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Re: Farmall M No spark Trouble shooting guide

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Owen Aaland

06-20-2013 20:13:18

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Nate V. IA said: (quoted from post at 12:17:59 06/20/13) John T and others

I am an electrical idiot but willing to learn. I set my meter to 200 ohms and tested the ballast resistor, the meter reads 3.0. Voltage ahead of the ballast is 12.8, after ballast is 4.0. The voltage readings vary everytime I check. I bypassed the ballast and am only getting 10 volts to the + terminal on the coil????
The alternator is wired correctly I think, #2 wire goes to + terminal on the coil and when removed it doesnt effect any readings.

The coil is 12v w/o resistor so, the way I understand it, it needs to have a resistor ahead of it.
I am only getting 4 volts to the coil today and had 6v last night.
The points are opening and closing as they should. Test light shows power to the coil + terminal. No power to the - terminal or the high power wire to the dist. cap.
Thanks for putting up with my ignorance.


The wire from the coil to the alternator will need to be switched at the alternator. If that wire is on the #2 terminal then the #1 terminal is connected to the battery post and will be live all the time. That will keep the alternator energized all the time and will drain the battery when the engine is not running. Terminal #2 is the voltage sense wire. The regulator monitors the voltage on this terminal and regulates the alternator voltage to maintain the set voltage on this terminal. Power to this terminal is coming from the ignition switch through the resistor and then to the coil. The readings you are getting on that side of the resistor are 4-6 volts. The alternator will boost the output voltage until it sees 14.2 - 14.8 volts on terminal #2. That is why I said that the alternator was going to be putting out at least 18 volts and probably quite a bit more. For now just remove the wire between the coil and the alternator and unplug the other small wire that is currently connected to the #1 terminal. Just leave them off until we get the other problems sorted out.

You said you had a reading of 3 ohms on the resistor. That may be correct, if it is it is probably a bit high. I would expect to see it in the range of 1.2 to 1.8 ohms. Short the leads of you test meter together after you switch it to the 200 ohms scale and make sure that it reads 0. If it is more than 0 then subtract that value from the reading of the resistor. Then remove one wire from the coil and take a resistance reading of the coil. The resistance of the coil should be close to the resistance of the ballast resistor. If you have about 3 ohms resistance across the primary leads of the coil you have a 12 volt coil for use without an external resistor. If the resistance of the coil is somewhere around 1.5 to 2 ohms you have a 6 volt coils and will need the external resistor. Electricity will see the coil is a resistor just like the ballast resistor. Since you want to reduce the voltage by 1/2 you want each resistor to use half of the voltage.
You can also check the points with your meter. With the points closed you should see zero resistance from the wire on the coil side of the distributor to chassis ground. When the points open you should see infinite or a #1 on your meter.
This post was edited by Owen Aaland at 20:18:05 06/20/13.

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