Installing an Alternator in the CJ-3B

by Eric Lawson


The Jeep's wiring harness and alternators are a lot more versatile than most people realize. Although the stock CJ-3B uses a generator, all that nearly any alternator system needs to function is a charge indicator light as it is wired in the later CJ-3B. A "one wire" alternator is the way to go in pre-1956 Jeeps that are equipped with an ammeter instead ofthe indicator lamp.

The additional output current that a modern alternator can provide ascompared to a generator or an old alternator is the best reason for making this change to a Jeep, and it may be required if you are converting to a newer engine.

Extra lightsThe stock generator was sized to be able to operateall of the electrical equipment that was factory installed in a Jeepand still have some spare capacity left over to charge the battery. Unfortunately, this spare capacity was kept to a minimum. Adding even a modest amount of extra electric load to the system, such as additional lights, will likely exceed the capacity of the stock generator or alternator.

If the electric powered devices you have operating exceed the capacity of the generator or alternator, the battery will make up the difference--for awhile. If this situation is kept up long enough, the battery will become discharged. To recharge the battery, you will have to turn enough equipment off to get below the rated capacity of the generator or alternator. The more you turn off, the faster you would recharge the battery.

The larger capacity alternator will save you the possibly tough decision about what to turn off!

Note: These instructions assume you already have a 12-volt electrical system. If you are converting from a 6-volt generator, there are some additional steps to avoid component damage or a potentially dangerous battery explosion. See Converting a 6-Volt Jeep to 12-Volt.

Also on CJ3B.info, see Generator Regulator Troubleshooting.

Generator vs. alternator

Both a generator and an alternator perform the samefunction -- supplying electrical energy for the electrical equipment inthe Jeep.

The major differences between the two are related to which of the twocoils of wire within the device generate the current that issupplied to the Jeep's electrical system, and the method ofconverting the AC voltage generated by the device to DC voltage.

In the generator, the armature rotates, and the field coil isstationary. The field coil creates the magnetic field needed for thegenerator to operate. The armature is the coil in which thecurrent that is output to the Jeep's electrical system is created.

To convert the AC voltage to DC voltage and to provide anelectrical connection from the rotating armature to the Jeep'selectrical system, brushes and a commutator are used. Thecommutator is a copper cylinder that is split into severalseparate segments. It is located at the back of the armature. The brushes are blocks of carbon that rub against the commutator.

The brush and commutator system is not very efficient at highelectrical currents such as those needed by the Jeep's electricalsystem, and much of the electrical energy is lost as heat. This iswhy most automotive generators are limited to 35-45 amps output current. To significantly increase this current would require a much larger generator.

In an alternator, the field is rotated and the other coil of wire,called the stator, generates the current provided to the Jeep'selectrical system. Since the stator is stationary, no commutator /brush system is needed. The high current generated in this coil isconnected to the Jeep's electrical system with wires.

The conversion of AC voltage to DC voltage within the alternator isdone with the use of diodes. Not only are the diodes much moreefficient at this task than the commutator / brush system used ingenerators, but they can also handle higher currents.

Diodes are the electrical equivalent of a check valve in an air line. Like the check valve only allowing air to flow in one direction, the diode only allows electricity to flow in one direction.

Wiring a Motorola alternator in a post-1956 CJ-3B

In the blue Universal Jeep Service Manual (available from Original Reproductions), figure H-23 on page 196 is actually the wiring diagram that shows how to hook the CJ-5s stock Motorolaalternator to a post-1956 CJ-3B. It does not say this in the book, but it willwork. Since the diagram illustrates testing the alternator, ignore the"DC Volts" meter that is shown attached to the AUX terminal of thealternator:

Fig. H-23 Alternator/regulator hookup.

The wire colors mentioned in figure H-23 are the colors of the wirescoming out of the regulator. These wires are a few inches long andend at a 3 wire connector. I have found these wire colors to beconsistent among 3 different manufacturers of replacement regulatorsfor Motorola alternators. If you are converting to a Motorola alternator, make the connections between the alternator and regulator as shown in figure H-23 above.

Figure H-38 below assists in identifying the terminals on Motorola alternators:

Fig. H-38 Motorola alternator terminals.

Wiring a Delco alternator

The standard Delco SI alternator uses three or four wires depending on "purity of the user". The fourth wire is a ground wire and is usually not needed.

The "one wire" alternator kit that Bruce mentions in the CJ-3B Engine Conversion Manual is a slightly modified Delco SI alternator which can be simpler to install, particularly in a pre-1956 CJ-3B with an ammeter on the dash. One thing that I have noticed on the one wire alternator is that I have to rev the engine to get the alternator to begin charging. Onceit begins charging, it continues charging even with the engineat idle. I asked about this at the alternator rebuild shop and theytold me that this was normal. (Now that I think about it, and how one wire conversion works, it seems normal to me too.)

