Other Articles - October/November - 2017

Acura DCT 8-Speed Transaxle

Modern Honda and Acura vehicles have their fair share of transmission options. The most familiar transmissions are the 5- and 6-speed units, with constant mesh gearsets that resemble manual transmissions. This is how we’ve come to know Honda and Acura transmissions and, along with the many aftermarket enhancements, the Honda 3-shaft, 5-speed is a good moneymaker.

So what does the future hold for Honda transmissions? For 2018, there are no more 5-speed transmissions, but the 6-speed still shows up in certain Honda Odyssey, Ridgeline, Acura RDX, and RLX models.

Honda also leans heavily on its CVT. In recent years, you’ll find the start clutch and torque converter equipped CVTs in many of their models. For 2018, Honda will only use torque converter equipped CVTs in the 4-cylinder Accord, Civic, CR-V, Fit, and HR-V.

Over the past few years, Honda has used the ZF 9-speed transmission in the Honda Pilot and Acura MDX. Their latest development is an in-house designed-and-built 10-speed, which appears in their 2018 Honda Odyssey.

Rounding off the Acura lineup is a 7-speed DCT that only shows up in the Acura RLX Hybrid, and the 8-speed dual clutch transmission (DCT), which appears in the popular Acura TLX and ILX. Let’s take a look at this 8-speed DCT (figure 1).

DCTs are becoming more popular in performance and fuel-efficient vehicles. The DCT offers a light and compact design, primarily because it’s more like a manual transmission than a conventional automatic transmission. The DCT contains shift rails and synchronizing assemblies, just like a typical manual transmission. The addition of hydraulics and electronics allows for automatic shifting.

The Acura DCT8 presents a different approach to the form and function of the dual clutch, as opposed to a conventional DCT. On a traditional DCT, the dual clutches sit in the bellhousing area and are actuated hydraulically (figure 2).

The traditional DCT is either a wet or dry clutch design. Common complaints regarding the traditional DCT involve the quality of low-speed engagements, harshness, and shudder. Honda addresses these issues with a design modification that provides the smoothness of a traditional automatic with the efficiency of a DCT.

The Acura DCT8 uses a conventional torque converter with a torque converter clutch. The dual clutches sit at the end of the mainshaft and the secondary shaft (figure 3). Both the mainshaft and secondary shaft consist of an inner and outer shaft assembly. When applied, the clutches lock the inner and outer shaft assemblies together.

The DCT8 attempts to improve upon the traditional DCT design by using a torque converter to provide smoother operation during vehicle launch and low speeds, but then apply the torque converter clutch once the vehicle reaches the proper speed.

Regarding the mainshaft (figure 4), torque travels from the torque converter turbine through the inner mainshaft and drives the mainshaft clutch assembly. The clutch hub of the mainshaft clutch assembly drives the outer mainshaft.

The outer mainshaft contains the synchronizing assemblies, which can lock the 2nd , 4th , 6th , and 8th gears to the outer mainshaft. When the mainshaft clutch applies, it locks the inner and outer shafts together.

Since hydraulic servos move the shift rails and engage the desired gear before the clutch applies, the module only needs to control mainshaft and secondary shaft clutch apply and release. The synchronizing assemblies use traditional brass synchronizing rings, just like a Honda manual transmission.

Torque to the secondary shaft (figure 5) comes from a fixed gear on the mainshaft, through an idler gear, and then to a fixed gear on the inner secondary shaft. The inner secondary shaft drives the secondary clutch housing, and the secondary clutch hub drives the secondary outer shaft.

The outer secondary shaft contains the synchronizing assemblies, which can lock the 1st , 3rd, 5th , and 7th gears to the outer secondary shaft. Just like the mainshaft, the synchronizing assemblies lock the gears to the outer secondary shaft. When the clutch applies, it locks the inner and outer shafts together.

Like a traditional DCT, the Acura DCT engages one clutch at a time, which allows the transmission controller to pre-engage the next gear, whether it’s for an upshift or a downshift. During the shift, one clutch engages and the other clutch releases. This allows for very quick and controlled shifts.

In the example (figure 6), first and second gears are engaged. Torque transfers through the mainshaft fixed gear, through the idler gear, and then to the secondary shaft fixed gear, which is part of the secondary inner shaft.

When the secondary clutch applies, it delivers torque through the inner shaft to the outer shaft and drives the 1st /3rd and 5th /7th synchronizer hubs. The sleeve locks the first speed gear to the hub, so torque transfers from the first speed gear to the large counter gear, and then to the drive pinion gear.

The 2nd /4th hub is already locked into the 2nd gear position to prepare for a 1-2 shift. When the 1-2 shift occurs, the secondary clutch releases and the mainshaft clutch applies. After the transmission is in 2nd gear, the control module can either upshift the secondary shaft into 3rd to prepare for the next upshift, or remain in 1st to prepare for a downshift.

The DCT8 internals look like a smashup of a manual and an automatic transmission (figure 7). There are four shift rails with permanent magnets attached. The magnets act on Hall Effect sensors to inform the control module of shift rail position.

The idler gear connects the mainshaft to the secondary shaft. It contains the reverse idler gear and clutch assembly (reassembled to demonstrate transmission layout; figure 8).

The DCT8 is a relatively simple transmission to rebuild. Some things worth noting: The bolt retaining the 5th and 6th countergear is a 12mm, lefthand thread (figure 8). You’ll need a two-jaw puller to remove the bearing, but it’s easiest to pull the gear and bearing together. Be careful not to chip the edges of the hardened gear.

There’s no way to strip the transmission shafts completely of the gears and hubs, so you’ll need to remove and install all four shafts and two shift rails together. Consider using a hose clamp (figure 9) to hold the shafts in alignment to keep them from separating during removal and installation. Be careful handling the shift rails so you don’t damage the permanent magnets and the nylon shift fork pads.

This transmission makes full use of feed tubes (figure 10). There are no less than 11 feed tubes and they stack in a specific order. Do yourself a favor and take pictures of the tubes before you remove them.

Next time, we’ll cover the electronic and hydraulic control of the Acura DCT8. This unit is solenoid and sensor rich, with 11 solenoids and 11 sensors, and the valve body is mounted externally.

As you might imagine, with this transmission relying so heavily on electronics, many issues may be resolved through a reflash. There’s already a reflash available to cure a “bump or hard shift when coming to a stop.” Now more than ever, transmission issues can be solved via software.