JD 70 coil

[8850dave]

I checked the voltage on the 70 tractor and had just under 12 volts on one side then checked the points and also had almost 12 volts so my question is should I put on a coil with a builtin ballast resistor or replace the ignition switch where the resistor is on the back or just put on a new ignition switch as well. This tractor has been as issue starting and running for the past few years.

 

[buickanddeere]

points open or closed when measuring?
How many ohm is the coil's primary ?

 

[M-MAN]

I prefer coils that do not need an external ballast resistor. That eliminates a fire hazard.

 

[8850dave]

Tell me how to check the ohms and I am touching
the point arm when reading. When points open coil
reading on the positive post feeding the distributor
is almost nothing

 

[Bob]

If you have access to a meter that will read low DC Amps accurately, the SIMPLEST way to determine is you need a primary resistor or not it to measure primary CURRENT with the switch on, points closed. You are looking for current in the 3.5 Amp to 4 Amp range.

If much more, a resistor is needed to prevent coil heating and reduced point life.

I significantly less, there's an issue such as a poor connection in the primary circuit, and spark output will likely be weaker than it should be.

(Do the test quickly, then shut the ignition switch "OFF" to avoid the breaker point and coil heating that will occur if you take several minutes or more to do this.)

 

[buckeye al]

My guess is when you checked these voltages your points were either open OR they are not making good contact.

I believe the 70 has a "bypass starting" circuit where the ignition resistor is attached to the back of the ignition switch. That means you should need a coil that is meant to be used with an external resistor!

Try this:
-Remove the distributor cap and make sure the points are open. If not prop them open with maybe a popsicle stick.
-Hook one side of your digital volt meter to some solid clean part of the frame and the other side to the post of the coil that is connected to the distributor.
-Turn the ignition switch to the run position. You should see 12 V (or very close to that.
-Using a screw driver or pocket knife or whatever, short the moving arm of the points to the distributor baseplate. Your volt meter should fall to zero volts or very close to that.
-With the points still shorted out, move the volt meter to the OTHER side of the coil. You will likely read about 6 volts. This means your factory original resistor is still in place doing his job. Remove the volt meter BEFORE you remove the screwdriver or pocket knife or before you turn off the switch.

If you measured 6 volts and you believe you are in need of a coil you should buy a coil designed for use with the external resistor.

While cranking, the ignition resistor is supposed to be shorted out & full battery voltage be applied to the coil for hotter than normal spark WHILE CRANKING to help reduce winter starting problems.


Hope that helps.

 

[buickanddeere]

Towards the tail end of 1954 production when LP was introduced. JD changed to starting switch AR11115R which is equipped with the bypass terminal.
According to the parts book the gas 70 always used a 12V coil and the LP 70 used a 6V coil with a ballast resistor.
There are rumours that the tail end of 70 production was equipped with bypass starting which was supposed to begin production with the 720 gas.

 

[Jim Guarino Agri-Services]

Hi B&D, you are correct that the 70 gasoline used a 12 volt coil with internal resistor, as did the A, G, early 50, and 60 gasoline. There are different 12 volt coils with varying primary resistances. The correct one for this application should have about 3.5 ohms on the primary circuit, however we have found some coils from different suppliers with 2.5 ohms, being sold as 3.5 ohm coils. It is likely that they were made by one overseas company, and sold to several USA distributors.
We now need to check each incoming order to verify that they are correct.
Agri Services