Honda's Automatic Transmissions: Peculiar Design Solution Reaches its Limit

Design solutions implemented in Honda cars frequently surprise the automobile community by peculiar technological approaches. One of the brightest examples is the design of automatic transmissions developed by Honda engineers.

Many owners of automatic Hondas may not be aware of the fact that transmissions in their cars differ substantially from gear shifting mechanisms implemented in other cars. Honda engineers develop their own transmission solutions, unlike many other carmakers that often buy transmissions from specialized manufacturing companies. In this article, we will consider the main features of Honda automatic transmissions, their pros and cons, as well as the specifics of their maintenance.

Why Honda transmissions are unique?

Most of Honda's automatic transmissions are uncommon because their designs do not include planetary gears in comparison to transmission solutions by other carmakers.

In contrast to manual gearboxes, main shaft/sliding shaft gears in Honda automatics do not combine into a single "cluster" gear but instead operate independently. Main shaft (sliding shaft) gears are attached to the main shaft (or to other gears) by means of separate clutches. In this configuration, the gears stay permanently meshed and unlike in conventional automatic gearboxes, there may be only 1 pair of gears per gear ratio. The design is also remarkable because it has engine braking due to removing a sprag between 1-st and 2-nd gears.

Unlike standard automatics where the R mode is engaged by blocking the planet carrier in 1 planetary set, Honda uses a selector fork and collar to engage a separate R-gear. Honda transmissions, unlike planetary transmissions, do not require brake bands (however they may have more clutches). Thus, the design of Honda transmissions turns out to be more space-saving if compared to other automatic transmissions, but they are still larger than manuals.

Honda engineers had to deal with one important issue related to the design of clutches. The thing is that the clutches should not be larger in radius than the spacing between shafts. Therefore, clutches in Honda automatics are smaller than in standard planetary solutions where the clutches may be nearly the full diameter of the gearbox. Due to the abovementioned peculiarities, automatic transmissions manufactured by Honda hold a special place in the field of transmission solutions.

Types of automatic transmissions

There are 2 types of hydro-mechanical automatic transmissions:

• Shaft-driven transmissions (provided by Honda and Mercedes);
• Planetary transmission (more common type of automatic transmissions).

How shaft-driven transmissions operate

Shaft-driven automatic transmissions are widely used in buses and heavy-duty vehicles. The definition of this transmission type (shaft-driven) refers to the manual gearbox inserted in the automatic transmission. The “mechanical” unit, in turn, can be the following:

• multi-shaft;
• two-shaft;
• three-shaft.

The process of gear shifting in vehicles equipped with this type of automatic transmission is performed with the help of the multiple-disc clutches dipped in special oil, while the reverse gear and the 1-st gear in some cases are engaged by the gear clutch. The structure of such automatic transmissions allows you to shift gears with the help of friction clutches thanks to the crankshaft operation without any power loss and torque sagging. The standard two-shaft design includes drive and driven shafts that bear gears. The three-shaft design also includes an intermediate shaft (lay shaft) which has the gear connected to the main (drive) gear.

5-speed Honda automatic transmission with 2 shafts

Shaft-driven transmissions also found a limited application in passenger cars (Honda and Mercedes models). The use of such transmissions is associated with certain technical difficulties: rear-wheel drive cars have to comply with coaxial alignment requirements, and the shaft-driven automatic transmission must have at least two meshes for each speed on its gears. As a result, it reduces the transmission efficiency.

6-speed transmission with 3 shafts for Mercedes

Another disadvantage of this transmission sub-type lies in high disc friction loses, if the total number of gears is more than 3. In addition, multi-shaft transmissions are quite large and weighty - it significantly limits the free space under the hood, increases operation noise, and reduces reliability of the transmission. Partially, this issue was solved by development of the three-shaft transmission with more shorter and more reliable shafts.

About planetary gearboxes

The design of conventional hydro-mechanical automatic transmissions necessarily includes a torque converter and planetary gears. For half a century of progress in the field of automatic transmissions it became clear that the combination of these components (from a technological point of view) in automatic transmissions is almost ideal, thanks to high quality of their operation characteristics.

Lear more about planetary gear set

Automatic transmissions with planetary gears are generally distinguished by excellent weight-and-dimensional characteristics (it was not for nothing that the legendary Ford T had planetary gears implemented in the manual transmission). For decades, transmission manufacturers have been putting in great efforts to improve characteristics of designed automatic transmissions (with torque converter and planetary gears). Only in recent times, automatic transmissions became challenged by other types of transmission solutions (CVT, DCT, hybrids).

Taking into account all advantages of automatic transmissions with planetary gears, they have several significant drawbacks.

