Re: 12v ign system voltage & resistance test?

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Posted by Bruce (VA) on April 10, 2018 at 07:25:51 [URL] [DELETE] :

In Reply to: Re: 12v ign system voltage & resistance test? posted by Mike Stanaway north mi on April 09, 2018 at 18:13:29:

Thank you for the tutorial.

Lets start with your assumption that reducing voltage to the points will increase their longevity.

It wonít.

The points will operate just fine on 6 or 12 volts; if they wouldnít, you would see different part numbers for 12v points or 6v points.

The issue isnít voltage to the points, itís voltage to the coil, and specifically, that 1932 designed front coil.

Ford put the front coil on his first V-8 in 1932 and on the 9N in 1939. The first time I ever saw one was in the early 1960ís helping the neighbor work on his 40 Ford. Those coils have a poor design by todayís standards. The oem tractor coil had an internal resistance of about .5 ohms; that allowed max current to the points for start up on a cold morning when battery voltage was reduced. But, while running at 7.5 volts, the low internal resistance of that square coil would result in over heating. Ford solved the problem by adding a ballast resistor in the circuit w/ a value of .3 ohms cold and 1.7 ohms hot. So, at start up, you got about 6.8 amps of current to the coil. But, as the resistor heated up, running current was reduced to 3.4 amps.

I asked about the values of the coil and resistor because it is very easy to add too much resistance. If your tractor didnít have the oem ballast resistor and you added one, thatís not a problem. But, if you added the usual ceramic resistor, it could easily have too much resistance in the circuit, depending on the internal resistance of your coil.

The good news is that your coil will last forever; the bad news is that a slightly discharged battery on a cold morning wonít get you enough voltage to the coil to fire the plugs.

The key to points longevity as you noted is quality points. But randomly adding resistance in the ignition circuit could easily result in a weak spark.


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