Universal CJ-5

Around the clock,
around the year,
around the world.

EarlyCJ5.com Technical Library

A technical library for classic Jeeps

Electrical

Electrical Related Articles

Wiring Diagrams

The wiring diagram for the F134 are provided here both in PDF and SVG format. You need a plugin to view either one. The SVG can be made larger or smaller with no loss of quality and displays in the same page. The PDF will display in another browser window. Links to the plugins are provided here. Thanks to Jim Williams, Tigershark, for the original drawings.

PDF Files

SVG Files

Electrical FAQ

Why would I have spark in a cylinder but it is not firing?
I have a Jeep with a Buick 225 V/6 engine. It also has the Prestolite/Delco distributor. What is that funny looking white thing with two wires mounted/hanging on the firewall? What does it do? What is its purpose? Do I really need it anyway?
My Jeep V/6 runs rougher at idle and/or seems to wear/burn out the points quickly. What could be causing this?
I just installed an HEI distributor in my V/6 Jeep, and the engine seems to run rougher and miss at higher speeds. I have double checked everything and re-read the instructions twice. What could be wrong?
Ballast Resistor: What does all this mean to you?
How do I install an ammeter?
How do I wire and operate the ammeter?
I am installing a Ford alternator and external voltage regulator. How do I wire them?
Where does the coil get its power from?
Can I use a 6v starter on a 12v system?
When I go about 1/2 mile, engine shuts down. Wait 5 minutes, she will start again. Sounds like coil overheating?
How do I wire a 'one wire' alternator as a replacement for an regulator?

Why would I have spark in a cylinder but it is not firing?

I *can* tell from the tune-up machine scope that the plug is actually firing (gets the voltage spiking that you'd expect). Compressed air shows the valves are working properly, and both wet and dry compression checks come out ok (about like the others). The thing seems to run pretty much as it should at idle, and then if you short out 1, 2, or 4 you get a miss and drop in RPM. With #3 shorted, it runs the same as not shorted - I even tried different distributor caps, plugs, plug leads - same results. Its almost like its actually firing on 3 with or without the plug shorted :) (of course, this is at idle - driving it with the plug wire off, you can definitely notice a power loss) The system is still 6 volt. Why is it not firing Your symptoms a highly suggestive of a ***weak*** spark as compared to an absence of spark. I say this because you have a 6 volt system, which means you have a generator. Almost all generators have larger pulleys than do alternators, and therefore spin at a lower speed than an alternator does for a given engine RPM. As I think back about the generator equipped cars and trucks I have owned (four of them), it seems to me that the ammeter would register a slight discharge when the motor was idling. This means that it is putting out at a lower voltage than the battery, probably around 5.5 volts.

With a 6 volt system, you are undoubtedly running the breaker point/capacitor/coil system for creating the high voltage necessary to fire the plugs. This system uses energy stored in the form of a magnetic field:

The cycle begins when the ignition points *close*, completing an electrical circuit from the power source (the battery), through the ignition switch, to the coil, to the distributor points, which connect to the grounded motor block, hence back to the battery.

This completed electrical path, or circuit allows current to flow through the "primary" winding of the ignition coil. This in turn creates a magnetic field, with is concentrated by the iron core of the coil. This represents stored energy that can be used to create a high voltage output. The current flows for as long as the points are closed, which is referred to as the "dwell time," or simply the "dwell." It is important that you have the proper dwell. Too short a dwell time, and the core will not have sufficient time to fully charge with magnetic energy, or "flux." Too long a dwell causes unnecessary heat build up in the coil, which can shorten its life expectancy. The proper dwell is designed into the cam lobes in the distributor, but this can change if the lobes become worn down. In an ideal situation the lobes will allow the points to remain closed long enough for sufficient flux to build up in the core of the coil at the highest design RPM of the motor, but not so long as to excessively overheat the coil during periods of prolonged idle. This wear on the lobes is one reason why you should replace or rebuild a point equipped distributor periodically. I like to do it about every 60 to 75 thousand miles. Good cam grease can prolong the life of a cam substantially.

