Manual Transmissions Explained

This topic will provide a basic understanding of some of the most popular transmissions that are available in today's tractors. This explanation will include gear selections, multiple countershafts, gear timing, and auxiliary range gear systems. We will also address preventative maintenance considerations, the lube oil system, and recommended lube oils.

Transmission Model Identification

Each transmission OEM identifies the model number with an identification plate riveted to the side of the transmission. Each OEM uses minor variations in its model-numbering system. As you can see from the tables shown below, there are some similarities and also some differences:






Typical Single Countershaft 5-Speed Transmission

All manual transmissions use a countershaft to obtain gear selection options. Shown below is a diagram of how a single countershaft transmission functions. The input shaft which connects to the clutch, is labeled IS. The output shaft which connects to the driveshaft is labeled OS.

The input shaft is attached to the top left red gear, which turns all the time that the clutch is engaged. This drives the lower left red gear at all times. The lower red gears are all attached to the counter shaft and are called the cluster. Any time that the clutch is engaged, the entire cluster spins.

Notice the lower right hand red gear. This is two gears, one stacked on top of the other gear which is a part of the cluster. That top red gear is called a reversing gear. The reversing gear has its own pivot shaft which is not shown in this drawing. The reversing gear simply spins on its shaft and reverses the direction of the 1-R blue gear when it slides above the reversing gear.

The bright white arrows in the diagram show the direction of gear rotation and power flow. The faint white arrows represent no power flow, just gears spinning freely.

The drawing above shows the transmission in neutral. In this neutral position, none of the blue gears are engaged, so the black gears just spin freely on the output shaft (driven by the lower red gears), and the output shaft does not transmit any power.

Notice the 5-speed shift pattern to the left. There are three fore and aft throws on the shifter. Each throw moves a separate shift fork, which moves a blue gear. When the shifter is moved to the left throw, then forward selects reverse gear, and a shift fork moves the 1-R blue gear to the right. Moving the shifter backwards selects first gear and the shift fork moves the 1-R blue gear to the left. Likewise, the middle shifter throw moves the 3-2 blue gear, and the right shifter throw moves the 5-4 blue gear. By selecting a gear with the shifter, the selected blue gear is moved in the appropriate direction.

The drawing below shows the transmission in first gear. The 1-R blue gear is slid to the left and now provides first gear power to the output shaft.

The drawing below shows the transmission in second gear. The 3-2 blue gear is slid to the right and now couples second gear power from the largest black gear to the output shaft.

The drawing below shows the transmission in third gear. The 3-2 blue gear is slid to the left and now couples third gear power from the middle black gear to the output shaft.

The drawing below shows the transmission in fourth gear. The 5-4 blue gear is slid to the right and now couples fourth gear power from the smallest black gear to the output shaft.

The drawing below shows the transmission in fifth gear. The 5-4 blue gear is slid to the left and now couples fifth gear power from the input shaft red gear directly to the output shaft.

The drawing below shows the transmission in reverse gear. The 1-R blue gear is slid to the right and now couples power from the red reversing gear to the output shaft, forcing the output shaft to turn in reverse. Notice the gear ratio difference between first and reverse red gears. The reversing gear is about 1/3 the diameter of the red cluster gear used for first gear. Since the gear diameter ratio or the gear teeth ratio both determine the gear ratio, the reverse gear will drive the 1-R gear (and the output shaft) about 1/3 the speed that first gear would drive the output shaft.

Multiple Countershaft Transmission Types

The single counter shaft transmission places high forces upon the gear bearings and upon the gear teeth. For increased transmission life, twin counter shaft and triple counter shaft transmissions were developed. The most popular twin counter shaft transmission is the RoadRanger. The drawing below shows an end view of the three countershaft types. The twin counter shaft divides the forces between two countershaft gear clusters, and the triple countershaft divides the forces between three countershaft gear clusters.

Notice that the twin countershaft has twice as many cluster gear teeth meshed, and the triple countershaft has three times as many cluster gear teeth meshed. The more teeth that are meshed, the longer the teeth wear. The input shaft bearing loading of the twin is lighter because the input gear is balanced between both countershafts and is not trying to move sideways. The input shaft bearing loading of the triple countershaft transmission is even less because it is balanced on all three sides by countershafts. It is not trying to move up or down either. The triple countershaft transmission is primarily manufactured by Mack Trucks.

Multiple Countershaft Transmission Timing Requirements

These multiple countershaft transmissions have a unique requirement for timing while being assembled. If you look at the drawing below and right, you will notice that gear A is floating around the output shaft. This design was incorporated to reduce unused gear drag on the output shaft. The teeth from the surrounding countershaft gears, hold gear A in proper alignment around the output shaft. Gear A floats on the countershaft gear teeth.

