Historic Military Vehicle Ignition Systems Part I

by Steve Turchet

The Basics

Some speak of the “graying” of the historical military vehicle (HMV) hobby. While it is true that many HMV enthusiasts have been collecting, restoring and driving their vehicles for decades, my experience and the letters I receive indicate that many younger people are joining the ranks every year. For many of them, vehicles with breaker-point distributor ignition systems are something new, since most of the civilian vehicles they grew up with had “breakerless” and “HEI” electronic ignition systems, which first began to appear in the 1980s. Consequently, there are a lot of younger people today who’ve had no experience with mechanical point systems... until, perhaps, they acquire their first vintage HMV. For them, this two- part article may serve as a primer, though “old dogs” may also learn a few new tricks.

REVIEW: INTERNAL COMBUSTION ENGINE

To begin at the beginning, an internal-combustion engine is so-called because combustion – the burning of fuel to produce power – takes place within the engine itself. This is different from an external-combustion engine where the burning of fuel takes place outside, usually in the firebox of a boiler, producing steam which powers the engine.

In an internal-combustion engine, some sort of volatile fuel, along with air – which provides oxygen for the fuel to burn – is introduced into the combustion chamber at just the right time. This can either be done through a carburetor (another archaic item to many younger people), which introduces both fuel and air at the same time, or by a fuel-injection system, which works about the same way in either a gasoline or a diesel engine, injecting a measured amount of fuel into the cylinder, while air is brought in throughintake valves... as in a carbureted engine.

On engines powered by gasoline, diesel, or Multifuel (that most people will encounter in common HMVs), this air is drawn in by the vacuum created when the engine’s pistons go down on the intake stroke. On a gasoline engine with a carburetor, air and fuel are drawn into the cylinder together, and have already been mixed by the carburetor in the right amounts for proper combustion, engine load, and RPM. In a turbo-charged or super-charged engine, whether gas, diesel, or Multifuel, air is blown in by the turbo or super-charger... but that’s another article.

To illustrate, the piston of a basic four-stroke engine has just gone down on its intake stroke, pulling in fuel and air. It’s now on its way back up on the compression stroke. It is at this point where it becomes necessary to ignite the fuel/air mixture to produce the power to push the piston down again. (It should be noted that there is only one stroke out of every four that produces power in either a gasoline, diesel, or Multifuel engine of this type.)

In a diesel or Multifuel engine, the compression ratio (how much the air in the cylinder is compressed by the ascending piston) is very high. As this air gets compressed, it becomes so hot that if fuel is injected into the cylinder at the right time, the heat of the air alone will cause it to ignite and burn (note the word, “burn” instead of “explode”). The expansion of this burning fuel creates the energy (pressure) needed to force the piston down on the power stroke. But this applies to a diesel or Multifuel engine.

Since this seems like a very simple system, you might wonder why we’re not all driving diesel-powered vehicles today? Some would say that Henry Ford was partly responsible, because he began mass-producing cars and trucks with gasoline engines. But another factor is that for a diesel engine to withstand the extremely high internal pressures needed for heat combustion, it must be built more massive than a gasoline engine to produce the same amount of power. For example, the Dodge 230 in an M37 runs quite well with a cylinder compression of 80 psi, but 500 psi is common for diesel engines. Simply said, the power-to-weight ratio is lower for a diesel engine than for a gasoline. With today’s technology, however, this gap keeps shrinking.

Engine mass and weight is an important factor for cars and small trucks, so the gasoline engine was a more practical choice than a diesel in those early automotive days. Though, as stated, that gap has narrowed considerably.

Back to business: The piston of a basic gasoline engine is on its way up on the compression stroke squeezing the fuel and air mixture into a smaller and smaller space. The mixture is getting hot: however the compression ratio of a gasoline engine isn’t high enough to make it ignite by itself. You obviously can’t light a match and drop it into the cylinder, but why not fire off an electric spark to ignite the fuel-air mix and force the piston down again?

TWO TYPES OF IGNITION

There are two basic types of ignition systems found on most vintage HMVs: The magneto system and the battery system. The magneto system was the first to be invented, in the late 1800s, and was used in most WWI vehicles. It’s the simplest of the two, and is also usually the most trouble-free. It is still in wide use in everything from airplanes to lawnmowers, leaf-blowers, chain saws, outboard motors, ATVs and snowmobiles. The main advantages of a magneto system are that it has fewer components than a battery system (which is desirable when weight is a factor) and is usually self-contained... meaning it needs no battery to operate. It has another advantage, too, which we’ll learn about later.

The magneto system was a logical choice for early cars and trucks because these vehicles didn’t have any electrical accessories, including starters, or even lights. Therefore, they didn’t need batteries, or generators and voltage regulators to keep a battery charged. Magneto systems were also used on farm tractors, construction equipment, stationary, marine and aircraft engines, which either didn’t need electrical accessories and/or lights, or where the added weight of a battery and generator was a disadvantage.