You can test to see if your cam is worn by using a dwell meter in conjunction with a timing light and a set of feeler gages:

Install a new set of points, setting them as accurately as possible to the manufacturer's specified gap. This may take you several tries. Now, start the motor and set the timing to the factory specification. Use a known good (read accurate) dwell meter and check the dwell reading. If it falls within the manufacturer's specification, your distributor cam is OK. If it does not, you need to replace or rebuild your distributor.

Next, use the timing light and observe the timing mark for several seconds. If it is perfectly steady, your distributor is in good shape mechanically. If it moves around, or dances, something is worn in the system. Possible causes of dancing timing include excessive wear in the bearing between the drive shaft of the distributor and the part that supports the rotor (I find this to be the most common problem); a loose breaker point mounting plate (another very common problem - check the pivot where the plate moves under the control of the vacuum advance unit - Note that the early jeeps do not have a vacuum advance); weak, damaged, or missing springs or weights in the centrifugal advance unit; warped or improperly mounted points; or occasionally a worn distributor drive dear or timing gear/timing chain.

It is important to know how the coil and points work to create the high voltage needed to fire the plugs. This happens when the points ****OPEN****. When the points open, the electrical circuit that was supplying current to the primary of the spark coil is interrupted. Without the energy supplied by the current flowing through the primary, the magnetic field in the core of the coil begins to collapse. I will not go into the physics involved, but suffice it to say that the collapsing magnetic filed induces a voltage in the windings of the coil that is trying to sustain the collapsing magnetic field. This induced voltage will be opposite in polarity to the voltage that sustained the current that created the magnetic field in the first place.

This induced voltage is called "magnetic kick." The amplitude of the "kick" is dependant on the *rate* at which the field collapses. The faster the collapse, the higher the "kick" voltage. The speed of the collapse (and therefore the magnitude of the "kick") is highest if there is a very high resistance between the terminals of the windings, and correspondingly is lower as this resistance increases.

There are two windings in a spark coil. The primary winding is composed of relatively few turns of relatively heavy wire, and the secondary winding is composed of a relatively great many turns of relatively fine wire. "Relatively" is used to compare the two windings with each other, not necessarily with any other frame of reference. The exact gage and number of turns is determined by the design parameters of the coil. The Primary winding has its ends connected to the two terminals on top of the coil marked "ign" or "+" and "gnd" or "-". The secondary winding has one end connected to the case of the coil and the other end terminates at the high voltage output where the wire that goes to the distributor cap is attached.

An inductive "kick" is created in both the primary and the secondary windings. Because there are a lot more turns in the secondary, the magnitude of the "kick" is much higher in the secondary. It is this voltage that is used to fire the plugs.

However, provision must be made to deal with the primary "kick." If the primary "kick" is allowed to dump itself across the points as they open, the resulting arc will very rapidly destroy the points! The capacitor is used to store the energy from the inductive "kick" in the primary winding.

When the points close, the capacitor stores an electrical charge at the voltage of the system (about 6 volts). When the points open, the voltage in the primary begins to rise in response to the collapsing magnetic field. This rising voltage is stored inside the capacitor where it is held until the points close again, shorting out the capacitor. Because the energy is bled out from the capacitor over a relatively long period of time when compared to the time it took the magnetic field to collapse, the voltage is lower, and the points are protected.

It is important that the capacitor have the right value to work properly. Too much capacitance, and it will slow down the collapse of the magnetic field, causing a weak spark. Note that both the primary and secondary windings draw the energy for their respective inductive "kicks" from the same source - the collapsing field in the core of the coil. Too little, and it will not be able to store enough of the energy induced in the primary when the field collapses, resulting in poor point life. The exact value of capacitance needed depends on several factors, including the specific parameters in the particular coil installed (inductance, resistance of the windings, etc). I do not change out a capacitor when doing a tune-up if the points are wearing uniformly, as it is properly balanced to that system. on the other hand, I have been known to buy a half a dozen capacitors and try to find the best one by substitution. Yes, it costs some money, but it gets the best performance from the system.