The Delcoalternator is a great improvement over the stock Motorola and Prestolitealternators. Changing the alternators is something to be considered ifone is doing an engine swap. It is much easier to find replacements forthe Delco should it fail, and depending on the model, the Delco has asmuch as three times the output current to run the electrical system.This is very important if one is considering extra lighting as the stockMotorola and Prestolite can provide only 35 amps. About 30 amps areneeded to run the headlamps, stop lights, engine ignition system, hornand heater blower motor.

Delco terminals.The steps for wiring the Delco in a post-1956 CJ-3B:

Finding the indicator lamp wire

The only trick to the whole thing is to find the charge indicator lampwire. In looking at the wiring diagrams of the CJ-5 or post-1956 CJ-3B (see the Parts Illustrations page), itwill be the only wire that attaches to the alternator or regulator fromthe wiring harness that goes through the firewall. (The wiring diagram of the early CJ-3B might as well be the wiring diagram for a CJ-3A with turn signals.)

Before tearing apart the wiring to find it, you can try this test:

If you want to be absolutely safe, go to the parts store and get aninline fuse holder with a 5 amp fuse in it. Connect one wire of the fuse to what you believe is the charge indicator lamp wire and touch the other end of the fuse to the Jeep's body. If you have the correct wire, the lamp will follow the description at the end of the previous paragraph. The incorrect wire will either do nothing, or blow the fuse.

If you don't want to go through all this, you can take apart the wiring harness and follow the wire back to the proper bulb in the instrument cluster.

Note: Instead of attaching to the starter solenoid's battery terminal, thewire that feeds the Jeep's under dash electrical system is sometimesattached to one of the terminals on the regulator. If your Jeep iswired like this, you will find two terminals on the regulator withtwo wires connected. If you do not take care of this "extra" wirethat is connected to the regulator your Jeep's electrical system willnot have any power.

If you examine the wiring connected to these two terminals, you will find that one of the terminals has a wire that connects to the starter solenoid's battery terminal and a wire that goes into the wiring harness that goes through the firewall. I'll call this terminal B. The other regulator terminal with two wires connected will have a wire that goes to a terminal on the generator and a wire that also goes into the wiring harness that goes through the firewall. I'll call this terminal A. If you can read the printing onthe regulator, the B terminal will be labelled B, BAT or BATT. The Aterminal will be labelled A, ARM, G or GEN.

The wire from the wiring harness that is connected to theregulator's A terminal should be the charge indicator lamp wire. Thiscan be verified with the tests described above.

When you get ready to remove the existing regulator, the two wires that connect to the regulator's B terminal should be connected together. This will ensure that current can flow to the Jeep's electrical system. The wires that go between the regulator and the generator can also be removed.

Mounting the alternator on a 134 engine

Bracket front
Recently I visited Willys Works, in Tucson, Arizona (telephone 520-888-5082), and found that they are making a bracket to mount an alternator to the front plate of a 4-134 Jeep engine. This bracket mounts into two existing holes in the front plate of the engine. The acompanying photos show the mounting arrangement.

According to the enclosed instructions, the generator belt tensioning arm, along with an included spacer, is used to tension the belt. I don't have a generator belt tensioning arm yet, so I can't tell for certain, but it looks like the arm would need to be attached under a water pump bolt--just like is done when a Motorola alternator was installed on the engine.

To mount an alternator on another 4-134 engine, I used a "universalalternator mounting adapter kit". I bought mine about 8 years agofrom a local alternator rebuild shop, but I called there a few monthsago to get another one and they no longer sell the kit. You may be able to find a kit for the alternator you have, or you can fabricate a universal one as follows:


The drawing shows the brackets in the kit and how they are arranged. The kit I bought used four 1 inch wide, 1/8" thickL shaped brackets arranged as shown. To make the kit universal, themanufacturer drilled several holes in the lower two brackets andmachined slots in the upper brackets. This allowed great freedom tomove the upper two brackets around as necessary so the alternator's pulley could be lined up with the other pulleys on the engine.

As an alternative to the highly curved support arm, a nearly straight support arm can be held in place by the waterpump bolt nearest to the alternator. Again there will be some offset between the alternator mounting ear and the support arm. This is how it is done on the Jeep engines with the stock Motorola and Prestolite alternators.

Using this type of adapter bracket and the 100 amp Delco alternatorsmay present some difficulty with the alternator clearing the bottomof a large military style oil filter (the cover on this filtersays "military standard filter"). Using a street L fitting on the bottom of the filter canister and moving the filter up in the mountingbrackets may provide enough clearance to accommodate the alternator. I did not have this problem because my engine uses a smaller oilfilter that uses a Wix 51010 filter element. -- Eric Lawson

Thanks to Eric for a very useful guide. -- Derek Redmond

Elsewhere on the web, see Ric Meagley Sr.'s One Wire Alternator page.

Return to CJ-3B Specs and Tech Tips.

Go to the CJ-3B Engine Conversion Manual.

FacebookVisit CJ3B.info on Facebook.

CJ3B Home | Contents | Search | Links | Bulletin Board

Last updated 10 October 2003 by Derek Redmond redmon@post.queensu.ca