A magneto produces its own electric power, and this power is boosted by a coil (more on ignition coils later) up to the high voltage – around 15,000 to 20,000 – needed to fire the spark plugs. A magneto can also be built into an engine’s flywheel, as in outboard motors, lawnmowers and such. Note that, just like the distributor in a conventional battery ignition system, whether civilian-style or waterproof military, a conventional magneto has breaker points and a condenser (capacitor) in addition to a coil. These components wear out, and the points need to be cleaned and reset or replaced at intervals – just as in a conventional battery ignition distributor.

A minor disadvantage to the magneto is that since it generates its own power, it must be turning relatively fast in order to produce a spark at the spark plugs and start the engine. For small engines, such as outboards, chain saws and so forth, this isn’t a problem because a quick pull on the rope or starter cord spins the magneto fast enough. But anyone who has ever tried to use the emergency crank on their military truck (or the starting bar on a DUKW), knows that you can’t spin a large engine very fast by hand.

This problem is solved in one of two ways: the first is to use what’s called an Impulse Magneto. These have an internal spring mechanism which “loads” and then “fires” while the engine is being cranked, spinning the magneto fast enough, and for just a second, to produce a hot spark. You may have noticed in old films that when airplanes are started by someone on the ground. They don’t spin the propeller like a windmill, but instead find the position where the magneto is “loaded,” then give the propeller a quick pull to “fire” it. So, too, on many old trucks, farm tractors, or industrial engines: You don’t stand there cranking as if hoisting a bucket out of a well. Instead, you find the position where the magneto is loaded, then give the crank a fast pull.

After the engine has started, the impulse mode is automatically disengaged and the magneto operates normally.

The second way to overcome this problem is with a small booster battery, such as the dry cells used on early Model-T Fords and many WWI military trucks. These batteries are switched on while the engine is cranked. They provide enough current to fire the magneto’s coil, producing a spark at the spark plugs, until the engine is turning fast enough for the magneto to generate its own power.

Of course, if the engine has an electric starter, this can often spin it fast enough to fire a conventional magneto. Or, an impulse mag can be used. But, if a vehicle has an electric starter, it also needs a large battery to power it,plus a generator to keep the battery charged. And if it has these things, the battery and generator can also be used to power lights and other accessories. And, since there is now plenty of electric power provided by the battery and generator, why should the vehicle need a self-contained magneto ignition system? Why not use a system that runs off the battery? And that is how the battery-ignition system came into being. For many years, though, cars and trucks retained the magneto ignition system and only used the battery and generator for starting and lights. Some large trucks and luxury cars even had both a magneto and a battery ignition system.

You might say that the magneto system became obsolete on most automotive vehicles. Not because it was inferior to battery ignition, but because such a self-contained system was no longer needed.

Actually, a magneto system is superior to battery ignition in several ways. One, it doesn’t usually need a battery to get the engine started. This is a nice feature when out in the bush or during combat. Secondly, a conventional battery ignition system “peaks” at high engine RPMs. In other words, it can only deliver a certain amount of voltage to the spark plugs at any time, and voltage requirements increase as engine RPM rises. On the other hand, a magneto’s output keeps on increasing the faster it spins, which is why many race cars used them.

A lot of collectable military gear, such as generators and pumps, have magneto ignition systems, but since most common HMVs have conventional battery systems, we’ll now move on to those. Despite outward appearance, there isn’t much basic difference between the civilian-type 6- and 12-volt ignition systems used on WWII HMVs and the M-series type 24-volt waterproof systems, except that the waterproof systems are sealed and therefore harder to work on and troubleshoot. Also, most of their components won’t be found at a neighborhood auto mart store.

Distributors are the hearts of these systems. In many ways, they’re like magnetos, in that they have breaker points, a condenser (capacitor), and use a coil to boost the vehicle’s low voltage – 6, 12, or 24 – up to the 15,000 or 20,000 volts needed to fire the spark plugs. They just don’t generate their own power to do so, but use the vehicle’s battery instead.

Like a magneto, a distributor is usually turned by the engine’s camshaft in some way (which, on a four-stroke engine, rotates at half the engine’s crankshaft speed). And, like a magneto, a distributor must be “timed” so that it fires each spark plug in each cylinder in the correct order and at just the right moment to ignite the fuel and air mix and produce the most power in the most efficient way.

For most car and truck engines, including those of most common HMVs, this timing must change with engine speed. In other words, the time at which a spark plug fires must change as engine RPMs increase.

Why? Because it takes time for the fuel and air mixture to ignite and burn. And it takes the same amount of time to burn,no matter if the engine is idling at 500 RPM or roaring down the highway at 3,000.