It is possible that the capacitor, points, etc. are working to specification, but you will still have a weak spark. A high resistance short anywhere within the coil itself that could be siphoning off power from that should be going to make a good, hot spark. Also, a short to ground anywhere in the high voltage side can rob a lot of power. Other possible sources of "power theft" include: A cracked or delaminated core in the coil; bad high voltage (plug) wires, including the insulating boots; cracked or moist cap; cracked rotor; or a cracked porcelain on one or more plugs. There is a simple "farmer's method" test to see if you are getting a good, "hot" spark. It was taught to me by my father:

With the engine off, remove a plug wire and insert a short (about 2-1/2" to 3" long) 1/4" bolt into the boot. Push it in far enough so that it is fully inside the connection in the boot. Rest the bolt on the block someplace to short it to ground. Start the motor and carefully move the bolt away from the block, very slowly. WARNING - SHOCK HAZARD! Observe the spark that jumps between the head of the bolt and the block. THE FOLLOWING COMMENTS APPLY ONLY TO A BREAKER POINT IGNITION SYSTEM. A modern computer controlled ignition has a much different characteristic appearance to the spark. All of the tests are done with a warm motor (the battery has had a chance to recharge after starting the motor) with the motor at idle speed...

First, observe the *length* of the spark. Move the bolt away from the block until a spark no longer jumps across. Now move it back towards the block until the spark starts up again. You are looking for the longest spark you can make *that will fire every time.* Eyeball estimate the length as best you can. The 1/4" bolt makes a good reference for you to use.

"Minimum" spark - You should be able to produce a spark ***at least*** 1/4" long. It takes a lot more voltage to cause a spark under the pressures inside the cylinder on the compression stroke than it does in free air, as you are testing. A 1/4" arc represents the minimum voltage needed for good ignition, particularly if the motor is old and pumping oil.

"Average" spark - Most breaker point systems in good shape will throw a spark about 3/8 of an inch. This should be enough to do the job.

"Strong" spark - well optimized breaker point system can throw a spark a half inch or more. I used the "swap the condenser" method to get the best possible spark from a '53 Dodge WM300 "Power Wagon" I use to own. It would throw a spark almost 5/8 of an inch once the generator had sufficient time to recharge the battery.

Now, observe the **color** of the spark. I think it is due to the duration of the spark, or how long it takes the field to collapse to the point where it can no longer sustain the arc.

A "Cold" spark - This type of spark is thin and light blue in color. This is the kind of spark you will see with modern computer controlled motors, which throw a very long, light blue spark. They use a spark of relatively short duration because this type of spark can be controlled more precisely.

A "Hot" spark - A "hot" spark appears orangish or yellowish, and is wider than a "cold" spark. Dad said that it represented plenty of current flowing in the arc. All I know is that any time I saw a Hot spark at least 3/8 of an inch long, the motor would generally start easy and run well with good gas mileage.

There are several reasons why a "weak," "cold" spark would produce the symptoms you observe. Because there is little difference between the engine performance at idle with #3 wire on or off, it is obvious that, while the spark plug may be firing in air, it is not firing hot enough (or what is more probable regularly enough) to ignite the mixture in that cylinder. Variations in compression pressure may allow the other three cylinders to fire with a spark that is too weak to work #3 which would almost certainly have the highest compression pressure of the lot. There may be an air leak in the manifold (check the gasket, and the manifold itself for any cracks, no matter how small) which is resulting in a leaner mixture feeding to #3. It takes a better spark to ignite a lean mixture than it does a rich one. A very small crack in either valve in #3 cylinder could be too small to really show up under the average compression test, but could dilute the mixture enough to make it hard to light with a weak spark. The same holds for the valve seats. #3 could be burning a tad more oil than the rest. Oil droplets in the mixture can make a cylinder much harder to light. A very small crack in the head or the block could be allowing a bit of water to get into the mixture, making it harder to light. Some of the symptoms will be more noticeable when the motor is cold than when it is warmed up. For example, a lean mixture will be easier to light after the motor is warmed. You should do all of your testing with a fully warmed up motor.

Once the motor speeds up, the generator is turning fast enough to produce appreciable output. The additional voltage supplied by the generator will create a spark that is "strong" and "hot" enough to properly ignite the mixture, which is why there is a loss of power when you pull the wire and drive at a higher speed. This is a bigger problem with 6 volt systems, as it takes a good, low resistance circuit to push enough current through the coil to give a good spark. Make sure that all of your connections are tight, all of the wires are good, etc. You may learn something if you use a piece of at least #14 gage wire and connect directly between the "ign" or "+" side of the coil and the positive terminal of your battery. If this clears up your problem, you will need to investigate the entire ignition circuit (wires, ignition switch, etc.) and find and cure the bad spots. Note that higher engine speeds tend to create a more uniform mixture in the cylinders, overcoming problems due to air leaks, etc.

