How to tell if a 12v coil has an internal resistor or not

How do I tell if a 12v coil has an internal resistor or not?

I have a Ford 3000 gas burner 3 cylinder I suspect the coil is bad. I would like to replace it. Looking at coils on line there are two types of coils for sale. One with no external resistor and one with. I can't tell which I have on the tractor. I seem to remember years ago the visable difference is how tall the center of the coil is.

On a side question could the resistor be in the wire instead of a ceramic type?

How do I tell if a 12v coil has an internal resistor or not?

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Most coils do not have an internal resistor. They all have some amount of internal resistance due to the type and gauge of wire and number of windings in the primary side. Those that are designed for a 6 volt system or for use with an external resistor in a 12 volt system have less internal resistance, but they do still have internal resistance.

If you're asking how to determine if the coil should have an external resistor when it is not marked as to whether it requires one or not, measure across the two small posts with an ohmmeter. A coil that requires no external resistor in a 12 volt system should measure about 3 ohms. One that requires an external resistor in a 12 volt system should measure around 1.5 ohms.

Your 3000 originally came with an external resistor wire between the key switch and the coil. The resistor part of the wire is thicker than the rest of the wire and is covered in heat shrink and sort of looks like a fusible link. If it still has that resistor wire it's possible that the resistor wire has failed, as most of them have over the years, but you need to measure the resistane between the key switch and the coil to be sure. The wire and the coil should measure about 1.5 ohm each.