HMV Transmission Basics

by Steve Turchet

As Snoopy might have written, “It was a dark and stormy night,” and I was driving a 1947 Ford ex-Railway Express Agency 2-ton truck on a lonely mountain road. While attempting a double-clutch from fourth to third to spare the brakes on a steep downgrade, the shift lever came out of the transmission! I found myself holding the lever while rapidly gaining speed in neutral. I did what seemed logical: I stuck the lever back in the hole and completed my down-shift. As soon as I was able to stop, I found the shift lever retainer cap had come loose, so I screwed it down tight and finished the trip without further adventure.

I share that story as an illustration of just one thing that can happen if you don’t regularly inspect and service your vehicle’s transmission.

Motor vehicle transmissions may be divided into two basic types: manual and automatic. With the exceptions of some tanks and other types of tracked vehicles and the GMC M211, most common U.S. military vehicles from the turn of the 19th century through WWI, WWII, and into the 1970s, had manual transmissions — vehicles such as jeeps, MUTTS, Dodge WCs and M37s, GMC CCKWs and DUKWs, and Reo M35s. Late model deuces, most CUCVs, and HMMWVs have automatics.

Beginning with the technical basics, the purpose of a motor vehicle transmission is to provide the driver with a selection of gear ratios between the engine and driving wheels so a vehicle can be operated at best efficiency in a variety of terrains, speeds and loads.

There are three basic types of manual transmissions: sliding gear, planetary, and friction disk (though the friction-disk is mostly obsolete).

The planetary (as was fitted to Model T Fords) is also no longer in general use, though its design is incorporated in many automatic transmissions using fluid couplings or torque converters, as well as in overdrives and in some dual-ratio rear axles.

The sliding-gear type is the most common today, and is what most people think of when manual transmissions are mentioned.

There are two basic types of sliding gear transmissions. The first and earliest type is commonly called a spur-gear (or “crash-box”) in which the gears themselves actually slide. The second is the constant-mesh in which the gears are fixed in place and are engaged by a sliding mechanism. More about these later.

There are also two basic sub-types of sliding-gear transmissions. One is the progressive, in which it is necessary to pass from one gear through another in a definite order. This means, for example, that with a four-speed progressive transmission it is impossible to shift from first gear to fourth without going through second and third. The use of this system is limited mostly to motorcycles.

The other sliding-gear type is known as selective. In this system the operator can select any gear without going through any intermediate stages. In modern passenger cars and light vehicles, the disadvantages of sliding-gear types are overcome by use of constant-mesh or “synchromesh” transmissions. These eliminate the noise and clashing common to spur-gear transmissions.

Manual transmissions share certain fundamental components. The case is usually of cast iron though often made of aluminum on today’s light vehicles. It houses the gears and shafts.

Then there is the control cover, which houses the shifter mechanism. Most three-speed transmissions, such as used in most U.S. military jeeps, have three shafts. They are, in the order of the flow of power, the input shaft, the countershaft, and the main shaft.

The input shaft has an integral main drive gear and rotates with the clutch driven plate or disks. That is, the shaft rotates any time the clutch is engaged and the engine is running.

The main drive gear is in constant mesh with the countershaft drive gear. Since all the gears in the countershaft cluster are either made integral or keyed on, they also rotate whenever the clutch is engaged. The transmission main shaft is held in line with the input shaft by a pilot bearing at its front end, which allows it to rotate or come to rest independently of the input shaft.

The transmission second-and-third and first-and-reverse main shaft gears have grooved hub extensions into which the shift forks are fitted that slide them back and forth on the main shaft splines. Thus, the second-and-third speed main shaft gear can be shifted rearward to mesh with the countershaft second-speed gear.

The second-and-third main shaft gear also has internal teeth that mesh with the external teeth on the rear of the main drive gear when the gear is shifted forward into the direct-drive position. Most conventional three and four speed transmissions are direct drive in top gear, meaning the transmission’s output shaft RPMs to either a drive shaft or a transfer case equal engine RPMs.

The first-and-reverse speed main shaft gear can be shifted forward to mesh with the countershaft first-speed gear or rearward to mesh with the reverse idler gear. The countershaft reverse gear is usually in constant mesh with the reverse idler gear. In some transmissions the reverse idler gear is shifted to mesh with the countershaft reverse gear at the same time that the first-and-reverse speed main shaft gear is shifted to mesh with the reverse idler gear.

The main shaft, countershaft, and input shafts, with their respective gears, are mounted in bearings in the transmission case. Shift rails and forks move the gears when the control lever is moved by the driver to change speeds. The countershaft is generally placed below the main shaft. This permits a deep, narrow case, which retains a considerable quantity of oil without danger of leakage, since the oil level is maintained below the oil seals.