A 5-speed transmission would have five of these floating gears. Because each of these five floating gears has a different number of teeth, the associated gears on the countershafts have different phasing positions. These differing phases are required to float each gear centered about the output shaft properly. If the twin countershafts are out of time (one or more teeth out of position), one or more of these floater gears will operate slightly out of position with the output shaft. The misalignment will usually allow all the gears to be selected, but that transmission is now doomed for failure.

When a floater gear is slightly out of position, it will wear on gear B as shown in the diagram to the left. Gear B is what couples power from gear A to the output shaft when that gear is selected. The out of alignment floater will cause much friction, which will breakdown the lubricant properties, and transmission failure will occur within about 200 miles of use after untimed transmission assembly. Always refer to the manufacturer's timing requirements during assembly of multiple countershaft transmissions.

Auxiliary Shift Units

So far we have only talked about the main 5-speed section of the transmission case. When more shift gears are required, then an auxiliary gear case is bolted on behind the primary 5-speed case. These auxiliary gear cases usually employ 2, 3, or 4-speed gears. The drawing to the left shows a 3-speed auxiliary gear case, which is shown selected in the low gear range.

The output shaft of the 5-speed is replaced with a shaft which splines to the input shaft of the aux unit. The input shaft of the aux unit is now connected directly to the output shaft of the primary 5-speed gear case. The output shaft of the aux unit is now splined to the driveshaft.

To preclude the requirement of a second shift handle for the aux unit, air pistons inside the aux unit are controlled by air valves on the primary gear case shifter handle. The driver flips a low switch to set the low aux gear range as shown above. While the low gear is selected, the high and mid gear air piston is locked out in the middle position and can not be selected.

Once the driver switches out of the low range, then the other switch on the shift handle allows shifting between mid and high gears.

The next diagram to the right shows the mid gear selection for this aux unit. While in the high or mid gears, the low gear air piston is locked out and can not be selected.

Many highway trucks have the 2-speed aux unit which does not have the low gear range. They just have the mid and high gears. These 2-speed aux units are commonly called splitters.

Although these diagrams only show single countershaft units, the same principles apply for twin and triple countershaft aux units. If the primary 5-speed is multiple countershaft, then the aux unit is also multiple countershaft.

The next drawing to the left shows the aux unit in high gear. Notice that this is just like the 5-speed in that the input shaft is coupled directly to the output shaft. No power flows through the countershaft.

Some aux units have an overdrive output. Instead of making the top gear direct, they simply gear the output shaft to turn faster than the input shaft. In this situation, there is usually a direct and over gear choice. This once again permits splitting of the 5-speed gears to simulate 10 speeds, but it now offers overdrive gearing.

One thing you can always count upon in the good ole USA, is a lot of choices. You have probably heard of 5-speeds, 10-speeds, 13-speeds, 15-speeds, and 18-speeds. With the new E-engines (electronic diesels), performance and fuel efficiency has forced the engineers to keep the engine RPMs lower while using more torque output. This combination reduces the need for many forward gears, so fewer speed transmissions are in the trucker's future. In fact, this leads us into the next transmission type which is called the automated top gear transmissions. Below is an exploded view of a typical twin countershaft transmission with a 3-speed auxiliary unit.

Automated Top Gear Transmissions

Today's heavy duty highway rigs run on smaller percent grade hill climbs, have high torque producing computer controlled engines, and get by with 10-speed transmissions operating in cruise control mode most of the time. In fact, most linehaul drivers spend 93% of their time in either 9th or 10th gear.

With the wide use of electronics on both engines and transmissions today, automatic top two shifters have come into play. The cruise control computer, engine computer and transmission computer all share information and manage the auto shift of 9th and 10th gears.

We explained earlier how the auxiliary shifter was air operated to preclude a second shift handle. The top-2 transmissions simply allow the engine computer to operate the air pistons which perform the shifts between 9 and 10. The transmission computer reports the input shaft RPM, and the engine computer measures the engine RPMs. The cruise control decides when a shift is required. When a shift is required, the engine computer breaks torque on the transmission input shaft (adjusts engine throttle to remove the power applied to the transmission), orders a transmission deselect of the current gear, matches engine RPM to the new gear input shaft RPM, and then selects the new gear, and then reapplies torque (adds power back). No clutch action is required because of the computer engine RPM gear matching. This entire process takes place when the cruise control computer determines that a change is necessary.

To operate in the automated shift mode, a couple of requirements must be met. The driver shifts through the first 8 gears, clutching in a normal manner. When the vehicle speed is above 40 MPH and the engine speed is 1400 RPMs or greater, and the driver shifts from 8th gear to the A position, and the cruise control is active, then the engine computers take control of the top two shifts and supports the cruise control requirements.

One problem that can occur is called transmission hunting. For a given road speed, throttle position, and engine load, the system will shift back and forth between 9 and 10. When this occurs, just change your road speed a little bit and the hunting will stop.

