Note: The following details apply mainly to civilian models. Some of the information, but not all, is also applicable to military versions. Please remember parts are easily swapped between vehicles, so expect all sorts of grief if troubleshooting based on what the manual says was originally installed, as opposed to what it actually present.
Many people have a fear of the electrical system. They expect to find a little wizard living under the cover of the voltage regulator. Should you offend this little guy, he might hurl lightning bolts at you. Even if you don't provoke him, he might still refuse to cooperate and reward you with a dead battery.
I can safely report there is no wizard at work, yet mystifying things remain. Your generator isn't charging properly, so you ask for advice. You may be greeted with the admonition to polarize the generator since it worked for the neighbor of your uncle's barber many years ago. Short of hiring a shaman, how can you learn about the process? What is polarization, how is it accomplished, and is it even needed? Since generators were replaced by alternators many years ago, much of the know-how has fallen by the wayside.
A bit of background about generators is in order. The spinning armature creates electricity as its coils rotate through a magnetic field. The strength of the magnetic field, and thus the generator's output, is controlled by varying the current through the stationary field coils. The voltage regulator controls the field coils by sensing the generator output. The end result is the generator's output hopefully matches the needs of the electrical system as determined by the voltage regulator. This includes charging the battery and powering the rest of the electrical system. In a nutshell, that is how a generator and voltage regulator work together as a package. Pretty simple, yet how come the battery is still dead?
I can safely say that if your generator was working fine recently, polarization is not needed. Polarization means nothing more than restoring the residual magnetism in the field coils. Over a period of several months of inactivity, the residual magnetism can fade and prevent the generator from working properly. If installing a new or rebuilt unit, or one that has been sitting for months, it would need to be polarized. If you swapped a working generator from one vehicle to another, it would NOT need to be polarized. I work with electricity for a living and can honestly say I don't completely understand the importance of residual magnetism. It would seem that normal current flow through the field coil windings (via the voltage regulator) would be all that is needed, but this is not the case. A brief surge of current through the field coils is needed to do the trick. Remember, you don't have to understand why it works, only how to do it and when polarization is needed.
Polarization is quick and simple, yet there is more to the story. There were two main types of generators produced simultaneously over the years. For example, Auto-Lite and Delco-Remy produced both types, so the brand doesn't help determine what you have. Both types worked equally well and neither had any benefit over the other. Each had their own technique for polarization. Without going into gory details, using the wrong method can either reverse the generator's polarity or create a dead short that will damage the commutator and brushes. Knowing what type of generator you have is important.
To the best of my knowledge, Willys only used one type in all of their post WW2 civilian production, regardless of the generator's manufacturer. (I don't know about the military versions.) You could scroll down to the actual how-to of polarization
and hope your generator was never swapped out. Since these two types of generators appear identical externally, proper identification is a must. If the original generator failed years ago, a low-cost repair might have been to swap in whatever generator/regulator combo was handy. Don't assume (Danger! Danger!) your generator is original simply because it has been there as long as you or any previous owner can remember. Even a cross-referenced generator part number may not be enough, as the guts from one type could have easily been swapped into the other. The internal wiring connections are the only difference. That would result in a FrankenGenerator that would defy all troubleshooting methods unless you could determine what type it really was.
This image shows the two main types of generators. The only difference is how the ends of the field coils are connected. The regulator, of course, must be a matching type. The image on the left shows an externally grounded system, as used by Willys. On the right is an internally grounded unit:
A simple check with an ohmmeter will let us determine whether the field is grounded internally or externally. The terminals on the generator must be identified first. Of the three terminals, the non-insulated one is the ground connection. The two insulated terminals are different sizes, the larger of which is the Armature connection. The regulator is also shown for reference. Click here
for more details about how the regulator terminals are labeled incorrectly in the Willys service manual. On the end of the generator, away from the pulley, note the sheet metal cover over the brushes:
The generator shown above is an Auto-Lite, a swap meet mystery find. It is similar but not identical to the Auto-Lite on my CJ-2A. The same three terminals are arranged in a different fashion :
The ohmmeter check requires taking a reading, and then comparing what happens after removing either brush. The brush cover is secured with a single screw. In this picture, a small awl is keeping the brush holder back. A piece of wire was used to hold the awl for the picture. The brush slips right out of its holder:
By removing either brush, we are breaking continuity between the brushes via the armature. The heavy windings in the armature connect the two brushes and offer nearly perfect continuity. There is no need to determine which brush is which, as either will do. Look again at the side-by-side comparison
of the two types of generators. On an externally grounded style like used by Willys, removing a brush won't break the continuity between the Armature and Field terminals on the generator. Conversely, with an internally grounded generator, continuity will be lost when either brush is removed. The battery and the leads to the regulator must be disconnected for these checks. (The ground wire is okay.) This picture shows the expected results for a stock externally grounded system on a Willys with the brush removed. The ohmmeter will show several ohms of resistance for the field coils, so don't be alarmed if the continuity is not perfect. We are only concerned with any change after a brush is removed:
Rearrange the test leads for a second check which will eliminate any false results from damaged internal insulation. (Don't ask how I know this.) On an externally grounded generator, pulling either brush will break continuity between the Field terminal and the case ground. On an internally grounded system, continuity between the Field terminal and Ground won't change. As above, this picture shows the expected results for an externally grounded system with the brush removed:
Before proceeding, it is vital to observe that pulling a brush changes the continuity between either ARM-FLD or FLD-Ground, but not both. A change observed for both tests, or none at all, indicates an internal wiring problem that needs repair. As a recap, a change between FLD-Ground indicates an externally grounded system like used by Willys. A change in continuity between ARM-FLD indicates an internally grounded system used on other vehicles.
Once you know whether the system is externally or internally grounded, polarization is simple. Reconnect the wiring and battery cable. For an externally grounded field like on a Willys, momentarily connect a jumper between the BAT and ARM terminals on the regulator, resulting in a spark. Don't clip the jumper in place, as an instant is all it takes. Battery power is present at the BAT terminal, and a surge of current will travel through the field coils via the common connection at the ARM terminal. With externally grounded field coils, the ground path is through the voltage regulator's internal contacts. For safety, do not connect the jumper in the presence of any gasoline vapor or hydrogen fumes from the battery. Look for stamped identification letters on the regulator terminals in case they are arranged in a different order:
Accessing the regulator terminals can be a bit tricky. The same exact surge of current can be accomplished with a jumper lead straight from the (+) battery terminal. Run the jumper directly to the ARM terminal on the generator, once again making only momentary contact. Make the final connection on the generator so there are no sparks near the battery:
Should you need to polarize an internally grounded system, whether a retrofit on a Willys or original on some other vehicle, that method is equally simple. Disconnect the Field wire at the regulator. Momentarily touch that wire to the adjacent BAT terminal. This will send the desired surge of current to the proper end of the field coils:
If you aren't sure if your generator needs to be polarized, it won't hurt to try as long as the proper method is used. The only harm that could come is if you used the wrong procedure. Please note that some brands of regulators will have the label GEN (Generator) instead of ARM (Armature) but it is the same thing with a different name.
Also, keep in mind that it is normal to see a spark as the jumper is connected. Of course you must observe the previously mentioned safety precautions. Should no spark be observed, further troubleshooting is warranted. This would indicate an open connection somewhere in the circuit, such as the power supply or ground, or within the generator or regulator.
Note: Remember, the preceding details apply mainly to civilian models. Some of the information, but not all, is also applicable to military versions. Please remember parts are easily swapped between vehicles, so expect all sorts of grief if troubleshooting based on what the manual says was originally installed, as opposed to what it actually present.