How to Test a Tractor 6-Volt Generator

The 6-volt DC generator was used prior to the advent of the 12-volt alternator. In 1957, the industry standard for automobiles and heavy equipment changed, but many of the old tractors remain in service today. As a result, 6-volt DC generators are still manufactured. If you think the electrical problems with your tractor may have something to do with your generator, the following instructions explain how to test it. There are two reasons why your generator will not run if it is not completely defective: bad brushes or lack of polarization, both which can be fixed relatively easily.

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Prior to 1957, many tractors' electrical systems ran on 6-volt generators.

Step 1

Place the voltmeter on the battery while the engine of your tractor is off. If the battery is charged, the voltmeter should indicate 12.6 volts, plus or minus a few tenths. If the battery is charged, start the engine and take a second reading. If the generator is working properly, the voltmeter should indicate around 14 volts, plus or minus a few tenths.

If it does not change, the generator's brushes are probably overly worn. Put on your rubber gloves and push down on the brushes with something non-conductive -- like a stick -- and recheck the voltmeter. If it does not indicate 14 volts, the problem probably is not your brushes.

Step 2

Remove the wires connecting the generator to the regulator. Run your jumper wire from the DF terminal on the generator to the frame of the tractor. Start the engine and connect your voltmeter to the D+ terminal and take a reading. Increase the rpm of the engine to 3,000. The voltmeter should indicate about 35 volts. If it does, the generator is good. If not, but you know the brushes are good, the generator may not be polarized.

Step 3

Kill the tractor's engine. Remove the tractor's fan belt with your ratchet and wrench set. Leave your jump wire where it is, connected to the DF terminal on one end and to the tractor's frame on the other. Connect the second jumper wire to the D+ terminal and start the tractor. Wait for the generator shaft to begin to spin. When it does, wait three seconds and kill the engine. The generator should now be polarized. If the shaft does not begin to spin, the generator is probably defective.