• The first problem is related to the fact that ratios of all transmission gears (speeds) are interrelated, i.e. it is impossible to change the ratio of one gear without affecting the others.
• Planetary gears are exposed to different loads with the engine torque. These loads are distributed unevenly. The most loaded gears frequently get out of order and as a result the complete transmission unit may fail.
• In terms of technology, planetary automatic transmissions are much more complex than manual transmissions used in passenger cars. Manufacturing of automatic transmissions with planetary gears requires sophisticated production lines, calculation systems, and technologies. Thus, automatic transmissions significantly increase the price of the car and its maintenance.

However, it should be admitted that advantages of automatic transmissions with planetary gears outweigh its disadvantages and most transmission manufacturers still consider this type of transmission to be an ideal solution. However, Honda decided to go against this trend.

Insight into History of Honda Transmissions

Honda has always been notable for its peculiar design solutions, especially in the field of automatic gearboxes. Only recently Honda has started following general trends in design solutions for car components, but in the 70’s-80’s Honda engineers set the pace in new transmission technologies. The first automatic gearbox designed by Honda known as Hondamatic was presented at the London Motor Show on October 18, 1967 and became a sensation. The first Hondamatic was installed in the mini-car Honda N360 АТ and it was the 1-st time when the automatic gearbox was paired with the small engine. The Hondamatic trade name is used on all automatic gearboxes from Honda.

Honda N360 ‘1970–72

The Hondamatic gearbox was designed without the use of technologies owned by BorgWarner (BW), the largest patentee in the field of gear shifting technologies of that time, due to disagreements between the companies. Honda could not design a standard planetary gear automatic transmission without violation on any patents. Honda eventually asked BW to develop a prototype gearbox for their new cars. However, BW refused. The main reason for that was the fact that BW did not have transmission techspecs compliant with small engines. Thus, Honda had to design its own gearbox solution. Honda bought a gearbox from BW aiming to create an original gearbox design.

The initial structure of Hondamatic, like all Honda automatics, includes sliding gears on parallel axes rather than planetary gears. The design preserved engine braking by removing a sprag between the 1-st and 2-nd gears. Honda's older gearboxes relied on pressure circuits to regulate line pressure to shift gears. The company's early solutions also included a patented torque converter, which applied stator force to decrease hydraulic losses by means of a reaction arm to build up the hydraulic pressure when the stator would stall. The reaction arm affected the regulator valve, thereby enhanced pressure was available to the clutch discs when torque multiplication was greatest.

The Honda torque converter also had a lock-up feature, thus the company offered the original Hondamatic (which had just 2 forward gear ratios) as a 3-speed. The real 3-speed solution was presented in 1979. The shaft-driven design was chosen by Honda for its fuel economy and high efficiency. Hondamatic transmissions were installed not only on cars with small engines, but also on motorcycles.

The classic Honda transmission was notable for its unique design of the control system. It is clear that automatic transmissions must “know” the torque power transmitted by the engine to upshift or downshift properly. At that time, it was impossible to obtain correct data on the torque rating from the engine, because engines were equipped with carburetors with no electronics.

The “classic” method of torque calculation was to measure the pressure in the intake manifold before and after the throttle opening and to control the position of the gas pedal. This method gave some idea of the current torque. But the system estimated the torque rate inaccurately and was quite sensitive to failures of the control system, and therefore it had to be adjusted to each specific engine. Therefore, a well-known engineer Torao Hattori came up with an alternative approach: he used the simplest physical principle of the torque converter.

The torque at the torque converter stator is always proportional to the torque conversion coefficient. Thus, if you connect the torque converter to the transmission control system, it will be able to shift gears automatically, depending on the load and conversion coefficient in the torque converter. This solution in combination with a shaft-driven gearbox turned out to be quite efficient and found successful application in cars with small engines.

The shaft design and control system with a hydraulic pressure regulating valve turned out to be an ideal match. In shaft-driven automatic transmissions, compared to planetary transmissions, it is not necessary to release friction clutches to engage a higher gear. It is enough to install an overrunning/freewheel clutch between the 1-st and 2-nd gears, and the process of shifting between these gears will be performed thanks to engagement of the 2-nd gear clutch. The overrunning/freewheel clutch is a special element of the automatic transmission design, which ensures that one of the shafts rotates relative to the other one in only one direction. If there was no overrunning/freewheel clutch (when the 2-nd gear gets engaged), it would be necessary to disconnect friction clutches of the 1-st gear. The overrunning/freewheel clutch solves this issue and significantly simplifies the control process.

Check out how Honda transmissions are assembled

The application of such alternative design solution allowed Honda to use their automatic transmissions with small engines, and the speed of gear shifting from the 1-st to the 2-nd gear had a good impact on the car acceleration up to 100 km/h that supported the sport image of the company. The 4-speed Honda transmission had more traditional pneumo-hydraulic control system. The freedom of action in relation to gear ratios allowed Honda 4-speed transmissions to be much more efficient than the planetary analogs.

