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Diagnosis: Charge the battery with a 1-5 amp charger over night to top off the charge. Accurate diagnosis depends on this condition. If headlights on causes the amp meter to go to discharge (about 5-10 amps) then the meter is probably ok. if the unit has a coil/points ign. turning on the ign. button (assuming a letter series) will not always draw current, the engine may have stopped with the points open. When running the ign. draw is usually less than 1-2 amps, so the meter will move just slightly toward discharge if the generator/charging system is not on line. If this is the case, the system voltage measured across the battery will be a smidge lower than the voltage when everything is shut off/not running.
The voltage in the following test should be taken between the tractor ground/frame, and the "A" terminal on the generator. While running, grounding the F terminal of the generator (as mentioned in the prior followup)will energize a good generator. Do this for a max of 10-15 seconds, as the output is unregulated and could heat components, and or burn out bulbs. Voltage in the area of 6.8-10 volts indicates a probably healthy generator. (replace the regulator and be happy)
If no voltage, or no change in voltage, occurs when F is grounded, check the following(done when everything is shut off/not running).
Disconnect both wires from the generator. Using RX1 scale on the MMeter, test the resistance between ground/chassis and F terminal. it should be very low .8-1.5 ohms. (not zero, and not more than 20 ohms, cretainly not infinity/open). If this is in spec. (checking book for field resistance spec is a good idea), the field windings are probably ok. Next check resistance between ground/chassis and the "A" terminal if this resistance is high, above one ohm, or open (infinity) the armature windings are open, or the brushes are bad. Spinning the pulley can make momentary contact, causing a resistance change. Bad brushes probable. If brushes, replacing them is not hard, sand the commutator bars (round-and-round, not along them) with 220 paper. clean all grit, it can cause wear and shorts. Put in new bearings in the end frames at the same time if you are energetic, and want a complete job. Lube the bushing in the armature end with an aspirin sized tip-of-finger dab of clean grease gun grease, do not over do this, more is less. If you repair the Generator and have a bad regulator, your efforts will be wasted. The bad regulator can toast the gen. If you know a real electrical specialist (with generator experience) they can evaluate the voltage regulator and adjust it if needed.
I recommend a new regulator. A new regulator is the solution to a good generator that still won't charge, or charges excessively. An old automotive book on generator systems is nice.
Good luck
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