Tractor Generators

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Tractor Generators
By Chris Pratt

As a companion to the articles on three-brush and two-brush generators, it seemed fitting that we should provide our readers with a description of how a generator works in lay terms. The difficulty with all those "theory of operation" texts is that they border on principles of electricity or physics and such. Since I know nothing of either, you will have to put up with looking at the common sense side of how generators work which means we "just accept" the principles that others either strive to understand or sometimes really do. You may be thinking "now why would I even want to understand how the thing works?". The answer is that after you understand it, troubleshooting it becomes possible. You may find that you can determine the make or break point on whether to "core" your old generator and buy a rebuilt or fix it yourself. Surprisingly, several of the common failure problems can be fixed with a soldering iron, new wire, brushes, and sandpaper.

Magnetism Generates Electricity?

Right off the bat we are into an area we just have to accept. I'm told that when you move a conductor (something that will allow electricity to move?) through a magnetic field , electricity starts to move... enough said. The generator is simply a conductor (actually a bunch of them) in the form of the armature that gets moved (spun) inside of two or more magnets (the field coils are electromagnets attached to the generator case). The electricity moves through the armature and is allowed to flow though the brushes.

The Magnets

First of all the magnets or Field Coils aren't just permanent magnets like we were amazed with when we were kids, they are electromagnets. Electromagnets do the same thing but are really the right kind of metal with coils of wire wrapped around them. When you apply electricity to the coils, they start to generate a significant magnetic "field" (must be where the term "field" coils came from). It is this magnetic field that the armature is spun within. The field coils on a generator are attached to the outside casing. The wire that makes up the coils begin at the F terminal of your generator, winds its way around the case and terminates either at a 3rd brush (on 3 brush generators) or is connected to the wire going to the output brush or A terminal on a 2 brush generator. On most tractors, there are 2 coils in the case though some generators had 4 coils for high output applications (There were no high-output applications on tractors until the 1980's and by then all machines used alternators). The magnets are actually a two-piece arrangement with the coil of wire that fits around the pole shoe. The pole shoe is the "right kind of metal" mentioned above. The large screws on the side of the casing are what hold the pole shoe in place.

The Conductor

The conductor that our generator spins through the magnetic field is the wire coils of the armature. As it is spun, it begins to generate electricity some of which we can use to charge the battery (or directly run electric lights as on very old non-battery systems). I said "some of which" because, the rest of that electricity can be run back into the field coils making the magnetic field even stronger which in turn makes the armature generate even more electricity (which in turn makes the field stronger which in turn...). You can quickly see a thermal overload coming in this scenario and the need for controlling it. I don't want to say much about the control (called the regulator) because this is an article on generators except for one thing. That is that regulator controls the generator by manipulating the ground connection of the field coils. See the other articles mentioned to learn about the many ways this is done.

Grounding the Field Coils

It seems that the Field Coils need to have a ground to generate their magnetic field (which of course allows the armature to generate electricity). The more ground, the more magnetic field. This is in reality because the coils increase their current with increased ground. Regulation is accomplished by either "crowbarring" (or directly connecting) the field coils to ground or allowing it to find ground through a resistor (a device to limit the amount of ground).

Tapping the Electricity

In order to tap into all this electricity the generator produces there are brushes that contact the commutator. The commutator is made up of the copper bars that are soldered to the conductors or coils of wire wrapped around the armature. The brushes ride on the commutator and allow the generated electricity to go somewhere. The "somewhere" is the battery or other load and back into the field coils.


Field Coils

If you can follow my description, it is pretty simple to troubleshoot the coils. First of all don't remove them unless you have to. If you take them out, it will be difficult (though possible) to get them back in without the proper tools. The only two things that can go wrong are either that the wires have lost their insulation somewhere and are touching ground (such as the generator casing) or they have broken leaving you with an open circuit. To test, remove the generator from the tractor, pull out the armature (because the test won't work with it in) and pull the backing plate away from the generator casing (simply because you will need to see the wires and brushes clearly). With everything spread out like this, first test for an open circuit by locating the F terminal (if its not marked, you can identify which terminal is F by noting which terminal wire goes directly to the field coils) and then locating the other end (follow the brush wire that leads to the field coils). Put your continuity tester (a voltage ohm meter set to "low ohms" such that touching your leads together will register on the meter) leads on the F terminal and on the brush that appears to lead to the coil. If your meter doesn't register anything, you have an open circuit. If you are lucky (and most of the time you will be), the open will be in the wires external to the coils and you can solder and reinsulate them. This open will probably be nearly obvious since an open in the stationary coils themselves is less likely than an open in the wires that are free to move around. Years of brush replacement usually moves these wires around and leads to an open. The reason this causes the generator to fail is that with little or no current flowing though these wires, no magnetic field and thus no electricity is generated.