These transmissions also have a hold switch which allows the driver to override the auto-shift, and when this hold switch is activated, the transmission remains in the current gear.

Failures of the system are identified when the system fails 3 attempts to shift within a 9 second time period. Failures result in trouble codes which are particular to the truck engine. Each engine has its own trouble codes for transmission failures. Once the failure mode has been detected, the transmission resorts to manual mode and will not return to auto-shift mode until after the truck has been stopped and the ignition key has been turned off for at least 10 seconds.

Refer to the specific transmission and engine manufacturer documentation for specific fault codes and troubleshooting tips. We hope to publish some of the fault codes as soon as they become available to us.

Transmission Oil Pumps & Oil Coolers

Transmissions operate under high torque at speeds below 30 miles per hour, such as during a steep grade climb. The high torque conditions generate considerable friction which can drive the oil temperature above 250 degrees. Oil temperatures above 250 degrees can breakdown the lubricating properties of the oil and result in shortened transmission life. Synthetics oils reduce these negative effects.

In some transmission models, the gear, bushing and bearing lubrication is obtained by oil thrown off of the countershaft cluster gears spinning in the oil. When this transmission operates at an angle steeper than 12 degrees, poor lubrication can occur with serious component damage resulting.

Many multiple countershaft transmissions use an oil pump and oil cooler to overcome the above problems. The oil pump can circulate the transmission oil through an external oil cooler, and the oil pump can ensure proper lubrication during steep grade climbs.

Oil pumps can be mounted inside the transmission or they can be mounted on the power takeoff mount of the transmission. When used with an oil cooler, the combination can reduce the transmission oil temperature by 50 degrees. This greatly reduces the possibility of the high torque condition taking the oil temperature above that dangerous 250 degrees. The oil coolers can be water cooled or air cooled. Oil coolers are recommended for engines above 350 HP, and are required for engines above 399 HP.

Transmission Gear Oils

Many lube oil brands are readily available for use in truck manual-gearshift transmissions. Transmission oils specified by an OEM are categorized by the American Petroleum Institute (API) and the Society of Automotive Engineers (SAE) classifies the viscosity of lubricants. A "W" appearing after the oil viscosity grade (15W-40) indicates that the product has met requirements for winter use.

Transmission SAE viscosity grade requirements can vary between SAE 30 and SAE 90 based upon the ambient temperature of the transmission oil. The following table lists the oil recommendations for the heavy duty Rockwell twin-countershaft transmissions. These same lub oil specifications are adopted by Eaton/Fuller, Spicer, Mack, Volvo, GMC, and ZF.

Lubricant Type Grade (SAE) Outside Temperature
Heavy Duty Engine Oil
Above 10 deg F
MIL-L-2104B, C, or D
or API-SF, -SG, -CD or -CE


Above 10 deg F
Below 10 deg F
Mineral gear oil with rust and
oxidation inhibitor (API-GL-1)*
Above 10 deg F
Below 10 deg F
Synthetic oil, Rockwell spec. 0-81*
*Multiweight and EP gear oils are not recommended. Do not mix oils in the transmission.

Synthetic Lubricants

Synthetic lubricants have such great advantages that they are used in all jet aircraft and space vehicles. These same advantages are the basis for use by many transmission manufacturers. The CD50 classification is widely accepted by Eaton/Fuller, Spicer, and Rockwell products. Synthetic oils have been chosen as lubricants of choice for transmissions and differentials. Some OEMs fill this equipment with these oils at the factory. Some OEMs also offer a longer warranty if the proper synthetic lubricant is used instead of mineral oils.

Most people have heard that synthetics are better than petroleum based lubricants but they don't know why. Well, here's why. Because synthetic oil is composed of molecules that are uniform in weight and shape, its heat of vaporization is much higher (more than 300C compared to conventional oil which evaporates at temperatures as low as 175C). This means that you will burn less oil and have less sludge, which is the result of evaporated oil.

Added slipperiness is another positive attribute of synthetics. The uniform length of synthetic oil polymers allows them to more easily slide over one another. Mineral-based oil has a film strength of about 500 psi, synthetics are closer to 3000 psi. This means that the oil is less likely to be pushed out from between two metal surfaces where a lot of pressure exists, like bearings and gears. The advantages of synthetics are so great that they out weigh the initial high cost. A synthetic lubricant will sell for 2 to 6 times the price of petroleum based oil. The synthetic lubricants resist oxidation and will not create varnish and sludge at high temperatures. This feature is important in the newer truck designs where the transmissions are smaller (can dissipate less heat), where aerodynamics reduces airflow over the transmission, and where the high torque E-engines increase the loading and torsional forces on transmission gearing.

In cold weather operation, the synthetic lubricant provides better low temperature fluidity to improve startup protection. Synthetics also posess improved wear protection and oil film strength, and extended drain intervals due to the oil's resistance to thermal breakdown.

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