Honda/Acura transmissions have evolved quite a bit over the past couple decades. When looking back to the five-speed transmissions from the early 2000s, you’ll find a fairly complicated electronic and hydraulic system that uses shift solenoids to position shift valves, where either line pressure or clutch pressure control (CPC) pressure is directed to the clutches.
The shift solenoids states changed frequently because there were in-gear states and in-between gear states. Line pressure was directed to the clutches when in a gear, and CPC pressure was directed to a clutch when the transmission was between gears.
After a decade or so, Honda engineers figured out that the CPC solenoids could always control pressure to the clutches, so their next generation of transmissions primarily used CPC solenoids for total clutch control. This simplified the hydraulic operation, making the hydraulic schematic easier to read.
Unfortunately, the electronic and hydraulic control for the Acura 8-speed DCT is back to being fairly complex.
There are eleven solenoids in the valve body (figure 1). The five shift solenoids are normally closed, on/off design. The shift solenoids control the torque converter lockup valves (solenoids A and B) and they shuttle shift valves (solenoids C, D, and E), which direct pressure to the appropriate hydraulic servos. Check out the October 2017 issue of GEARS for an overview of how the Acura DCT-8 is constructed, including mechanical powerflow.
There are six, pulse width modulated, linear solenoids that control the mainshaft and secondary shaft clutch pressures (CPC A and B), servo piston pressures (CPC C and D), torque converter clutch pressure (CPC E), and line pressure.
All of the linear solenoids, except the line pressure solenoid, are normally closed, so when the solenoid is electrically off, it’s also hydraulically off, or not passing fluid pressure.
The CPC solenoid A is in charge of the secondary clutch pressure, and the CPC solenoid B is in charge of the mainshaft clutch pressure.
The mainshaft and secondary shaft clutch pressures alternate as the transmission upshifts (figure 2). The light blue waveform shows the secondary shaft clutch pressure sensor; the red is the mainshaft clutch pressure sensor; dark blue shows the shift command.
The graph demonstrates the transmission shifting from first through seventh gears. As the blue trace steps up, the transmission upshifts into the next gear. The PCM controls the duty cycle to CPC A and B to gain the proper clutch overlap and apply/release rates. CPC A and B modulate line pressure to the clutches, and the line pressure is controlled by a normally-open line pressure solenoid.
As of now, there are no pressure specifications listed in the service information, but while driving a known-good vehicle, the pressures maxed out in drive at about 1000 kPa (145 PSI); in reverse it reached 2000 kPa (290 PSI). While cruising in gear, the pressures were about 550 kPa (80 PSI).
CPC C and D control pressure to the servo assemblies that shift the four shift rails (figures 3 and 4). To allow the shift rail to shift in both directions, each shift rail is shuttled by two servo pistons. There’s a shift rail for 1st – 3rd , 5th – 7th , 2nd – 4th , and 6th – 8th gears, with eight servo pistons shifting these four rails.
Since there are only two CPC solenoids controlling the four shift rails, the shift solenoids C, D, and E position the shift valves to allow the proper CPC solenoid to control the appropriate servo piston.
For example, the 1st – 3rd shift rail is shuttled by the 1st and 3 rd servo pistons. The 1st servo piston shifts the rail into first gear, and the 3rd servo piston shifts the rail back to neutral and into third gear when needed. The shift solenoids allow CPC C to move the 1st – 3rd shift rail back and forth.
One interesting discrepancy is that the shift solenoid tables in the service manual don’t come anywhere close to matching the graphs, and when graphing out the solenoid sequence, the shifts don’t always have the same solenoid sequence.
Since this is a dual clutch transmission and the PCM pre-shifts the non-applied shaft into a gear that it anticipates you’ll want next, the shift rails on the non-applied shaft might be in an upshifted, downshifted, or neutral range.
The point is that the shift solenoids position the shift valves to allow CPC solenoid C to control the 1st – 3rd and 6th – 8th shift rails, and to allow CPC solenoid D to control the 2nd – 4th and 5th – 7th shift rails.
Most of the time, the PCM actuates the shift solenoids right after a shift command (figure 5). While reviewing the graph, the red steps represent shifts 1st through 7th . All other traces are shift solenoids electrically cycling off and on.
When the shift solenoids energize, the CPC solenoid delivers pressure to the appropriate servo and, once the shift is complete, the CPC solenoid and shift solenoids turn off. The PCM pre-shifts the transmission right after the shift command, so, for example, you’ll see the 1st – 3rd shift rail shift into 3rd right after the transmission shifts into 2nd gear.
Look at the operation of CPC solenoids C and D, and the 1st -3rd and 2nd – 4th shift rail stroke sensors (figure 6). Notice how the CPC solenoids energize momentarily after the shift and only apply as long as needed to stroke the shift rail.
The shift rail position sensors monitor the shift rail position. These are digital sensors that alter a voltage provided by the PCM to indicate shift rail position.
When the 1st – 3rd and 2nd – 4th shift rails are in their lower gear, sensor voltage will be around 1 volt; when they are in their higher gear, sensor voltage is around 4 volts; in neutral, sensor voltage is around 2.5 volts (figure 7). The opposite is true for the 5th – 7th and the 6th – 8th rails because of the way the gears are configured in the transmission.
In addition to the four shift rail position sensors, the DCT8 has a range sensor, ATF temperature sensor, three pressure sensors, and four speed sensors (figures 8 and 9). All sensors and solenoids mount externally.
The speed sensors produce a digital signal. The even and odd shaft speed sensors create a signal using tone wheels built onto the mainshaft and secondary shaft clutch hub (figure 10). These tone wheel blades are fragile, and the sensors measure the tone wheels from the side of the blades, so be careful when working with the hub assemblies.
The mainshaft and countershaft speed sensors create a signal from the tips of the helical gears. The mainshaft speed sensor creates a signal from the fixed gear on the mainshaft, and the countershaft speed sensor uses the 7th speed gear, which is in constant mesh with the counter gear assembly.
When reviewing the electronics on the Acura DCT8, it’s obvious that, with 14 sensors and 11 solenoids, this transmission is very sophisticated. To make seven upshifts and lockup control feel seamless, the engineers have designed a complex arrangement of solenoid control and input monitoring.
With this heavy emphasis on electronic control, don’t be surprised to see updates correcting shift quality issues as time goes on.