Street Smart - September - 2018

GM’s AF50 8-Speed vs. Toyota’s U880 8-Speed: The Same But Different!

Let’s take a look at some of the changes GM made with the Aisin 8-speed FWD.

One of the first things that’s different is GM integrated the park neutral switch into the TCM (figure 1).

With the integrated position sensor, we now have the following advantages:

  • Reduced number of parts
  • Simplified wiring harness
  • Enhanced reliability because there’s no harness circuit between the TCM and the position sensor

The AF50 has only two speed sensors: the input speed sensor (NIN) and the output speed sensor (NOUT). The input speed sensor detects RPM of the intermediate shaft C2 drum as input shaft speed. The output speed sensor detects the RPM of the counter drive gear as output shaft speed.

These signals are transmitted to the TCM, based on the signals the TCM controls: shift timing and lockup (figure 2).

The U880 has three speed sensors:

  • Input/turbine speed sensor B (NC3) reads the C3 and C4 clutch drum
  • Input/turbine speed sensor A (NT) reads the input speed
  • Vehicle speed/output sensor (NC) reads the speed of the counter gear.

Notice that the AF50 doesn’t use the NC3 speed sensor.

Moving on to the valve body, the AF50 added an electromagnetic oil pump (EMOP) mounted on the upper valve body. The structure of the electromagnetic element is the same as that of a linear solenoid (figure 3). The piston in the electromagnetic oil pump operates in response to on/off signals from the TCM to create pressure. Hydraulic pressure generated by the pump is solely used to engage the C1 clutch during an idling stop, smoothing the transition from a stopped engine to a driving state.

AUTOMATIC TRANSAXLE FEATURES

Lockup Control, Slip Control — The system maintains smooth lockup control based on output RPM signals (NOUT), signals from the engine control unit (engine RPM and throttle opening) and vehicle speed. In addition, the system maintains slip control by adding input RPM signals (NIN) to monitor slip rate.

Self-Learning Control — The TCM helps provide smooth clutch engagement at gear shifting and smooth and delicate shifting while driving by performing shift control learning and garage shift control learning.

Neutral Control (N Control) — When the vehicle is at low speed or stopped in drive, the transmission enters neutral by releasing the clutch. Reducing drag loss of the torque converter lightens the load on the engine, enhances fuel economy, and reduces idle vibration.

Manual Shift Control — By moving the shift lever from drive into the manual shift position and shifting into + (upshift) or – (downshift), you can select the desired gear, enabling sporty driving that feels like a manual transmission.

TRANSMISSION ADAPTIVE FUNCTIONS

After performing repairs on the AF50, you’ll need to perform system relearn procedures.

The transmission control module (TCM) uses speed sensor data during commanded shifts to determine if a shift is occurring too fast (harsh) or too slow (soft). Transmission build variation and normal wear of the clutches can cause the shift time (the time required to apply a clutch) to be longer or shorter than desired.

To compensate for these changes, the transmission control module (TCM) adjusts the current output to the various pressure control solenoid valves, to maintain the originally calibrated shift timing.

This self-learning process is referred to as adaptive learning, which provides consistent shift feel and increased transmission durability. Failures to perform the relearn procedure could cause poor transmission performance, set DTCs, and dissatisfy customers.

We’ll be taking a closer look at the AF50 at the 2018 Powertrain Expo. Make sure to reserve your seat right away.

Because that’s not just smart… that’s street smart!