Transfer Case Basics

by Steve Turchet

Although this article is intended as a primer for transfer cases in general, it will focus mainly on the Dana Spicer Model 18 as found in most U.S. military jeeps, including the WWII Ford GPW and Willys MB, as well as the M38, M38A1, and M606. The Dana Spicer 18 was also fitted to Willy/Kaiser civilian jeeps, such as the CJ2, CJ3, CJ3B and CJ5, as well as used in four-wheel-drive Willys pickups, station wagons,and the FC-150 and FC-170 Forward Control vehicles. Additionally, it was used many foreign-licensed Jeep models, including those built by Mahindra Jeep in India, and Mitsubishi Jeep of Japan, as well as in many International Scouts.

The Spicer Corporation was founded in 1904 by Clarence Spicer, who was soon partnered with Charles Dana. The Dana Spicer Corporation manufactured automotive powertrain and chassis components through the most of the twentieth century.

THE DANA SPICER MODEL 18

Designed in 1940, the Dana Spicer Model 18 was in production for thirty years. Even after it was finally updated to the Model 20, it remained basically the same, making it one of the most successful vehicular components ever made. Even today, all of its internal parts are still being manufactured and are readily found anywhere in the world.

When viewed as a whole, the final design was somewhat elegant for a piece of machinery—almost having the same of 1930s-look of a Douglas DC-3 airplane. Like that famous aircraft, itwas massively overbuilt and astonishingly durable.

Throughout the years of production, the only significant changes were increasing the size of the intermediate gear shaft, which was originally 3/4” in WWII and early CJ2 jeeps, then increased to 1-1/8”, and finally to 1-1/4”. The main reason for the changes was to provide more surface for the intermediate gear’s roller bearings.

TRANSFER CASE BASICS

To begin at the beginning, let’s answer the question, “What is a transfer case?”

I bought my first jeep, an M38, when I was 16—back in the day when jeeps were so rare that their drivers waved and smiled at each other out the road. I had taken it for a lube job, and the gas station mechanic—who couldn’t understand why anybody would want such a weird vehicle—told me a transfer case was, “...another transmission and something else to go wrong.”

One could indeed say that a transfer case is another transmission in the sense that it transmits engine power to the driving wheels. A conventional manual transmission in a rear wheel drive vehicle transmits engine power to the rear axle only, while giving the driver a choice of gear ratios, usually three or four in forward and one in reverse. However, for an all-wheel drive vehicle such as a jeep or an M37, a transfer case is needed to also transmit engine power to the front axle. On many small vehicles (such as jeeps), the transfer case is bolted to the rear of the transmission. On most larger vehicles, the transfer case is a separate unit mounted behind the transmission and coupled to it by a short drive shaft.

A single-speed transfer case, such as those used in the half-ton WWII Dodges and many three-quarter ton Dodge carryalls, is basically a directional gear box that transfers power from the engine after it leaves the main transmission to all of the vehicle’s wheels. Except for the White Scout Car, it also gives the driver a choice of powering only the rear wheels, or both the rear and front. If the main transmission is a 4-speed, there are still only four gear ratios forward and one in reverse, either in or out of four-wheel-drive.

You might ask, “ Wouldn’t it be better for a tactical military vehicle to always be in four-wheel-drive?” The answer, “Yes,” but the problem with powering a vehicle’s front and rear wheels on a full-time basis is that they turn at different speeds. For example, when going around a corner, the outside wheels have to travel farther—make more revolutions—than the inside wheels. That’s why axles have differentials. This works fine if only one axle is powered. The differential can compensate. But, if two or more axles are powered together, and especially if one is a steering axle, they fight each other. Why? Because each of the vehicle’s wheels is turning at a different speed. This also happens to some extent while traveling in a straight line, since the driver is always making corrections to keep the vehicle on course. In addition, tires are seldom precisely the same size, and almost never the same size if of different ages, states of wear, or if made by different companies. Tire size may also differ due to unequal inflation. A a taller tire rolls more slowly (makes less revolution) to travel the same distance as a smaller tire.

