In 2013, when I presented the Honda 6-speed at Expo in Washington, D.C., I speculated that Honda was pushing their traditional helical gearset design to the max. Their 6-speed transmission weighs about 250 lbs without fluid, it’s about 15.5” long, and it has a whopping five shafts to deliver six speeds and reverse.
With the competition moving to eight and nine speeds, something had to give. Well, Honda engineers haven’t been resting on their laurels. In recent years, Honda has engineered new CVTs for their smaller vehicles and the 4-cylinder Accord, a dual clutch transmission (DCT) 7-speed for the Acura RLX hybrid, and an 8-speed DCT fitted into the Acura ILX and TLX models.
The DCT8 transmission is covered in the Oct/Nov 2017 and Jan/Feb 2018 issues of GEARS and you can also find it online. Even the Acura NSX has a new 9-speed DCT. This is all in addition to the ZF 9-speed transmission used in their Acura MDX, Honda Pilot, and Odyssey.
How can they top all of that? Well, Honda plunged head-first into planetary gearsets by designing and manufacturing a 10-speed automatic transmission with four planetary gearsets and a special 2-way clutch that allows a ratchet-type mechanical clutch to either lock in both directions or allow the gearset to freewheel in one direction.
Honda introduced the 10-speed in the 2018 Honda Odyssey and Acura RLX. Compared to the 6-speed, the 10-speed is a solid inch shorter and weighs about 30lbs less! The gearset, including the clutches, is a mere 10” long. The gearset and clutches are very compact (figure 1).
Trying to understand the powerflow for all ten speeds and reverse is a headache in the making. To make some sense of it (and any new transmission that I take apart), I often draw up a simple schematic representing how the gears connect to each other and how the clutches drive/hold specific gearset combinations. This transmission didn’t end up being so simple (figure 2). There are four planetary gearsets shown, with gearset one (P1) to the far right and gearset four (P4) to the far left.
The illustration shows the transmission operating in first gear. Brake 1 (B1), brake 2 (B2), and the 2-way clutch (TWC) are holding everything colored in black. Some common connections you might make out include:
- The input shaft has the P3 sun gear splined to it, so it’s always considered an input. The input shaft also drives the C1 clutch and the C3 clutches. These are all indicated in red on the illustration (figure 2), and the bottom of figure 3 shows the input shaft, P3 sun gear, and C1 and C3 clutches.
- P1 internal, P3 carrier, and P4 carrier are all splined together. This is indicated in purple and it’s the middle gearset in figure 3.
- P1 carrier and P2 carrier are splined together. These carriers are held stationary by the 2-way clutch in reverse and first and they freewheel in all other gears. The illustration shows these gears in black.
Have a look at the P1 planetary, P2 planetary, and the TWC (figure 4). Also shown is the P1 sun gear and hub for B1, the P4 sun gear and C2 clutch (and hub) which allows the P4 sun gear to either be held by the B2 brake or be connected to P3 internal and P2 sun.
Confusing, isn’t it? Nobody ever said squeezing ten forward speeds into a transmission with six hydraulic clutches and one mechanical clutch would be easy.
The two-way clutch is an interesting piece (figures 5 and 6). It operates as a ratchet, or diode style one-way clutch, but on the other side of the clutch is a mechanism that locks the mechanical clutch and prevents it from freewheeling in either direction (figures 7 and 8).
This transmission uses three hydraulic driving clutches and three hydraulic brake clutches. The C1 clutch has a pressed-on metal clad seal in the drum, which you can pop out using a pry bar. At first I thought this seal was molded into the drum, but after I destroyed it in the name of research, I was able to pop it off (figure 9).
Aside from that, the clutches are unremarkable. All the driving clutches use balance pistons and don’t have orifices or checkballs to drain the fluid when not applied.
It’s evident that the engineers were making every effort to reduce the overall length of the transmission. Examples include:
- The design of C1 piston seal.
- An internal retaining snap ring for the carrier in P1 to the housing for P2.
