Posted by Bob on February 17, 2009 at 10:37:09 from (69.178.228.68):
In Reply to: Ford 12v conversion posted by sandcrabsteve on February 17, 2009 at 10:19:26:
You MUST use a ballast resistor with that coil. WHY would you think they would print that information on the coil if a resistor were not required?
Fact is, a "12 Volt coil" labeled "use WITH external resistor" is actually a 6 Volt coil.
The reason for using a coil such as that is to aid in starting. When the battery voltage is drawn down by the starter during cranking, spark voltage is reduced.
By using a 6 Volt coil, a "positive temperature coefficient" ballast resistor or a "starting bypass" with a fixed resistor can be used to give full battery voltage to the coil during cranking, then dropping the voltage off as the PTC resistor heats up over the first minute or so after the ignition switch is turned on, or, in the case of a "starting bypass", it drops out a soon as the starter is no longer activated.
I'm NOT familiar with your specific tractor and how it was set up, or if that coil is original, but the fact remains you MUST use a resistor with that particular coil.
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