The second problem is the possibility that they are grounded. With the generator disconnected and laid out as we discussed in the last paragraph, touch one continuity tester lead to the F terminal and another to ground. Make certain that your 3rd brush (on 3 brush generators) or output brush (on 2 brush generators) is not allowed to touch the generator case. If there is a circuit (noted by your meter registering), you have a short to ground. As with the open, you should look for the most likely source which are the wires that are loose. The coils themselves are stationary and probably won't have a problem but the wires that are loose may be frayed from misrouting, contacting the armature or just have worn out insulation. Even if you don't show a ground with your meter, check the wires for bad insulation since the ground may not show up while disassembled. Be sure to check both terminals since these could also be a likely source of a ground due to being exposed to the elements for 50 years or more. If you have looked at all the wires and can't find a source of the ground, your coils may have worn through their insulation and you have to make the decision whether to pull them off and look them over. This may be beyond most of our skills and signal the time to trade it in for a rebuilt. One other potential showstopper is that if your coils were finding ground at the F terminal, the generator may have been running wide-open and may have fried the armature (mostly on 2 brush generators since the 3 brush type can't actually run wide open unless the 3rd brush is misaligned). See below, under Armature, for detecting this condition.

Note that on a very few oddball generators, the regulation is internal via a set of points and a resistor. If you have a resistor in the generator case and no external regulation (aside from a cutout relay), the above won't apply since you will always show a ground through the resistor. In this case, not finding a ground would be the problem.


The armature is clearly more difficult to troubleshoot by the shade tree mechanic, but a few things will be readily noticeable. The first and most common is the condition of the commutator bars. The commutator bars are the start and end point of the conductor. If they have scoring or are worn down to the point that the mica between them holds the brushes away from the bars, you will get little or no electricity generation or flow. In my Grampa's and Dad's auto electric shop, I can remember as a 5 year old, wandering through the curtain into the generator and starter repair area where the pride and joy of the business sat. On a bench, too high for me climb on, sat the lathe that was used to clean up all those commutators. When a generator came in to be repaired, usually the armature was destined to be removed, chucked up the lathe and the commutators bars turned down till they were shiny and smooth. Unfortunately, most of us don't have lathes. Most old tractor manuals tell you to take very light (00) sandpaper (all say do not use emery cloth), pull back the brushes, slip it around the commutator and turn the generator pulley or armature until the commutator is just cleaned up and smooth. While this may be time-consuming, it is easy enough to do. The problem may then arise that the mica material separating the commutator bars may end up being at the same height as the commutator bars. When this condition exists, there is no chance that the brushes will ride on the commutator as they should. Instead they will ride on the mica and no electricity will be generated. To get around this, the mica must be undercut. The shade tree method of doing this involves extreme patience and care. Using a very thin-kerf hack saw blade (grind the sides of the teeth till they fit between the commutator bars), gently cut between the bars until you have a clearance of 1/32 of an inch.

The second noticeable problem with the armature will be that the solder that connects the coils of wire with the commutator bars may have heated up and spun off if the field were continually crowbarred and the generator ran at full output for too long. On very unusual large generators, these may be welded connections and can be wire feed welded with low amperage but for likely all tractors, a soldering iron and rosin core solder should be used. Just be sure to get a good solder joint and ensure that you don't leave blobs of solder that will bridge two sections together.

As with the field coils, if the problem is that the wire coils are internally open or shorted, it is probably beyond the shade tree capability to correct. It would seem that winding an armature or field coil is a lost art though it was commonly done by small shops through the 60's. Additionally reinsulating with resin was possible but again, I can find no one with the necessary knowledge (my Grampa passed on in the 70's and my Dad insists that 30 years as an accountant blurred his memories of the techniques). The other common method of correcting these problems would be to simply replace field coils or armatures with readily available replacement parts. It would seem that this option is no longer available since your local parts store does not have these available nor will the person behind the counter have any clue what you are talking about ("I need a set of field coils and pole shoes for a Delco 3rd brush generator 1103075"... "3rd brush... huh?... oh the car wash brushes are on aisle 5.").

Mechanical Problems

Not to be overlooked are simply the bushings and bearings of the generator. These can still be obtained and may be a source of grief. Recently, I opened up a 3 brush generator from an Allis-Chalmers IB that had sat outside for many years, the hood was gone and the cover band that normally protects the brushes from the elements was missing. There was considerable rust on the case and the armature. Surprisingly, I ran some tests and found that there was nothing electrically wrong with the wiring or coils. Still it was clear that the generator would not operate satisfactorily because the armature would not turn smoothly. In this case there is no choice but to pull off the front pulley, remove the armature and replace the bearing. The jury is still out on whether this one will be saved but right now the front bolt and pulley are soaking in diesel and hopefully will be removed without damage. If successful, I'll replace the bearing and this generator will rejoin the ranks of the functional.

If you are like me, figuring out how to rejuvenate non-functioning parts adds to the enjoyment of the hobby. Even if your tractor has nothing to do with enjoyment but is a working machine, you can't argue with not putting out the money to replace a part that, with a small amount of effort, can be returned to service.

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