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Ford Tractors Discussion Forum
Show Parts for Model:

Re: Coil 3400

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Author  [Modern View]
Keith Williams

11-15-2012 04:49:59
24.142.137.38



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Billy the kid,
First lets start with: "Well I have power at the coil on start and run. I tested with a test light. I guess I need to put a volt meter on the coil input wire to see what I get.
When I jump the coil the old girl will run."

When you jumped the coil, You supplied power. It must have not had enough power already!

"Why would you need a resister with a 6 volt coil
but I can replace with a 12 volt coil and not need one?"

Long story short. You need a certain amount of current to run through the circuit (as in Amps) to create a spark. Amps equals volts divided by resistance. The problem is volts are different when you start vs when running. Maybe 8 volts left when you crank the engine. Around 14 volts when the engine is running and the charging system is working.

So the old way to do that on a 12 volt system was to run two wires to a low resistance coil (called a 6V coil). Wire number 1 came from the starting circuit, since the voltage is low when starting, no additional resistance is needed. Wire number 2 comes from the running circuit, you need resistance to limit the current, so they add a ballast resistor. In both cases the current is enough to create a spark.

Now the coils (called 12 volt) can be placed directly in the circuit and are able to produce a spark and limit current under both the start and running conditions with out additional resistance. These coils are marked "No resistance needed".

I hope this brings to light some of the info you need.
JMOR or several others can explain more if you need it. Just let us know if this is enough.
Keith

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