Was wondering how hot do the ignition coils get while running. I was in the process of putting a timing light on the newly restored 70 had turned on the ignition when the phone rang. By the time I got back to the tractor thought I would to check to make sure the distributor bolts were loose and noticed the coil was so hot could not touch it. Up till then never thought about them running hot at least like that. Anyone ever notice how hot there ignition coils run?
Ignition coil running very hot
- Thread starter mdross1
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With ignition on and not running, the coil will get very hot if the ignition points are closed. If the points were open, power would not flow through the coil. When engine is running of course, the points are constantly opening and closing so the coil doesn't have constant flow of voltage. You should find that the coil does not get nearly as hot in normal operation with engine running. .
buickanddeere
Super Member
The coil is a 45 watt heater when left energized. It's going to heat up until reaching equilibrium .
Redneck in training
Elite Member
or burned up.
So no resister should be needed? I also noticed the 60 coil heats up when running but never left the ignition on without engine running to compare against the 70. What is the resistance supposed to be on these coils at the studs?
beenthere
Super Star Member
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- JD_4x2_Gator, JD_4300, JD_425, JD_455 AWS, added JD_455, JD_110, JD_X485(sold)
So the trick is to not leave the ignition on so the coil doesn't get "very hot". As long as the ignition coil still works after "running hot" then the solution should be easy. 
Redneck in training
Elite Member
The resistance will vary between different coils. In general it is low. Because of inductance the current doesn't increase instantly when the points are closed but gradually until the magnetic field becomes saturated. When points open the magnetic field collapses generating high voltage pulse in secondary winding connected to the spark plug. The capacitor in ignition has two functions. It limits arcing when points open and together with primary winding makes an oscillator so the coil generates several sparks in fast succession for each points opening.
Since the coil is autotransformer you can measure secondary winding between posts and sparc cable connector. The resistance of secondary is way higher then primary.
Probably over thinking it but knowing these coils are oil filled and laying on their sides has to expose part of the transformer has me wanting to fabricate a bracket to stand the coil up for piece of mind.