For the purpose of simplicity, the transmission described above is a spur-gear type. In modern vehicles, helical gears are generally used because they run more quietly than spur gears. However, there is a side thrust on a helical gear, due to the angularity of the teeth, which tries to slide the gears out of mesh. This difficulty is avoided in constant-mesh transmissions because the gears don’t slide on the shaft. When sliding helical gears are used, the splines on which they slide are also cut helically to the same angle as the teeth, which offsets the side thrust.

The three-speed transmission described is operated by a control lever extending from the control housing. The lever has a balI fulcrum fitting into a socket in the housing. It is kept from rotating by a set-screw or pin in a slot in the side of the ball fulcrum but is free to move backward, forward, and sideways. The end of the lever below the ball fulcrum engages both slots, but there is an interlock device (usually a ball or pin engaging notches in each shifter shaft) which permits only one shifter shaft to move at a time. This prevents two gears being engaged at once.

When the control lever is pressed to the left, the slot in the first-and-reverse shifter shaft is engaged and the fork can be moved backward or forward. After the first-and-reverse shifter shaft has been returned to the neutral position, the control lever can be pressed to the right and the second-and-third shifter shaft and fork can then be moved forward or backward. The shifter shafts are held in the different speeds and the neutral position by spring-loaded balls or poppets engaging notches in the shifter shafts.

Sliding-gear transmissions, usually four or five speed, as used in many WWII HMVs such as Dodge WCs, the Chevrolet G506, Ford GTB, and CCKWs and DUKWs, use stub-tooth (spur gears); consequently, these transmissions are noisy and become noisier as they wear.

Beginning around the late 1940s, when most U.S. auto manufacturers came up with new designs. The sliding-gear transmission was mostly replaced, especially in light trucks and passenger vehicles. In the new system, the gears were always in mesh with their mates, and selection was made by sliding other components into and out of the gears rather than moving the gears themselves. Two of the most common forms are the constant-mesh and the synchromesh. These possess an additional feature to prevent clashing of gears.

In a constant-mesh transmission some of the countershaft gears are constantly in mesh with gears on the main shaft. The meshing gears on the main shaft are fixed so they cannot move fore and aft, but are supported on roller bearings so they can rotate independently of the main shaft.

If all this seems as clear as mud, it could be simply said that the main difference between most synchromesh transmissions and “crash boxes” are that in the latter one moves the gears when shifting while with the former the gears are engaged via a “slider.” In addition, most synchromesh transmissions use internal clutch mechanisms to bring each selected gear up to speed to prevent clashing, while with most spur gear transmissions this matching of gear speeds must be done by the driver — it requires practice and attention to do properly.

The construction of a typical synchromesh clutch is shown in the accompanying illustration. The driving member consists of a sliding gear splined to the transmission main shaft with bronze internal cones on each side. It is surrounded by a sliding sleeve having internal teeth that are meshed with the external teeth of the sliding gear.

The sliding sleeve is grooved around the outside to receive the shift fork. Six spring-loaded balls in radially drilled holes in the gear fit into an internal groove in the sliding sleeve and prevent it from moving endwise relative to the gear until the latter has reached the end of its travel.

The driven members are the main drive gear and second-speed main shaft gear, each of which has external cones and external teeth machined on its sides to engage the internal cones of the sliding gear and the internal teeth of the sliding sleeve.

The synchromesh clutch operates as follows: When the transmission control lever is moved by the driver to the third speed or direct-drive position, the shift fork moves the sliding gear and sliding sleeve forward as a unit until the internal cone on the sliding gear engages the external cone on the main drive gear.

This action brings the two gears to the same speed and stops endwise travel of the sliding gear. The sliding sleeve then slides over the balls and engages the external teeth on the main drive gear, locking the main drive gear and transmission main shaft together.

When the transmission control lever is shifted to the second-speed position, the sliding gear and sleeve move rearward and the same action takes place, locking the transmission main shaft to the second-speed main shaft gear. On most older transmissions a synchromesh clutch is not fitted to first or reverse gears.

First speed is engaged by an ordinary dog clutch when constant mesh is employed, or by a sliding gear. Reverse is always engaged by means of a sliding gear. In a few older types of transmissions it was possible to engage reverse gear while moving forward… though this could generally be done only once.

The first mass-produced standardized U.S. military tactical cargo trucks with automatic transmissions were the G-749 family, designed in 1950, in which the M211 is probably the most common example. Built by GMC, the transmission was basically a Cadillac Hydramatic fitted with a rear-mounted two-speed auxiliary unit, providing eight speeds forward and two in reverse. These trucks used a single speed transfer case, since high and low ranges were in the transmission, and power take-off units for a winch or other equipment were mounted on the transfer case.

