With the ignition on, the points closed (or ground the distributor side coil lead), take some voltage readings. Work quickly so not to overheat the coil!
The line from the ignition switch: Where it connects to the resistor, should be near battery voltage to ground. If it is low, check the ignition switch, other connections from the switch to the amp meter, amp meter to battery, etc.
The + terminal of the coil: Should be around 6-8 volts to ground. If low, you may have the wrong resistor or a bad coil, drawing too many amps.
You can also take some ohm readings.
With the coil disconnected, the resistance across the + and - terminals should be around 1.5 ohms for a 6v coil.
With the resistor disconnected, it too should have around 1.5 ohms.
With the coil and resistor connected in series, the ohm reading across both should add up to around 3 ohms.
This can be accomplished with the 6v coil/resistor combination, or by replacing the coil with a 12v coil that already has 3 ohm resistance, no resistor required.
If you replace the coil, or discover the resistor is the wrong resistance, when you go to buy another, take your ohm meter with you, measure what they offer before buying.
Old school coils and resistors are slow moving items. They are commonly traded and reboxed from one supplier to another. Not uncommon for them to be boxed wrong, and a confusing subject for the typical millennial employee.
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Today's Featured Article - 12-Volt Conversions for 4-Cylinder Ford 2000 & 4000 Tractors - by Tommy Duvall. After two summers of having to park my old 1964 model 4000 gas 4 cyl. on a hill just in case the 6 volt system, for whatever reason, would not crank her, I decided to try the 12 volt conversion. After some research of convert or not, I decided to go ahead, the main reason being that this tractor was a working tractor, not a show tractor (yet). I did keep everything I replaced for the day I do want to restore her to showroom condition.
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