This isn’t much of a problem for a four-wheel-drive vehicle as long as the tires can slip a little, such as in mud, gravel, dirt or snow. However, if the vehicle is used on dry pavement it’s difficult for the tires to slip, and this puts a strain on the whole drive system from the transmission and transfer case to the drive shafts, differentials and axles. It also rapidly wears the tires, and doesn’t improve gas mileage either. A modern full-time four-wheel-drive system has what amounts to a differential built into the transfer case, which compensates for these different wheel speeds, but for most vintage HMVs and civilian 4x4s never engage front-wheel-drive on hard dry surfaces where the tires can’t easily slip.

Early M35 trucks had an automatic-engagement transfer case, and we’ll get to those later. But on most WWII and smaller M-series vehicles such as jeeps and MUTTs, the driver had a choice of manually selecting either rear wheel drive or all-wheel-drive.

As mentioned, with a single-speed transfer case there are only the gear ratios available from the main transmission, either in or out of all-wheel-drive. Most of the time this was sufficient: the WWII Dodges had a very low first gear which usually gave them enough power to crawl through mud or climb steep grades. Still, there were times when an even lower gear was needed: When the trucks were heavily loaded or towing trailers and had to get through swampy terrain, across a river, or up a mountain.

INCREASING POWER

There are several ways to increase a vehicle’s useful power: You can give it a bigger engine—but then it may need a stronger transmission and heavier axles. You could give it a lower axle gear ratio—but that will decrease the vehicle’s top speed. You could give it a transmission with an extra-low first gear—but most of the time, a gear that low would only be needed in four-wheel-drive. So, why not put a lower gear into the transfer case? Then, if the main transmission is a 3-speed, as in most jeeps, the vehicle will have six gear ratios forward and two in reverse. A four-speed truck, like a Dodge WC or M37, now has eight gear ratios forward plus two in reverse, while a CCKW or DUKW with a 5-speed transmission has ten gear ratios forward plus two in reverse.

Most 2-speed transfer cases, such as the Dana 18 and other units used in larger vehicles, have a lockout so that the lower gear ratio can only be used when the vehicle is in all-wheel-drive. This is to prevent excessive torque loads on the rear axle(s). As with most safety features this can be bypassed (very simply in the Dana 18), but there is little practical use in doing so. Furthermore, bypassing runs the risk of overloading the rear axle.

TWO TYPES

Transfer cases can be grouped into two basic types: Those that mount on the main transmission, as on most jeeps, and those that are separate units, such as on M37s, M715s, and M35s. Transfer cases may also be subdivided into gear-drive types (as in jeeps and most common HMVs) and chain-drive models. While the latter are often used in modern trucks and SUVs, they were also used on many old-time, large, all-wheel-drive trucks such as those made by Oshkosh, Coleman, and FWD.

COMMON QUESTIONS

By now, youshould understand what transfer cases are and what they do. As with any component on your HMV, if you have major work to perform or want to know how to service it right, get a manual. Here are some answers to often-asked transfer case questions, and various trouble-shooting hints:

Why is a vehicle’s parking brake usually on the transfer case and not on the main transmission?

Since most 2-speed transfer cases have a neutral position, a parking brake on the main transmission could not keep the vehicle from rolling if the transfer case was shifted to neutral.

Why does the speedometer usually drive off the transfer case and not the main transmission?

If it drove from the main transmission, it wouldn’t read correctly when the transfer case was in low-range.

Why should the transfer case and the transmission both be in neutral when towing a vehicle?

In some vehicles, the rear bearing in the main transmission doesn’t get oiled if towed with the transfer case in gear. You should consult a manual to be completely sure of the correct procedure before beginning a tow.

Is it okay to shift into four-wheel-drive with the hubs unlocked?

It is fairly harmless for a little while, but why would you want to?

Always make sure that both your hubs are fully locked or fully unlocked. It’s not going to be an instant disaster, but never drive your vehicle with one hub “in” and one hub “out.” And check them every so often, because inquisitive parking lot fingers will sometimes turn them.

