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How Can I Tell if My Farmall Cub Generator Is Charging?

by Don Bowman

Farmall Cub generators operate on 6-volt systems. A generator must be polarized properly every time the battery is low or disconnected. The poles in a generator are the two sets of field windings surrounding the inside of the generator. As the armature spins, power is supplied to the field windings, creating an electromagnet whose field strength is dependent on the amount of power supplied to it. Two wires to a commutator connect the armature windings -- these are the copper plates surrounding the back of the armature shaft. The power produced by the generator passes through these connectors to the brushes, which ride on the commutator and on to the regulator. Worn brushes or commutator surface will cause low or no voltage. Power from the battery goes to the regulator, which varies the voltage to the field windings to regulate the voltage produced by the generator.

Step 1

Check the generator belt to make sure it is tight. Polarize the generator so that you start on the right foot; the key does not need to be on. Connect the jumper wire to the B+ (battery supply) terminal on the voltage regulator. With the opposite end of the jumper wire, very quickly touch the G (generator) terminal, which will short the terminal and polarize the generator field windings. It only takes a millisecond to polarize the generator. Remove the wire.

Step 2

Start the engine. One major difference in a generator and an alternator comes into play here. Unlike an alternator, the generator will not charge well at an idle. The voltage increases with the number of rpm. The engine must be at 1,500 rpm to check the charging system. The next question is always “Why not install a undersized pulley to raise the generator rpm at idle?” The answer is, it doesn’t work. Yes, it will increase the voltage at idle, but as soon as the rpm increases, the brushes lift off the commutator due to their lack of tension and the generator ceases to charge.

Place the voltmeter's red lead on the positive terminal on the battery and the black lead on the negative one. Observe the voltage. Proper charging voltage is between 7.5 and 8 volts. If the voltage is other than described, a problem exists in the generator or regulator. Check the voltage regulator first, since it is the most common problem.


  • If the generator is not polarized, the generator will burn the voltage regulator points, putting the generator in series with the battery. The resulting voltage will double, systematically burning out the amp meter and instruments. At the same time, due to the low resistance to ground in the generator and the regulator, points welded together and the battery will drain as in a short directly to the block when the engine is off.

Items you will need

  • Voltmeter
  • Jumper wire
  • Set of wrenches

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