CJ3A Generators

CJ-3A Generators

Willys-Overland and Electric Auto-Lite had a very close relationship so the majority of Willys electrical components were made by Auto-Lite. Original CJ-3A generators were the two brush, Type A, variety. There were 3 different generators used on CJ-3As. The GDZ-4817A was used up to mid 1950. Then came the GDZ-6001D up to the beginning of the 1953 model year when the GGW-4801D entered production. See the Dating a CJ-3A article for more information.

How does a generator work?
A generator operates just opposite of an electric motor. It converts motion into electricity. Electricity is generated when an electrical conductor is moved through a magnetic field. The amount of electricity is determined by the strength of the magnetic field and the speed, size, and number of conductors moving through the field. The size and number of conductors is fixed by the design of the generator and the speed is determined by engine conditions. The strength of the field can be varied by changing the amount of current flowing through the field coils. The field coil current, and thus the generator output, is controlled by a voltage regulator.

When the generator armature revolves, its conductors cut through the magnetic field supplied by the field coils. This generates electricity in each of the armature conductors. As each conductor in the revolving armature passes first a north magnetic pole then a south magnetic pole, current is produced first in one direction then the other. The commutator and brushes act as a switch which changes the connections at the proper instant to deliver direct current at the armature terminal (sometimes called the generator terminal). The field excitation current is self-supplied by the generator. When the generator is stopped, there is no field current but there is a slight residual magnetism in the pole shoes. The residual magnetism is sufficient to start the generating process which in turn increases the field coil strength and therefore the generated electricity. This building up of the generator output continues until it reaches about 7.5 volts where the regulating action begins to control the field current. If the residual magnetism of the pole shoes is lost the generator must be polarized. The magnetism can be lost by long periods of inactivity or improper electrical connections.

Identifying the generator and components

An Auto-lite generator will have a tag with the generator number stamped on it, hopefully the tag is still in place. The terminals are labeled in the illustration. The armature terminal (also referred to as the generator terminal) is a larger diameter than the field terminal since more current flows in that circuit. The ground screw is simply threaded into the generator frame.

The voltage regulator must be matched to the generator, see chart below for the proper regulator number.

CJ-3As used Type A generators, other Willys Jeeps did use Type B. If the Type is unknown skip down to Determining the Generator Type.

On Vehicle Testing

First make a visual check of the wiring and battery cables. Repair any frayed wires and loose or dirty connections. Check the drive belt tension per the service manual. If everything looks good the following tests will require a high quality digital volt meter or an older analog meter. Inexpensive digital meters can give unexpected results in these circuits. Skipping any test can lead to misdiagnosis.

Connect the negative voltmeter lead to the battery ground terminal. Set the meter to read DC volts. Start the engine and run at a fast idle.
Touch the + lead to the generator ground screw and then the voltage regulator base. Since these are all at ground potential the voltage reading should be very close to 0.0 volts at both points. Anything approaching 0.5 volts indicates bad grounds, possibly loose or dirty connections.
Move the + lead to the positive battery terminal. The voltage should read about 7.5 volts DC. Anything below 7 volts is too low and indicates a low charge condition. Above 8.0 volts is a high charge condition.
Move the + voltmeter lead over to the battery terminal of the regulator. It should also be about 7.5 volts. Different voltage readings at the battery positive terminal and the regulator battery terminal indicates a problem with the vehicle wiring in the ammeter circuits.
If the above tests did not resolve the problem continue below in the appropriate section, low or high charge testing.

Continue here for low charge testing

With the engine still running at a fast idle put the voltmeter + lead on the generator armature terminal. If the voltage is 7.5 volts or higher the regulator or interconnecting wire is at fault. If the voltage is below 7.5 volts a "full field" test can be done to determine if the problem is caused by the generator or regulator.
Shut the engine off and remove the field wire from the regulator, leave the voltmeter + lead on the armature terminal.
Start the engine and run at a fast idle again.
Full field the generator by touching the disconnected field wire to a good ground for Type A generators (CJ-3A). For Type B generators touch the field wire to the battery terminal of the regulator. This will force the generator to produce maximum output. The voltmeter should immediately read above 8 volts.
If the voltmeter reading is above 8 volts the regulator is at fault. If the voltmeter reading is lower the generator or interconnecting wire is at fault. If the generator was replaced, or has not been used recently, polarize the generator and retest.

