34gpa,
The ignition dropping resistor used in the 520 was AA5791R (still available from Deere), and according to FSB 269, it has a value of 2.20 ohms. It was used with a 6V coil AM791T (currently listed in JDparts as a bearing...). If you are using a commonly available 6V coil labeled 505, the primary resistance varies from 1.02 to 1.10 ohms, according to my measurements. So, adding 2.20+1.05 (average value) gives 3.25 ohms for the total resistance. Assuming your battery voltage is ~12.6, the current is 12.6/3.25 = 3.9 amps, which is less than 4 amps (good), and 12.6 x 3.9 = 49 watts of power are generated in the system. However, if you use a 1.77 ohm resistor with the 505 coil, the current is 12.6/(1.77+1.05) = 4.5 amps, which is higher than normally recommended for a time-averaged primary current - and 12.6 x 4.5 = 57 watts are generated to be dissapated in the coil and resistor. So, what's the point? I would measure the primary resistance of the coil you are using, and calculate the value of the needed resistance using say 3.8 amps of current - this would give a resistance closer to 2.20 ohms. It's easy to google ignition resistors, including the part numbers John T gave, but none of them give the resistance values. I bought an "ignition resistor" a few years ago and when I got it home it was 0.56 ohms - correct for some application but useless for mine - hence in my opinion a bit of work on the front end to figure out what you need, and then ensure you are getting that (bring an ohmmeter to NAPA to measure it) will provide less headaches in the future....
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