I have a Jeep with a Buick 225 V/6 engine. It also has the Prestolite/Delco distributor. What is thatfunny looking white thing with two wires mounted/hanging on the firewall? What does it do? What is its purpose? Do I really need it anyway?

You are probably talking about the ballast resistor; most are off white in color and can be made of porcelain or some other heat resistant material.

The ignition ballast resistor is wired in series with the primary winding of the ignition coil. The ballast resistor helps regulate the flow of primary current through-out the speed range. At low engine speeds, when the point contacts remain closed longer, the ballast heats and increases in resistance, thereby limiting the flow of primary current (let's say from 12 volts down to 9 volts). At higher engine speeds,when the point contacts remain closed for shorter periods of time, the ballast cools and thereby decreases in resistance to allow more primary current(let's say from 9 volts back to 12 volts) and reduce the fall off in available voltage. During engine cranking/starting, the resistor compensates for the lowered battery voltage resulting from the starter load and permits an increase in primary current( 12 volts), resulting in a higher secondary voltage for starting.

The only test required of the ignition ballast resistor is a continuity check. Characteristics of the ballast produce wide variations in resistance with changes in ballast temperature. Therefore, checking voltage drop across the ballast would/can be misleading.

CAUTION: Never make a connection that connects the ballast across the battery as this will burn the ballast resistor winding.

My Jeep V/6 runs rougher at idle and/or seems to wear/burn out the points quickly. What could be causing this?

The ballast resistor could be bad. Loose or wobbly connections inside the unit can cause intermittent problems. If you are having to replace/re-adjust the point sets every 3-4000 miles or less, this could be an indicator that the ballast is bad. If your Jeep V/6 doesn't have a ballast resistor due to some PO, then purchase one and install it. They were used on Chrysler products as well I believe, so any good parts house should be able to get one for you.

I just installed an HEI distributor in my V/6 Jeep, and the engine seems to run rougher and miss at higher speeds. I have double checked everything and re-read the instructions twice. What could be wrong?

Answer: Although there could be many causes, one often overlooked is disconnecting/by-passing the ballast resistor. The HEI units (most) will require a full 12 volts all the time to function properly. With the ballast resistor left unchanged, the HEI is not getting the voltage it needs. Simply remove the 2 wires from the ballast resistor, and connect them together. It is not necessary to remove the ballast from the vehicle; your choice.

Ballast Resistor: What does all this mean to you?

Answer: The ballast can cause engine running issues at times; they do go bad. If the Jeep won't start, idles poorly, runs poorly, check it out.

Old time Jeepers (me included) always carry a spare ballast along with points and condenser. Cheap fix. In a pinch, on a trail somewhere, you can simply by-pass the ballast by connecting the two wires together, and continue on your way. No damage will occur other than premature wear on the points; so be prepared to file them or change them out, once your trip is over or at your convenience.

How do I install an ammeter?

The AMMETER is simple to hook up, and most of the wiring may already be in place. Here's the way to hook it up:

I assume that you have the standard type of alternator that will put out about 60 amps maximum, and that you have only the average vehicle loads (lights, a radio, etc.) If you have a winch, you will probably benefit from heavier wire. Post here if you do, and I'll try to help. For a standard vehicle setup, you will need to wire your ammeter this way:

You should use at least a number 10 wire (again check the package) for all the connections leading to the ammeter. There will be two terminals on the back of the gauge. They may or may not have any markings on them. You cannot damage the meter if you happen to get the connections reversed, so that is not a problem.

You should run a heavy (#10) wire from one side of the ammeter to the positive post on your battery. Most of the time, this is done by running the wire to the terminal on the starter solenoid that has the battery connected to it. Note that this wire will cause some considerable fireworks if it comes into contact with any part of the body, so make sure that it is well insulated where it runs through any holes. I like to use a short piece of vacuum hose for protection where ever any wire passed through a hole. This should be the only wire connected to this post on the ammeter.

There should be two wires connected to the other post on the ammeter. one will run directly to the "Battery" terminal on the alternator. This should be at least a #10 wire.