In addition the advantage of gear ratios, Honda transmissions are notable for their reliability. The thing is that speeds/gears in Honda transmissions operate independently of each other. If the 2-nd gear gets out of order you can freely drive at 3-rd, 4-th, or 5-th. Moreover, Honda transmissions have the minimum number of elements which are sensitive to the quality of lubrication. Thanks to the fact that Honda transmissions are based on manual transmissions, they are not so vulnerable to transmission oil contamination and high temperatures.

Friction clutches of Honda transmissions are more reliable, because they have a partial load mode of operation. Any friction clutches are loaded only at the moment when the gear is engaged, while all the rest of the time they do not work and cool down, and wear products are washed out of them. Friction clutches of the first gear as well as high gears are manufactured with a good margin of safety. Moreover, access to the 1-st gear is possible from the outside – it is not necessary to remove the gearbox when the repair issue occurs in the 1-st gear. As a result, the service life of friction clutches may exceed 300,000 kilometers.

Drawbacks of Honda transmissions

One of the most important drawbacks of Honda transmissions is a limited number of speeds that can be implemented in Honda-design transmissions (the maximum number of speeds in Honda transmissions is 5). Initially, when Honda produced 3-4-5 speed transmissions, engineers of the company did not have to change the established transmission design, it was enough to add another pair of gears and a clutch for them. But the design limit of Honda transmissions was reached in 2001, when the 5-speed Honda transmission was released.

Later, Honda engineers had to deal with the issue of limited dimensions and weight for designed transmissions. In fact, Honda transmissions have always exceeded planetary transmissions in terms of size and weight. Initially, Honda engineers managed to neutralize this drawback thanks to successful design solutions, but with the appearance of the 5-speed transmission it became a stumbling block in development of Honda automatic transmissions.

Conventional automatic transmissions with planetary gears do not increase in dimensions when more speeds are added to their design. Thanks to close gear ratios, the structure of automatic transmissions with planetary gears can be lightened, but in Honda automatic transmissions (shaft driven) addition of a new gear leads to increase in size and weight.

Another problem of Honda transmissions is the use of the freewheeling/overrunning clutch in the 1-st gear. Despite its numerous benefits, Honda transmission was easily damaged during sharp gear shifting – the freewheeling/overrunning clutch gets torn. Moreover, the engine braking at the 1-st gear simply does not function properly.

Review on the 10-speed automatic transmission designed by Honda

This is not to say that the abovementioned drawbacks are critical. To put it simply, each transmission design has its peak of popularity and the moment when competing solutions outrun them. The same rule can be applied to Honda automatic transmissions. The time of Honda transmissions technological leadership stayed in the 90s. This may be perhaps the reason why Honda has recently introduced (2017) an all-new, in-house designed 10-speed automatic that uses planetary gears.

That’s all you have to know about the ZF 9HP transmission installed in Honda Pilot

Moreover, in 2014, for the 1-st time in its history Honda decided to use the ZF 9HP transmission for its Acura TLX V6 model. Later the company even expanded the list of models (Acura MDX, Odyssey and Pilot were added) offered with the ZF transmission.

Maintenance of Honda transmissions

Honda specialists recommend car owners to change the transmission fluid together with engine oil every 10,000 km. Transmission fluid plays a crucial role in the transmission operation, therefore Honda recommends using original fluids for its transmissions. It is extremely important to check the fluid level regularly (at least once a month), since the temperature of operating fluid reaches 130-140Cº and its volume is constantly decreasing due to evaporation. A low level of transmission fluid leads to intensive wear of transmission components and consequently to their damage.

In addition to evaporation, the level of transmission fluid can decrease due to the following reasons:

• leakages in gaskets and seals;
• fluid suction via vacuum elements of the transmission;
• transmission fluid overflows through the transmission cooling system (transmission fluid gets mixed with the engine coolant and vice versa).

Learn how to change ATF in Honda cars

First signs of a low fluid level in the transmission:

• Delays in gear shifting;
• Noise during transmission operation;
• No movement when gears are engaged.

Nothing can prolong the service life of Honda transmission more efficiently than well-timed and correct change of the transmission fluid. Car owners should immediately change the fluid if it becomes dark or has a characteristic burning smell.

Honda transmission replacement

If you have to replace the Honda transmission, then it is necessary to take into consideration some of its peculiarities. All transmissions (manual transmissions, automatic transmissions, CVTs) have identification marks. In other words, the same Honda car model (with the same engine) can have several variants of markings for transmissions and they are not interchangeable! Thus, it is important to install the transmission which has the same marking as the original transmission unit. It is impossible to find the transmission marking with the help of VIN or frame numbers (using parts catalog). The identification mark can be found on the transmission case (on the paper sticker with a code bar).

All in all, automatic transmissions provided by Honda have been successfully used in Honda cars for a long period of time. However, the growing competition between automakers and rapid progress of transmission technologies pose challenges to Honda engineers which cannot be addressed with the help of old solutions. Therefore, nowadays Honda equips its cars with automatic transmissions based on planetary gears and actively introduces transmission solutions provided by companies specialized in automatic transmissions.