Incidentally, it’s usually easier to lock your hubs before you shift into four-wheel-drive. If the hubs won’t lock, just move the vehicle an inch or so. The hubs have gear teeth inside and sometimes those teeth don’t mesh at first.

If you’ve been off-roading, and the hubs won’t unlock, just back the vehicle up a little. This relieves built-up tension on the axles and drive train.

Unless a hub has been damaged, only use your fingers to turn their mechanisms. Don’t try to force them with wrenches or pliers.

LET IT WHINE

Yet another question, and one asked more frequently these days by younger HMV enthusiasts, is about transfer case noise. Most of us over 50, who have had experience with older jeeps or other all-wheel-drive vehicles, know that transfer cases whine. This is due to the design where three gears are running together. (Chain-drive transfer cases have their own special sounds.) But a whine, not a scream, is normal, and there’s usually nothing wrong. The sound will generally rise in pitch and volume as the speed of the vehicle increases and the transfer case oil warms up and thins out.

Speaking of oil, never use heavier oil than your manual specifies to quiet down a transfer case. As stated, some howl or whine is normal. For the Dana 18 especially, synthetic gear oil will usually provide maximum gear and bearing life, as well as quieter operation.

Loose transfer case control levers will transmit the whine and make it sound louder. Grasp the levers as you’re driving and see if this quiets things down. If so, figure out how to make the levers tighter.

On jeeps or other vehicles with Dana 18 transfer cases, the anti-rattlers or lever springs are sometimes broken or missing, so check and replace if necessary. One HMVer solved his jeep’s noise problem by simply stuffing a piece of foam rubber between the levers,

You can try replacing the control lever bushings on other vehicles. Loose linkage rods, bell-cranks, and worn clevis pins will also amplify transfer case whine. Also check the transmission hump or cover on the vehicle’s body to see if it’s loose, which may amplify whine.

Shift levers that touch the transmission hump or cover, sometimes unseen under the dust boot or bellows, will likewise amplify noise. New engine, transmission and transfer case mounts may lessen the noise, but there’s really not a lot one can do... those three gears running together just naturally give off a whine.

The Dana Model 20 resolved this by moving the rear drive shaft output from the right to the center. Besides being touted as a “silent transfer case,” it raised the rear output shaft above the level of oil inside and eliminated a potential leak source.

HOW STRONG IS IT?

How strong are transfer cases? In most applications, the Dana 18, if treated well, will outlast the transmission—often two or three of them in early jeeps. And if treated well, they are capable of handling much larger engines than those of most stock jeeps.

Which is better, a single control lever or the two “old-fashioned sticks?” Most serious off-roaders prefer twin sticks. The best single lever control I ever saw was on a ‘72 Scout II. The worst was on a ‘65 Jeep Gladiator. You can convert most single stick models to twin-sticks, and there are kits available.

What about automatic transfer cases, meaning those which engage the front axle without the driver’s effort or consent? These are most commonly found on early M35s, both gas and multifuel. There is no computer involved. They are based on a Sprag clutch principle: When the rear wheels start turning faster than the front, such as when spinning in mud or snow, the clutch engages the front axle drive. Then, as soon as all the wheels are turning at the same speed again, the clutch disengages and only the rear wheels are powered. There are a few situations where you might not want this to happen, but these transfer cases are long-lived and pretty much trouble-free, provided you lube and service them properly.

They are sometimes prone to clutch windup, which may unexpectedly spin a front wheel if it’s jacked off the ground. The best way to prevent this is to never let the vehicle roll backward with the transmission in a forward gear.

The GMC M135/M211 and variants used a single-speed transfer case and the low range was in the automatic transmission. This setup worked well, as long as things were kept adjusted.

WATCH FOR THIS

One of the biggest “little problems” youmay encounter with the Dana 18 is with the output shafts to the front and rear axles. They are located below the oil level. This puts more strain on the oil seals. And because no rotating seal is perfect, some oil will usually leak out. It’s usually not serious on the front drive shaft, but leaking oil from the rear output often goes on the parking brake, which then may not hold.