There should be another wire connected to the same post on the ammeter as the wire running to the alternator. This wire is used to provide power for the lights and ignition switch. A #12 wire will do, but if you have enough #10 on hand, you might as well use it. The simplest way is to run a heavy (at least #12) wire from the ammeter to the "BAT" terminal on the ignition switch, then run another wire from the "BAT" terminal on the ignition switch to the "BAT" terminal on the headlight switch. Most headlight switches have a circuit breaker mounted on them that has the "BAT" terminal on it. You can run a #14 wire from the other side of the circuit breaker to the break light switch, and the turning signals if you have them.

The horn relay usually does not connect to the ammeter, but is instead connected to the battery.

After you get everything hooked up, turn on the headlights. The ammeter should swing to the "D" or "DISCHARGE" side. If it does not, simply reverse the wires on the ammeter to make it swing the other way.

How do I wire and operate the ammeter?

The customary way to install an automotive ammeter is to have it monitor the current flowing *from* or *to* the battery. The important thing is to not allow your battery to become discharged, or "run down" as my father use to say. The ammeter will tell you whether or not power (current) is flowing into the battery or flowing from it.

You can tell a lot about the condition of a charging system *if* you know how to interpret what your ammeter is telling you. Since you are installing an alternator and its associated regulator, the ammeter should respond like this:

Ammeter Operation

After a large load (for example starting the motor):

The ammeter should initially show a fairly high rate of charge immediately after the motor starts and attains enough RPM to let the alternator put out appreciable current. With most alternators, this happens at *roughly* *about* *approximately* (note my choice of words) 1800 RPM. At that speed, the alternator should be able to put out enough current to supply the connected load (ignition, lights, etc.) and also replace the energy used to start the car.

Due to the chemistry that is going on inside the battery, it will initially accept a goodly quantity of electrons while it replaces the chemistry that was depleted in cranking the motor. This process is not linear with time, and you should see a large ammeter reading right after the motor starts (assuming it's turning fast enough). The exact amount of current shown depends on a lot of factors like how easy your motor starts; the capacity and condition of the battery; the capacity of the alternator; the motor RPM- you get the picture. It is very hard to predict the actual current, but we can talk in general terms. The ammeter should initially show a good rate of charge.

This will diminish over time, and should become very close to zero after one to two minutes, perhaps less, probably remaining at a small positive value (the battery is charging slightly). It should stay that way for the rest of the time you are driving around. This is the state of a properly functioning charging system. The alternator/regulator combination is maintaining the correct voltage (14.5 to 14.7 volts) to hold the battery at the fully charged point, and is able to supply all of the power required to operate the vehicle (lights, motor, radio, etc. etc.). As long as you see the ammeter swing to a good rate of charge then fall off to slightly above zero and stay there, your charging system is in good shape.

Two common charging system problems are failure to produce enough power, and improper voltage regulator action.

Failure to Show Enough Power

The ammeter will not show a strong charge immediately after starting, and/or the needle will show discharge whenever a load is turned on (for example, the headlights). Either of these conditions are a warning that something is wrong in the charging system. It cannot produce enough electricity to replace the chemistry used in starting, and/or supply the demands of the various components. There are several possible problems. The most common ones include:

Defective voltage regulator
Bad connections
One or more parts has failed in the alternator - the brushes and the internal "diode bank" are the usual culprits
A loose or broken fan belt!
Voltage Regulator Set Too High or Failed

This is not a common problem with the modern solid state regulators, but it use to happen fairly often with the old mechanical types. What happens is that the regulator is unable to control the output of the alternator, and it starts to overcharge the battery. This condition is diagnosed when the ammeter shows an unusually high initial charge rate and the needle fails to come almost to zero after driving for a while.

I am installing a Ford alternator and external voltage regulator. How do I wire them?

The Ford external volt regulator has the following harness tabs:

F - This is connected to the "Field" terminal on the Alternator

I - This is connected to one side of the resistor on the firewall. [see below]

A - This is connected to the "Battery" terminal on the alternator.

S - This is connected to the "Stator" terminal on the alternator.

All of the above wires carry relatively little current. A #14 wire (check the package at the parts store) will be plenty heavy for these wires.