Continue here for high charge testing

Shut the engine off and remove the field wire from the regulator, connect the voltmeter + lead to the armature terminal.
Start the engine and run at a fast idle again.
If the voltage is above 6.5 volts the generator or interconnecting wire is bad. If the voltage is below 6.5 volts the regulator is bad.

Determining the generator Type

It is important to know if the generator is a Type A or Type B for service and repair. The electrical difference is the internal connection of the field coils. On Type A the internal field connection is to the armature (+) and on Type B the field is connected to ground (-) internally. If the armature is electrically removed from the circuit a simple continuity check can be made to determine if the field is connected to ground or the armature terminal.

First set the multi-meter to the low ohms range. Connect one lead to the field terminal (small diameter) and then measure the resistance to the armature terminal (large diameter) with the other lead. See the photo to the right. A normal reading is less than 10 ohms, here it is 3.7 ohms.
Now move the lead from the Armature terminal to the Ground screw. Again the normal reading of less than 10 ohms is seen in the photo. If the resistance of these two checks is not less than 10 ohms each there is an internal problem with the generator that must be repaired first.

To isolate the armature for this test remove the brush cover band. Lift one of the brushes and slip a cardboard strip between the brush and the commutator. It does not matter which brush is insulated since either will break the armature circuit continuity. In the photo the generator housing, or frame, has been removed for clarity. The cardboard strip can be inserted through the brush access holes without generator disassembly.

With the armature out of the circuit repeat the first two tests. Check the resistance from the field terminal to the armature terminal. For a Type A generator this measurement will remain unchanged (less than 10 ohms) as seen in the photo. This is because internally the field is connected to the armature terminal. If this was a Type B generator the reading would be an open circuit, or no continuity.

Now move the test lead to the ground screw. For a Type A generator this is now an open circuit, or no continuity, as seen on the multi-meter. If this was a Type B generator this reading would be less than 10 ohms because internally the field is connected to ground.

Polarizing a generator

The residual magnetism remaining in the field pole shoes is what enables the generator to start charging. After a long period of inactivity, or during replacement or testing of the generator the residual magnetism may be lost, or the polarity could accidentally get reversed. A generator should be polarized after it is installed and connected, but before the engine is started. For Type A, as used on a CJ-3A, momentarily connect a jumper wire from the battery to the armature (generator) terminals of the voltage regulator. Just a touch of the jumper is all that is required. If you have a Type B, disconnect the field wire from the regulator and momentarily touch this wire to the battery terminal.

CJ-3A Auto-lite Generator Component Part numbers and Specifications

Generator Number

	GDZ-48l7A	GDZ-600lD	GGW-4801D
Armature	GDZ-2006F	GGY-2006F	GGW-2006F
Brush Set	GGU-2012S	GGY-2012S	GGY-2012BS
Commutator End Head Assembly	GDZ-2087A	GGW-2002S	GGW-2002S
Commutator End Bearing	GBF-79	GGU-38A	GGU-38A
Brush Arm (2 used)	GCJ-26	GCJ-26	GCJ-26
Brush Spring (2 used)	GBW-45	GGY-45	GGY-45
Field Coil Assembly	GDZ-1005	GGW-1005	GGW-1005
Drive End Head Assy.	GDZ-1088A	GGW-1003	GGW-1003
Drive End Bearing	X-295	X-295	X-295
Pulley or Fan	SP-1885	SP-1986	SP-1986
Voltage Regulator	VRP-6003A	VRP-6003A	VBE-6105A
Generator Specifications
Rated Volts	6V	6V	6V
Rotation at the drive end	Clockwise	Clockwise	Clockwise
Brush Spring Tension	35-53oz.	35-53oz.	35-53oz.
Field Coil Draw	5.0V, 1.3-1.5A	5.0V, 1.3-1.5A	5.0V, 1.4-1.5A
Motoring Draw	5.0V, 3.9-4.4A	5.0V, 3.9-4.4A	5.0V, 4.1-4.6A
Output	8.0V, 35.0A max, 2000 rpm max	8.0V, 35.0A max, 2000 rpm max	8.0V, 45.0A max, 2125 rpm max

Thanks to Karl Russel for the simple generator type identification test.
Part numbers from the 1955 Auto-lite Service Parts Catalog
Generator Specifications from 1952 Auto-lite Electrical Equipment Specification Book
Generator description of operation from TM 9-1825B Electrical Equipment (Auto-lite)