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Re: Farmall M No spark Trouble shooting guide

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Owen Aaland

06-19-2013 19:46:50

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gene bender said: (quoted from post at 22:19:42 06/19/13) You need more than 4v into the coil how many volts when cranking

If the wire from the coil to the alternator is going to the #1 terminal on the alternator he is only getting 4 volts into the coil because the exciter wire to the alternator is also connected to the coil. The resistance of the coil in the alternator rotor is added to the resistance of the coil resulting in about twice the resistance of the ballast resistor. The end result is the ballast resistor is dropping the the voltage 8 volts instead of 6. The battery at 12.3 volts is half discharged already and with the additional voltage drop while cranking the voltage is probably only about 3 volts.

On the other hand if he were to pull it to start it, it should start and run since 4 volts should be enough to energize the alternator if it spins fast enough. Once the alternator starts to charge it will send about 14 volts back down the wire to the coil so it should have plenty of spark then. Unfortunately this power source will also keep the engine from stopping when the switch is turned off.

On the other hand if the wire is connected to the #2 terminal on the alternator expect to see about 18 to 20 volts coming out of the alternator.

The solution is to remove the wire that comes from the alternator from the coil and connect it to the ignition switch side of the ballast resister. He will also need to add another resistor or a diode in this wire to drop the voltage enough so the engine may be shut off.
This post was edited by Owen Aaland at 19:51:52 06/19/13.

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