Ford Alternator has the following tabs:

Stator - This is connected to the "S" terminal on the voltage regulator.

Field - This is connected to the "F" terminal on the voltage regulator.

Battery - * see below

Ground - There should be a #14 wire from this connection to one of the mounting screws on the base of the voltage regulator.

You will need to make the following connections in addition to the above. The wires may already be in place, but you should check them to make sure the insulation is good and the connections are clean and tight:

From the other side of the resistor on the fire wall: Run a #14 gauge wire to the "accessory" terminal on your ignition switch.

If your switch does not have an "accessory" terminal, use the one running to the ignition coil.

Do not connect to the terminal on the ignition switch that runs to the battery. You will drain your battery when the motor is not running if you do!

Where does the coil get its power from?

In the modern vehicle, there is are two ways that are used to supply power to the coil. I guess I had better give a little background:

The old 6 volt system used a coil that was designed to operate at the normal charging voltage of a generator supported system. This was about 7.4 volts. The coil would function properly with this voltage applied to it. Because many of the early vehicles had a mechanical pedal on the floorboard that actually engaged the starter motor to the ring gear on the flywheel (no Bendix unit) and simultaneously closed a mechanical switch mounted on the starter motor, most early cars and trucks use the following ignition circuit:

A wire runs from the battery to the ignition switch. Another wire runs from the ignition switch to the "BAT" terminal on the coil. That's it - pure and simple.

With the invention of the Bendix drive unit, it was no longer necessary to have a direct mechanical linkage to engage the starter. This meant that it was possible to have a solenoid (relay) mounted someplace that could work the starter. This made it possible to start the motor by either a push button on the dash or a special "start" position in the ignition switch. Because the system was still a six volt one, the coil connections remained the same.

In both cases, there is a real problem with a loss of voltage available to the coil when the motor is being cranked. Depending on lots of factors, it is possible for a six volt system to drop to as little as four volts during cranking. When cars started to switch to 12 volts, a good solution to this problem was available.

The spark coil in a "12 volt" system is designed to operate at about 8.5 to 9.0 volts! Most "12v" coils will get pretty hot, and burn out in a few miles if they are connected the same way the old 6v coils are. They all require a "ballast resistor" in series with the ignition switch. You can buy a "ballast resistor" at any parts store. The are made of a white to off white ceramic material and have a wire wound element in a slot molded in the ceramic. Some parts people will call it a "coil resistor."

Here's how you do the basic hook up a 12 volt spark coil:

|------(Ammeter)----(Ignition)-----(Ballast )-----(Spark)

-------|- ( Switch ) (Resistor) (Coil )

| |

|Battery|

| |

I mentioned that the advent of the 12 volt system allowed a solution to the problem of low available voltage during cranking. This is done by adding a separate circuit that supplies power directly to the coil from the battery during cranking. This power is taken from a special terminal on the starter solenoid. This terminal is designed to supply power directly from the battery whenever the solenoid is in crank mode. The

connections look like this:

|-\----(Ammeter)----(Ignition)-----(Ballast )-----(Spark)

-------|- \ A ( Switch ) (Resistor) /(Coil )

| | \ |-----------| /

|Battery| \ | B|-----------------------------/

| | \ | |

--------- \--(Solenoid)---(Starter)

(Motor )

Where: A = Terminal on solenoid that goes to the ignition switch, and works the solenoid to start the motor. Note that this can have any one of several markings (a number, "S", and "start" are common). B = Terminal on solenoid that supplies the "boost" power to the coil during crank. Again, the markings are varied (a number, "I", "C" and "coil" are common). The wire from the solenoid connects to the same terminal on the coil as the wire from the ballast resistor. Because you have ignition only during crank, it is almost a given that you are only getting the "boost" power from the solenoid, and there is an open in the regular ignition circuit.

Can I use a 6v starter on a 12v system?

First of all, yes, the starter will work as is. Yes, it will get hot quicker, much hotter. Yes, it will work for a while if your motor is well-tuned and you don't have to grind on the starter. BUT there is a potentially *big* problem if the Bendix drive doesn't engage the ring gear on the flywheel.