The seal isn’t hard to replace, but you should also check that the sealing surface on the companion flange doesn’t have a groove worn into it. If so, you should replace the flange (not expensive) or get a “Speedy Sleeve.” Putting silicone sealant on the output shaft splines before installing the companion flange will also eliminate a potential leak. You

should check the flange nut for tightness every once in a while. A loose flange will wobble and wear out the seal. Most of the above tips also apply to other types and sizes of transfer cases.

Always read your vehicle’s manual, but most common HMVs use the same oil in their transfer case as in the transmission and differentials. One exception is the Dodge M880 which uses the same oil in the transfer case as it uses in the engine.

If your transfer case is attached to the main transmission, check the bolts for tightness, from time to time. Likewise, check the mounting bolts on separately mounted transfer cases, as on M37s and M715s.

Never try to force the shift levers either into or out of gear. If they seem to be stuck—and you know, for a fact, they’re not just rusted or packed with dirt—simply move the vehicle a little and they should slip into place. If the vehicle itself is stuck and can’t be moved, put the main transmission in gear with the engine running and gently let up on the clutch while keeping a hand on the transfer case lever. Itshould slip into gear. A transfer case that repeatedly jumps out of gear generally needs a rebuild—assuming the shifting mechanism is properly adjusted and the detents are working.

The Dana 18, besides being one of the best transfer cases ever made, is also one of the simplest and easiest to rebuild and repair at home. There are many web sites devoted to this job, most with good (though sometimes conflicting) advice, so there is no use in repeating it in this article; but here are a few tips based on my experiences:

*In most cases, no special tools except a good pair of snap ring pliers are required. You should have, or make, a brass rod (I use a piece of a boat propeller shaft) to drive out the intermediate gear shaft and other such tasks. Never hit the shaft or other parts directly with a hammer. They may chip.

*The intermediate shaft drives out through the REAR of the case! The shaft is slightly smaller at the front. Trying to drive it out through the front may crack the case.

*Check the case for cracks around the intermediate shaft bore. If there are cracks, it usually indicates that someone did try to drive the shaft out through the front. In civilized conditions, such cracks are not considered repairable, even thoughthe usual advice is to get another case. Under less civilized conditions, youcan try welding or brazing.

*Check the intermediate shaft for wear or galling. Most shafts (especially the 3/4” shafts) on vehicles that are driven frequently will require replacing. While there are replacement kits available that include a new shaft, bearings, and bushings, many such kits (including some with “USA”-stamped shafts) have soft shafts that will quickly wear out. Buy these kits from a reputable source.

*One way to tell if a shaft is too soft is to take a sharp punch and a medium-sized hammer, and try to made a dimple at one end of the shaft (the section that mounts in the case, not the bearing surface). Tap the punch smartly with the hammer—once. A good shaft won’t dimple.

*You can upgrade early Dana 18s to accommodate larger intermediate shafts, though in most cases there is no need to go above 1 1/8”. An easy way to install the intermediate gear, shaft and bearings is to make a dummy shaft (I found an old hardwood chair leg that was the perfect size).

*Use bearing assembly grease to hold the rollers in place. If youuses wheel bearing grease, it may not dissolve in the gear oil and could clog the oil hole for the intermediate shaft bearings. Vaseline will also work, and will stick better if put in a fridge overnight before using.

*Replacing the Dana 18’s intermediate shaft and bearings can be done with the transfer case in a vehicle, but it’s usually a test of one’s patience, dexterity and threshold of rage. The parking brake assembly must be removed before removing or installing an intermediate shaft.

*Don’t use a magnet to handle or fish out the roller bearings. They will become magnetized themselves. This will cause them to skid instead of rolling. Don’t mix old and new rollers(unless you’re out in the wilds and have to) because the worn rollers won’t carry their share of the load.

*While gasket sets are readily available, you can make your own. I use old-fashioned Permatex, not silicone. Gasket paper is best, but I made some gaskets out of manila folders ten years ago, and they are still fine.

*If your transfer case doesn’t already have one, get a magnetic oil drain plug.

SHIFTING GEARS

That’s about it for this basic transfer case primer. As mentioned earlier, there is lots of helpful information on the Web, and, as with your HMV itself, get the service manual!