The Hydramatic transmission consists of a fluid coupling and an automatic transmission. Slippage in the fluid coupling at engine idling speeds provides the cushioning action of a clutch. Gear changes are made automatically by hydraulic pressure and are governed by the speed of the vehicle and the extent to which the driver depresses the accelerator.

The planetary unit consists of planet gears (four in the front unit and six in the rear unit) which encircle and are meshed with the center, or sun, gear. These gears are, in turn, surrounded by a gear having internal teeth. One member of this planetary unit can be kept from rotating by a band wrapped around an integral drum. The band is applied or released by a servo mechanism which is a double-acting piston and cylinder operated by spring and oil pressure.

Each unit also includes a multiple-disk clutch which is applied by oil pressure and released by spring pressure. When the clutch is released and the band applied, the planet gears are forced by the driving gear to “walk around” the stationary gear; consequently the output shaft, which is connected to the planet gears, is driven at reduced speed and the unit is in reduction.

When the band is released and the clutch applied, the entire unit is locked together and rotates as a unit, providing direct drive. Each planetary unit is a complete two-speed transmission in itself, as it can operate either in reduction or direct drive. The transmission contains of two of these planetary units, giving a total of four speeds, since each unit has two speeds.

The hydraulic control units include two oil pumps, two servos that operate the bands, a governor assembly, and a master control valve assembly which directs the oil pressure to the various control units. The speeds at which the gear changes occur are controlled by oil pressure to the control-valve mechanism.

This mechanism consists of balanced shift valves against which oil pressure is exerted at one end in proportion to throttle opening, and at the other end by pressure from the centrifugal governor. Thisis in direct proportion to the speed of the vehicle. When the vehicle reaches a certain point in relation to governor pressure and throttle opening, the transmission up-shifts to a higher speed.

When the vehicle speed decreases due to brake action or ascending a hill, a point is reached when the various pressures cause the transmission to shift to a lower gear. As throttle opening follows engine torque very closely, it controls the shift valve positioning in accordance with the demands for power made by the driver.

The four transmission speed ranges are: In first speed, both planetary units are in reduction for a total ratio of 3.92:1. In second speed, the front unit is in direct drive and the rear unit in reduction for a ratio of 2.53: 1; in third speed, the front unit is in reduction and the rear unit in direct drive for a ratio of 1.55:1. Andin fourth speed, both units are in direct drive.

If you own one of these trucks, you should be aware there are many things about operating and maintaining the transmission that won’t be found in the manuals. Rule number one is: Never use high range reverse. It is also very important that the parking brake is in good condition. There is no “Park” feature in the transmission.

The three most common causes of failure in both manual and automatic transmissions are insufficient oil level, worn out oil, and use of improper oil.

These are the best assurance of long and trouble-free transmission life: Check the oil frequently and topping it off to the correct level. Check for leaks and fix them. Replace the oil, either when it has become contaminated with metal particles and/or water, has been severely overheated, or when it has simply aged (about five years as a general rule).

Of course, consult your vehicle’s manual to find the proper oil to use in your HMV’s transmission. You may become confused by the oil designations in older manuals, however.

As a general rule for most common collectable U.S. HMVs, 140 weight gear oil is suitable for most spur gear transmissions, such as the four speed used in the Chevrolet G-506. Most synchromesh transmissions, such as those in jeeps and MUTTs use 90 weight. But, again, read your vehicle’s manual, especially in regard to oil specified for severe operating conditions. You can seldom go wrong by following their vehicle’s manual, though you should check for updates(such as those found inPS Magazine bulletins.

Beware of advice from self-styled “experts.” These are most- often found on HMV websites rather than leaning over your backyard fence these days. While most are (at least) well-intentioned, the good intentions of others don’t matter if you destroy your transmission by taking their advice!

Be wary of advice to use either a heavier or a lighter weight of oil than specified by your vehicle’s manual. For example, unless you’re operating your vehicle in desert conditions with ambient temperatures consistently above 100 °F, it is usually unwise to use a heavier weight of oil than specified by your manual. While heavier oil may provide better lubrication in some transmissions at high environmental temperatures, it won’t lubricate properly when cold, including when first starting to drive, which will cause a lot of wear.

Similarly, unless you’re operating your vehicle in constant sub-zero weather, using a lighter weight oil than specified won’t provide proper lubrication at warmer temperatures, or even when the transmission itself heats up. Keep in mind that both the transmission manufacturers and the military did a lot of testing in extreme conditions before making lubricant recommendations. They probably destroyed a lot of transmissions in the process. But, they had hundreds of transmissions to experiment with. How many do you have?