CENTRIFUGAL ADVANCE MECHANISMS

But, as engine RPM increases, there is less and less time for the intake valves to stay open and the fuel and air mix to be sucked into the cylinder. Therefore, most distributors (and some magnetos) have centrifugal advance mechanisms. These advance the spark timing as the engine’s RPM increases. In other words, the fuel and air mix is ignited and starts burning sooner as the piston comes up on its compression stroke at high RPM than at low RPM. This gives it more time to burn completely, expand, and push the piston down again.

And remember, this fuel and air mix does not explode instantaneously (at least it shouldn’t). Instead it burns in a wave that sweeps across the cylinder, beginning where the spark occurs from the spark plug. Of course, this burning is pretty fast, but it does take time.

If the spark occurs too late, then not all the fuel gets a chance to burn and the engine isn’t operating efficiently. On the other hand, if the spark occurs too early, the engine is fighting itself, because the already burning fuel is trying to stop the piston before it reaches the top of its stroke.

Up until around the mid-1930s, many cars and trucks did not have a centrifugal advance built into their magnetos or distributors. You may have seen two small levers, usually located on or below the steering wheels on antique vehicles. One lever is the throttle, the other is the spark advance; and it was up to the driver to choose the best spark timing for engine speed, vehicle load, climbing hills and starting.

A modern distributor’s centrifugal advance is automatic and was calibrated at the factory for the type of vehicle in which the distributor was installed. Thus, a distributor meant for the 230 engine of a Plymouth car will not have the same centrifugal advance as a distributor meant for the same engine in a Dodge WC. This setting is controlled by the tension of springs, and unless you are really knowledgeable it’s usually best not to mess with it. These springs are simple to replace, but if the wrong ones are installed, they may degrade the engine’s performance.

Now we know that distributors not only fire an engine’s spark plugs, and in the right order, but they fire each plug at just the right time. We also know that distributors change the timing (when the spark plugs fire) as engine RPMs increase or decrease. These are the three basic functions of a distributor.

VARIABLE TIMING

For many vehicles, this is enough for efficient operation. However, an engine can be even more efficient if its ignition timing can also be changed to better suit the load it’s under. For example, a truck cruising empty at 50 mph on a level highway, can make better use of its fuel if the spark plugs fire at a certain time. We needn’t go into details of throttle settings and manifold vacuum in this article, except to say that manifold vacuum in this situation would be high because the throttle (butterfly) valve in the carburetor is only partly open.

On the other hand, this same truck, traveling at the same speed on the same stretch of highway, but heavily loaded, could benefit from a different spark timing. Why? Because although engine RPMs are the same, there is a much heavier load on the engine. The throttle is open a lot wider because more fuel and air are needed to maintain the same road speed and RPMs than if the truck were cruising empty. And manifold vacuum is low. But, since the centrifugal advance is controlled by engine speed, the distributor can’t “know” that the engine is now under a heavy load so a different spark timing would be better.

THE VACUUM ADVANCE MECHANISM

As mentioned, up until the mid 1930s, it was usually left up to a vehicle’s driver to choose the best spark timing for different speeds, loads and road conditions. Of course, there were people who could never learn to do this right or pay enough attention. So, their engines suffered.

As a result, the vacuum advance mechanism was invented. It didn’t replace the centrifugal advance, it merely supplements it to make the engine more efficient.

A vacuum advance is basically a little diaphragm control, which is linked to the distributor’s breaker plate (where the ignition points are mounted). In this type of distributor, the breaker plate is movable. A hose or tube runs from the diaphragm chamber and is usually connected to the carburetor just above the upper edge of the throttle valve, at the position where the valve is closed. This is so there is no vacuum advance when the engine is idling (and manifold vacuum is high).

Such a setup allows the vacuum advance to override the centrifugal advance. In situations where the engine is turning at high RPM, themanifold vacuum – instead of engine speed (or an inexperienced driver) – decides the best time for the spark plugs to fire. Some vehicles (most notably 1940s and 1950s GM cars and trucks) accomplished vacuum advance by having the diaphragm mechanism rotate the entire distributor instead of a movable break plate.

THE REST OF THE SYSTEM

So, we find that most modern distributors perform four functions: Not only do they fire each spark plug in the correct order, they fire the plugs at just the right time. Additionally, they advance or retard the timing to better suit the engine speed as well as changing the timing to better suit the engine load. Most waterproof distributors on common M-series vehicles – jeeps, M37s, etc. – do not have a vacuum advance but rely on centrifugal advance alone.

We now know what magnetos and distributors do. Both have breaker points which cause an ignition coil to produce a high voltage spark from the vehicle’s (or magneto’s) low voltage system. We could discuss how a coil works, but since it’s one of those things like a light bulb that most of us can’t repair, let’s say that it either works or it doesn’t... and if doesn’t work it must be replaced.