- The use of the P2 internal gear as a surface for the output gear.
- The elliptical torque converter.
- The off-axis oil pump, which is chain-driven (figure 10).
You can easily air check all the clutches with the valve body removed (figure 11). According to the service manual, there’s no line pressure test procedure since the TCM can monitor oil pressures from the various pressure switches. If you feel the need to check pressure, there are C1 and C2 pressure taps located on the case.
The valve body is loaded with electronics (figure 12). The top of the valve body includes eight linear clutch pressure control solenoids (CPC), four shift solenoids, and six pressure sensors. The underside of the valve body includes another three solenoids (for a total of 15 solenoids!), two stroke sensors, and a transmission fluid temperature sensor (figure 13).
The good news is that they don’t need a range reference sensor since there’s no manual valve. The TCM controls everything regarding clutch application and the park actuator electro-hydraulically engages the park rod to lock the vehicle in park.
This brings up an interesting point, which will likely become mainstream in the coming years. With the influx of auto-stop type vehicles (engines that shut off when the vehicle comes to a stop and restarts when you release the brake pedal), the automotive manufacturers are providing a means to electronically engage park for safety reasons.
Think about potential safety issues:
- The customer comes to a stop in their driveway or a parking lot and the engine enters auto stop.
- The customer gets distracted for a moment and forgets to place the vehicle in park.
- The customer gets out of the vehicle and the engine starts, allowing the vehicle to pull away.
In this example and without the use of safety features, when the brake pedal is released, the engine starts back up and the vehicle attempts to drive off. But, with an electronic park actuator, as soon as the driver unbuckles the seat belt and opens the driver’s door, the transmission automatically goes into park.
This feature should prevent accidental drive offs on these auto start/stop vehicles. It might also make for some interesting future transmission complaints regarding vehicles that cannot be pulled out of park. Now we have to think of seat belt sensors and driver’s door switches.
There are other “forced into park” situations, such as:
- Driver switches the ignition off when the vehicle is below 1 MPH and the engine running (driver forgot to switch to park).
- Driver doesn’t press the brake pedal when commanding a gear other than park.
- Driver presses the accelerator when commanding a gear other than park.
Likewise, the TCM will prevent a shift into park or reverse if the vehicle’s moving. It’ll prevent a shift into park if the vehicle is moving over 1 MPH, a shift into reverse if moving faster than 2 MPH, and a shift into drive if the speed is greater than 7 MPH.
The range reference chart shows the wide range of gear ratios and the use of four overdrive gears (figure 14). While studying the chart, you might notice that for any upshift or downshift, the TCM releases only one clutch and applies only one clutch. The CPC solenoids can control the rates of apply and release for a seamless shift.
The torque converter design will likely present some challenges to the converter rebuilding industry (figure 15). The elliptical construction of the converter makes for a tight squeeze of the impeller/turbine/ stator (figures 16 and 17).
The torque converter uses a captive clutch design with two sets of dampening springs on the turbine assembly: one set on the outer diameter and one set more toward the center. The damper and turbine assembly have 38 rivets holding the TCC hub, turbine shaft hub, and spring stops together.
On the converter that I cut apart, the TCC had very tight clearance. The multiple disc clutch assembly has selective plates as discovered by the stampings on the backing plate.
Regarding maintenance, here’s how you can check the oil level:
- Warm the engine up.
- Shift through all ranges and manually shift into second gear.
- Place in park.
- Shut the engine off.
- Remove the ATF Level bolt on the side of the transmission (figure 18).
There should be a slight drip from the hole. If no fluid is present, add Honda Type 2 ATF to the fill hole.
This mechanical overview of the Honda 10-speed transmission demonstrates that advances can be made to existing planetary gearset technology to make a transmission with more gear ratio options smaller and lighter, yet still hold the torque and horsepower of a modern powertrain.
Look for another article in the near future covering the electronics and hydraulics of this impressive transmission.