[Tom Wesenberg]

I'm asked quite often about brush adjustment and how they work, so hopefully these pictures will help.
The first picture is a cutaway taken at the Model A museum at Gilmore Auto Museum in Hickory Corners, Michigan. The second picture shows the 3 brushes. This happens to be the style with a ball bearing in the rear, as used until about March 1930. At that time the bearing was changed to a bushing and the output stud was moved from the front of the case to the rear.

The brush on the left is the output ground brush and has no insulators on the brush holder.
The top brush is the adjustable brush (third brush) and it sets the generator output amperage.
The brush on the right is the output brush and connects to the output stud in the generator case.
Notice all 3 brushes have a long and short side, and the short side is always closest to the brush holder pivot post.
Also notice the adjustable field brush is thinner than the 2 output brushes.

The adjustable brush connects to the field windings, and the other end of the windings is connected to ground. So, consider the ground brush as 0 potential and the output brush as 6 volts potential. Now you can see that as you move the adjustable brush (field brush) closer to the output brush, you will increase the voltage feeding the field coils. This will increase the magnetism of the field poles, which increases the generator output.

So, you can see this also becomes a runaway situation. As the field gets stronger, the output increases, and this also increases the output to the adjustable brush, and it becomes a vicious circle. However since the output is connected to the 6 volt battery, this keeps the output under control. If you have a poor connection in the charging circuit, then the battery is no longer connected and the voltage can rise to as high a 40 volts. This can smoke the field windings and cook the armature. If the lights are on they will also burn out.

With the generator on the car, as you face it from the front of the car, the adjustable brush will be about in the 2 o'clock position. Moving the brush UP lowers the output, and of course moving the brush DOWN will increase the output. For most daytime driving a couple amps should work fine. The more amps, the more the water is evaporated from the battery. If you do a lot of starts and short drives, or night drives, then you will need to move the brush down for more output.

This picture shows too much grease for the rear bearing, and I removed about half of it before the final assembly. I also put a thin coat of grease on the adjustable brush ring and 2 tension holders, and a drop of oil on each of the 3 brush pivot posts.