil. They are talking about a potential loss of performance IF your engine is operating at the ragged edge of some performance criteria, like high RPM and high compression ratio, or starting the engine in very cold temps, or using wide spark plug gaps, etc.