It's kind of complicated, but when an electric motor is running, the rotating armature (technically the "rotor") has electric current flowing through it which is proportional to the applied voltage and an opposing voltage called the "back EMF" that is generated in the windings by the fact that they are cutting the magnetic field produced by the field coils (the "stator"). A rotor will turn at the RPM required to create a back EMF that is equal to the applied voltage, taking into account the work being done by the rotor. Skipping all the physics, this happens at an RPM that is determined by several factors, the primary ones being the number of turns in the rotor, the current flowing though them, the work being done, and the strength of the magnetic field. If no work is being done, this RPM can be quite high. A properly designed motor can run unloaded at its rated voltage and the various parts of the rotor will be strong enough to withstand the centrifugal forces that are trying to make it fly apart.

If you double the applied voltage, the system will establish equilibrium at whatever RPM it takes to make the back EMF equal the applied voltage, again taking into account any work being done. If the starter is craning the motor, the load will keep the RPM low enough so there is no danger of things flying apart, which is why there is no problem *unless the Bendix doesn't engage the flywheel gear*. If this happens, the motor will run virtually unloaded (only friction and wind drag will supply the load), and the RPM will be quite high - possibly high enough so that the centrifugal force tears the armature apart!

Some of this can be mitigated by design choices. I think that the early jeep motors were what is called "shunt wound," which means that the fields are in parallel with the armature. If that's the case, the increased voltage will cause more current to pass through the field coils, which makes more magnetic field, which means that the armature doesn't have to turn as fast to reach equilibrium. Still, the equilibrium point may be at too high an RPM, and cause the armature to fail. The strength of the field is proportional to the current flowing through the coil, and the number of turns of wire in it. A field wound for 6 volts may not have sufficient number of turns to produce the necessary magnetic flux required to keep the armature RPM at a safe level, even with the increased voltage.

When I converted my jeep over to 12 volts, I took the starter motor down to a good "automotive electric shop" and had it completely gone through (new bearings, brushes, turn the commutator, etc.). When they had it apart, I had them rewind the fields so that they produce the proper magnetic field at 12 volts that will keep the unloaded RPM within safe limits. It will not keep the armature from overheating if I crank for too long, but it will keep it from flying apart if the bendix doesn't catch. They charged me $35 extra for rewinding the fields. I feel it was well worth it.

When I go about 1/2 mile, engine shuts down. Wait 5 minutes, she will start again. Sounds like coil overheating?

Five possible sources of this behavior come to mind.

A defective coil. It could have an open winding situated so that, as long as the coil is cold it is touching and current can flow. When the coil heats up (they all do in normal operation), CTE causes the innards to move around, opening up the winding. When the coil cools down, the CTE reverses direction, moving the winding so the circuit is closed.
A defective capacitor inside the distributor. This use to be a major problem when automotive capacitors were made with transformer oil and paper. I have not had this problems for many years, but a bad capacitor was a real possibility as late as the early 60's.
A defective series dropping resistor. As in the case of the coil, it could have a broken wire that opens the circuit when the resistor heats up.
A defective ignition switch.
A fuel starvation problem.

The next time your motor quits, leave the ignition switch on and measure the voltage at the coil terminals (both sides). This could tell us a lot...

How do I wire a 'one wire' alternator as a replacement for an regulator? I have a 64 cj5 that had the regulator replaced with a Delco alternator but the wires were not connected when I bought it; I'm running off the battery now. Thanks

My one wire alternator goes from the alternator thru the amp gauge to the battery post on the starter.

Jeep won't start when hot. Wait 5 minutes, she will start again. Sounds like coil overheating?

Five possible sources of this behavior come to mind.

A defective coil. It could have an open winding situated so that, as long as the coil is cold it is touching and current can flow. When the coil heats up (they all do in normal operation), CTE causes the innards to move around, opening up the winding. When the coil cools down, the CTE reverses direction, moving the winding so the circuit is closed.
A defective capacitor inside the distributor. This use to be a major problem when automotive capacitors were made with transformer oil and paper. I have not had this problems for many years, but a bad capacitor was a real possibility as late as the early 60's.
A defective series dropping resistor. As in the case of the coil, it could have a broken wire that opens the circuit when the resistor heats up.
A defective ignition switch.
A fuel starvation problem.

The next time your motor quits, leave the ignition switch on and measure the voltage at the coil terminals (both sides). This could tell us a lot...