Remember that while a heavier oil may quiet a worn transmission, it won’t fix anything. It will usually accelerate wear.

Other examples where having a manual is imperative are in cases such as M211 Hydramatic transmissions, some of which were modified to use ATF (automatic transmission fluid) while most were designed to use 30 weight non-detergent oil. Which type does your truck have?

While there are ongoing debates in regard to synthetic oils versus mineral oils for manual transmissions, in most cases synthetic oils are superior and will generally give longer transmission life. However, they will not “fix” a worn transmission, but they may extend its life. As far as transmission oil additives, experiment at your discretion. If you find one that seems to work, stick with it.

To change the oil in most manual transmissions, the oil should be warmed up by driving the vehicle until the transmission case feels warm to the touch. This will not only make the oil drain faster, it also assures that most contaminants, such as metal particles, drain out with it.

With a suitable pan or container below, remove the plug from the bottom of the transmission case and let the oil drain out. A few metal particles are normal, but if the oil is silver with ground up metal or if there are any large chunks of metal, you may have serious problems and should investigate.

Conventional gear oil should be amber in color and fairly clear. Oil that has been in service a long time may be grayish, brownish, or “light black.” Dirty black and/or a thick tarry consistency usually indicates very old, worn out oil. Greenish or milky oil usually indicates water. Some synthetic gear oils are red or purple, but that doesn’t mean they can’t be contaminated by metal particles or water. Most molybdenum gear oil additives turn the oil black, so expect this if you’re using them.

Some people flush a manual transmission by reinstalling the drain plug and filling the case with diesel oil or kerosene and then draining it again. Some go a step farther by jacking the drive wheels off the ground and running the engine at idle for a while with the transmission in gear to circulate the kerosene or diesel oil and wash everything inside.

In either case, be sure to let all the diesel oil or kerosene drain out before installing the new gear oil or the oil may be thinned. Never use gasoline for flushing a transmission! One little spark caused by a clashing gear can produce a very unpleasant event.

Oil level on most manual transmissions should be maintained at the level of the threads in the upper (inspection) plug hole (unless otherwise specified).

MUTT owners should be aware of the difference between the oil inspection plug and the bolt for the reverse idler. If you take out the latter instead of the former, you’ll wish you hadn’t!

Oil level should be checked whenever you perform a lube job, or more frequently if there is a leaking seal. Of course, a leaking seal should be replaced as soon as possible, and while you will generally have to pull the transmission to replace a front seal, replacing a rear seal is usually not a major undertaking.

Some HMV transmissions have vent fittings, and if these get clogged pressure may build up inside the transmission causing seals to leak and/or oil to come out around the shift lever. On M-series vehicles with fording systems, leaving the fording valve closed, or a clogged or crushed line somewhere in the system, can cause the same problems. Both of these possibilities should be investigated if transmission seals are leaking or oil is coming out around the shift lever.

As mentioned earlier, five years is a good interval between changing transmission oil — unless you put a lot of miles on a vehicle, in which case the oil may require changing more often. Keep in mind that gear oil ages and should be replaced at five year intervals, regardless of mileage.

Most of the above also applies to the automatic transmissions of the GMC M211, Dodge M880, various CUCVs and the HMMWV. Read your vehicle’s manual to determine the correct type of oil to use, check the oil level frequently, and change the oil at least every five years.

Most ATF is red in color and should be fairly clear. If it’s brown, murky, and/or smells burned, it should be replaced.

On most light and medium vehicle automatic transmissions, draining the oil requires the removal of the bottom pan, so you will need a new gasket. Even if a drain plug is provided, it’s still smart to pull the pan. Many such transmissions have an internal filter, which should be replaced at this time. Use of silicone gasket sealer is not wise because bits of silicone may get into the transmission and clog an oil passage. When draining the oil, look for metal particles, which may be a warning of trouble if many are found.

When performing a lube job or other routine maintenance, check the tightness of the bolts that hold the transmission to the engine or bell housing. Also check that the transmission mounts are secure and in good condition. One the the major causes of a vehicle shuddering when letting out the clutch are loose or broken transmission mounts.

For vehicles such as jeeps which have the transfer case attached to the transmission, check those bolts.

Likewise check that the two-speed unit is firmly attached on M211s.

Check all fittings and linkages for secure attachment and smooth operation, including any hoses or piping on automatic transmissions, as well as their cooler lines (usually to the radiator).

And don’t forget to check the shift lever cap or retainer — or the lever may come out on a dark and stormy night.