641 6 volt coil question

[Tony Jacobs]

No, you have not removed 12 volts. You have dropped the voltage to the primary of the coil by 6 volts. When you have two equal resistances in series, half of the total applied voltage to the circuit is applied to each of the two resistances.

So what you are saying here is that on a tractor that has a 12 volt system that YOU installed a 6 volt ONLY coil that has 1.5 ohms resistance built in to it plus you added a 1.5 ohm external resistor in the circuit to use it with 12 volts so that it doesn’t burn the points you now have only 3 volts left to fire the spark plugs when starting the tractor ? Since the saturation point of a 6 volt coil is only 6 volts that means according to you there is only 3 volts left to fire the plugs . After all the circut from the coil to distributor is now the same as a 6 volt tractor because of the 6 volt ONLY coil that you installed . So when we add in voltage loss from the draw of the starter cranking the engine who knows if there is even 3 volts left to fire the plugs . We won’t bother to get into temperature differences because as we already know that temperature affects voltage also and it also affects atomization of fuel . Fuel atomizes into smaller droplets the warmer the weather and larger droplets as the weather gets colder making the cold weather fuel much harder to ignite especially when lacking the voltage to produce the spark . This extremely low voltage is why a 6 volt tractor with a 6 volt coil seems to start every time just as you release the starter button because that little extra voltage spike from releasing the draw caused by the starter increases the spark just enough to start the tractor . This also explains why Soaked had to make a jumper wire to jump the resistor on the 8N tractor with the 6 volt coil that was converted to 12 volts because that tractor also does not have enough voltage to fire the plugs . He also stated that it ran ok once it was running and the alternator kicked in to raise the voltage back up . That was in his post claiming 6 volt ONLY coils work on 12 volt conversions . This is the same problem ole Fred has with his other tractors , the 961 and the 851 that he spoke about in his post if in fact they have 6 volt ONLY coils on them as he said and the same problem he will have with the 641 that he is using a 6 volt ONLY coil while converting it to 12 volts if you want to call it that .

 

[Sean in PA]

So what you are saying here is that on a tractor that has a 12 volt system that YOU installed a 6 volt ONLY coil that has 1.5 ohms resistance built in to it plus you added a 1.5 ohm external resistor in the circuit to use it with 12 volts so that it doesn’t burn the points you now have only 3 volts left to fire the spark plugs when starting the tractor ?

You apparently don't understand basic electric theory. First, I didn't install anything, Ford installed a coil with a 1.5 ohm primary AND a 1.5 ohm resistance wire on their 1965-1975 tractors. The primary of the coil has 6 volts across it all of the time. I did not say anything about 3 volts. The Ford engineers also installed a bypass wire from the solenoid to the coil primary so that the coil gets the full 12 volts while the starter is turning, but only while the starter is turning so that the points will last.