Both devices have a “rotor” and a “cap.” The rotor is mounted to a shaft that turns at half engine crankshaft speed, and this rotor lines up with electrodes inside the cap at the same time the breaker points open to fire the coil. The spark plug wires – whether civilian-style or waterproof M-series – carry this high voltage current from terminals on the cap to each spark plug. The spark plugs spark, and the engine usually starts. After this happens, it’s entirely up to the distributor or magneto to decide the best time for the plugs to fire... unless there are two small levers on your steering wheel.

FIRING THE SPARK – HOT OR COLD?

Why is such a high voltage needed to fire the spark plugs?

For one thing, the amperage is low since power only began with 6, 12 or 24 volts. This is fortunate, because if you get “bitten” by the spark while working on your engine, you won’t be electrocuted... though you can be so surprised that you fall off your stool or stick your hand in the fan, so be careful. Secondly, because of the high pressures within the cylinder, it takes a lot of volts to make a spark jump the gap at the spark plug.

In addition to the fact that voltage requirements increase as the engine’s RPMs rise, they also increase as spark plugs get dirty and their electrode gap burns wider. This is why you should clean and reset them at regular intervals.

Voltage requirements also increase as spark plug wires age, or get dirty, oily or wet. You may have found that cleaned spark plugs seem to get dirty again faster than new ones. The reason is that their electrodes and insulating porcelain become rough or pitted so carbon and combustion by-products accumulate on them more quickly. This doesn’t mean that you shouldn’t clean and reinstall them, just don’t be surprised if they get dirty again faster.

You may have heard about “hot” and “cold” spark plugs. Some people think this refers to the electric spark itself, so they naturally want the “hottest plugs.” Nope. What this means is that the spark plug itself runs hotter or colder. It has nothing to do with how “hot” or “cold” the spark is.

A hot spark plug has a longer lower body, so it takes more time to dissipate heat. The heat has to travel farther. Inversely, a cold spark plug has a shorter lower body.

Unlike civilian-style spark plugs, there isn’t a wide range of choices of waterproof plugs for M-series HMVs, but generally speaking – and assuming an engine is in good condition and not burning oil – you should use hotter plugs if you drive only short distances, and/or in low speed stop-and-go traffic (parade vehicles are example). Cold plugs will foul-up faster in this kind of service.

On the other hand, if you use your vehicle for long highway trips, you will usually benefit by using colder spark plugs, because hot plugs may burn out or even melt. However, if your engine is worn and burning oil, hot plugs may serve longer before they foul. Trial and error is often the best way to select the right spark plugs for your own vehicle and how it’s used.

THE CONDENSER (CAPACITOR)

We have just about covered – or at least mentioned – all the major components of a conventional battery and/or magneto ignition system. We’ll get into breaker points, their function, settings and replacement in Part Two. But, before ending this episode, let’s look at what seems to be the most misunderstood or mysterious item in either of these systems. This is the condenser (also called a “capacitor”).

A lot of people know that a condenser should usually be replaced along with the points when doing a tune-up. But they still don’t know why it’s there or exactly what it does. Will your engine run without it? Possibly... but only for a very short time.

While we didn’t go into detail of how an ignition coil works, it should be known that the high-voltage current to fire the spark plugs is produced by the sudden collapse of an electromagnetic field within the coil. This collapse is triggered when the breaker points open. In other words, the breaker points act like a switch, constantly opening and closing as the distributor cam rotates, which allows the coil to build up a charge (when the points are closed) and then firing that charge to the spark plugs when they open. Naturally, the rotor decides which spark plug will fire by directing the charge to the proper terminal on the distributor or magneto cap.

However, due to the way an ignition coil works, there’s a tendency for the breaker points to arc as they open. (If you’re curious, this is caused by the counter-voltage produced when the coil’s electromagnetic field collapses.) Although the breaker point contacts are made from hard steel alloys, this arcing will quickly erode and burn them.

What a condenser does is momentarily store this surge of counter-voltage and keep the points from arcing. You could say that the condenser acts like a shock-absorber for the points. By the way, momentarily storing the counter-voltage also makes the coil more efficient.

An engine might run without a condenser, but the points would burn out very quickly. In fact, burned or badly eroded points are almost always a sign that a condenser is failing or has failed (or you have the wrong condenser for your vehicle).

It’s possible for a condenser to outlast several sets of breaker points; and in theory they should be tested at tune-up time and only replaced if they need to be. But in practice condensers are usually replaced along with the points. But don’t throw the old one away. Instead, keep it in your vehicle, along with your other old but still usable ignition parts. Doing this has saved me from several long treks on foot through the desert, and from having to spend more than one icy night by the side of the road.

We’ll go into the service and repair of ignition systems in Part Two.