Quoting Removed, click Modern View to seeThe first rule is ...if it ain't broke, don't fix it, especially after it has been fine for 6 years.........
IF you really have a true 12v coil, which requires no resistor, it will have a primary winding resistance of about 3 Ohms, some as high as 4, which by design limits point current to about 4 amperes, which is a safe operating current for thew points, so no resistor required. Just because a coil is sold for a 12v application (for example a 1971 Ford PU) does not mean that it is a "true 12v" coil. The Ford PU is a 12v system, but the coil has about 1.3 Ohms resistance, which if used without an external resistor would produce about 10 amperes through your tractors points, leading to a very short lifetime! The PU has a resistor built into the wiring harness with a special high resistance wire & high temperature insulation and it has the current switched by a transistor, not points.
Hopefully, you can see why emphasis is placed on the words, "true 12 volt". It would be a lot more useful to use words such as a "1.3 Ohm coil", a "3.2 Ohm coil", or a "a 0.47Ohm coil", etc. than to say a "12 volt coil", because with the simple 12v coil description, you really don't know what you have in your hand. Some will have additional information, such as "use with external resistor", or "no external resistor required", but not all manufacturers are so good as to include this kind of marking on their coils. Hope educational for you.
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