Delivering the Goods - April - 2019

Not the Typical DSG! Acura’s Twist on a Direct Shift Gearbox

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Leave it to Honda to create a transmission that the world has never seen before. Ever since the H2 Hondamatic transmission was introduced in 1973, Honda transmissions have been dubbed as strange, different, or unique, depending on one’s point of view. Transmission builders worldwide either love them or hate them.

Holding true to this history, in 2015, Honda introduced an 8-speed, direct shifting gearbox (DSG) that holds an unexpected twist. At a glance, it looks more like a scaled-down version of one of their clutch-to-clutch, multiple shaft units. A second look at all the external components screams out, “Hey, grab a book before you lay a wrench on this baby!”

We’re going to look at what makes this unit different from other DSGs and how to identify issues by first knowing what “good” looks like.

Identifying the Unit

Honda introduced this transmission in the Acura line in 2015 in the TLX, mounted behind the 2.4L inline, 4-cylinder engine. It’s also available in the Acura ILX starting model year 2016. The unit has a tag on the bellhousing, facing the front of the car, indicating the unit type (figure 1). Its external appearance sets it apart from previous Honda units.

The torque converter measures 3.750” tall and 11.850” diameter, which compares with most Honda converters. If you plan to service this unit, you’d be better off either rebuilding your existing unit or purchasing one from the OEM. Although it looks like other Honda torque converters on the outside, it’s unique to the DSG application on the inside.

Engineering Behind the Transmission

Honda, like all other automotive manufacturers, strives to make vehicles that are fuel efficient, exciting to drive, and durable enough to satisfy their customers. They’re also sensitive to the complaints that their non-customers present as to why they prefer not to own a Honda product. One common concern that always loomed in the driveability realm was the firm or positive shift feel that seemed to be present even in the flagship, luxury models. As technology evolved and torque management strategies improved, overall shift quality improved for all Honda and Acura vehicles.

So why would Honda switch to a unit that has a more mechanical feel? By design, DSG transmissions are known to have a very positive shift feel. After all, they’re manual transmissions being controlled by computers and actuators.

Typical concerns with DSG units involve launching the vehicle from a start, throttle transitions (on the gas, off the gas), deceleration (trailer hitching), and low speed vehicle operation (such as heavy traffic or driving in 5-15 MPH zones). All of these conditions cause noise, vibration, and harshness (NVH) complaints.

But Honda engineers decided to join the best aspects of a conventional automatic transmission with those of a DSG transmission. DSG transmissions are the quickest shifting units on the market. This allows engineers to tailor programming around an engine with a narrow horsepower and torque band to feel exceptionally responsive under most driving conditions, without sacrificing fuel economy.

The conventional torque converter coupling adds a component of torque multiplication on demand to encourage a livelier low-RPM response. The torque converter is also used for smooth takeoffs, shift transitions, and to produce seamless low speed operation.

Unit Driveability Characteristics

The design of this unit makes it one of the smoothest functioning DSG-type transmissions on the market. On launch from a start, it uses the torque converter to transmit torque from the engine to the transmission in the same way as a conventional automatic transmission, where the fluid coupling acts as a dampening device.

Since the input clutches are inside the unit, away from the engine, they’re fully applied during vehicle launch from a stop. This takes away the complicated torque transfer computer programming necessary to accomplish a smooth launch using clutch slip rates.

When cruising, the torque converter clutch is modulated on and off to dampen shift sequences and minimize noise, vibration, and harshness under different engine load conditions. The torque converter clutch will fully lock as often as possible to conserve fuel.

Torque Converter

This is where the magic happens: A look inside this unit uncovers a compact assembly that is precisely constructed (figure 2). The impeller and turbine fins are furnace-brazed and very sharp to minimize oil deflection and turbulence.

It maintains converter charge pressure using a sealing ring on the stator-to-converter hub and input shaft-to-turbine. This gives the hydraulic circuit greater capability to precisely control the converter clutch apply and release.

The damper assembly is robust enough to minimize any noise, vibration, and harshness related issues when the torque converter clutch is engaged or in transition of applying and releasing (figure 3). The clutch material bonds to a piston, just like its conventional counterparts.

The total fluid capacity of this torque converter is relatively small, which means converter charge pressure is important. The low volume allows for more precise pressure control.

Unit Serviceability

The DSG follows suite with other Honda units and only offers drain-and-fill capability. The filter inside is accessible only by disassembling the unit. The fluid capacity is typical for a DSG unit in spite of the addition of a torque converter.

The fluid type is Honda H2 low viscosity ATF. The drain and fill plugs are accessible, but filling the unit will require some ingenuity (figure 4).

The input clutches are serviceable by removing the back cover of the transmission. Unless you can figure out a way to access the cover to pull it straight out of the unit, the transmission needs to be removed. There are feed tubes you can bend if the cover doesn’t come out straight (figure 5).

The clutch drums are marked “M” for mainshaft and “S” for secondary shaft. The clutch packs resemble typical, late-model Honda clutch packs. Be careful removing them. There are no snap rings holding the assemblies onto their respective driveshafts. The drums can rotate independent from their shaft.

Each drum has a hub that is splined to the shaft, which engages the friction clutches in the clutch pack. You’ll need to handle these hubs carefully so that you don’t damage the reluctor wheels (figure 6).

The clutch drums aren’t interchangeable. The friction and steel elements are the same dimensions for both clutch packs, but the mainshaft clutch pack has one less steel and friction than the secondary clutch pack. The sealing rings are solid, Teflon rings that ride inside the drum assembly and seal it to the shaft (figure 7).

The clutch pistons are molded rubber assemblies with a balance piston. It maintains a residual oil pressure in the clutch apply cavity to make sure the clutch apply circuit remains charged for fast apply and precise control.

A coated, metal gasket seals the end cover to the case (figure 8). For now, this gasket isn’t available. The gasket’s thickness affects the clearance of the input drums between the end cover and the case bearing inner race surface. Never attempt to replace this gasket with other sealants like silicone or gasket-making anaerobic chemicals.

Predicting Potential, Future Failures

The input clutches — This application is no different from other wet clutch DSG units. As the unit ages, the clutches wear. The only difference is that the clutches are inside the rear of the unit. Other OEM applications have their dual clutch assembly in the front portion of the unit with a coupling device that connects it to a dual mass flywheel. Since Honda isn’t slipping this clutch to launch the vehicle, this may extend the life of the clutch assemblies.

Solenoid and sensor damage — The control valve body faces the radiator. Most of the actuators and sensors mount on top of the valve body. The valence pan and undercarriage skid plates should ward off road debris like rocks and tire pieces, but a front collision will most likely cause transmission component damage. Be aware of these vehicles coming into your shop with fresh bodywork and transmission codes and issues.

CPC A solenoid electrical or mechanical failure — CPC A assists in the shift feel and remains electrically on in the top gear (8th). The fact that it’s energized for the majority of the traveling life of the vehicle makes it a prime candidate for failure due to fatigue or debris.

Torque converter clutch sealing rings — There are two sealing rings that control the torque converter charge and pressure: one on the stator and one inside the torque converter. If you install the torque converter incorrectly, it could damage the steel sealing ring on the stator. Even though the leading edge of the hub is amply tapered, be careful engaging the torque converter.

Overall, Honda appears to have found a new transmission design they’ll be able to use in many other applications in the future. With a seamless blend of comfort and efficiency, they’ve set a new bar of performance that’ll be noted by other OEM engineers.

For our industry, that means we’ll need to become familiar with this unit and what “good” looks like, because this could very well be Honda’s next common unit.