It Can Make for a Bad Day - August - 2017

Shift Time: GM’s New Electronic Shifter

An overview of GM’s ETRS system: a shift-by-wire electronic shifter that’s just begun to show up in 2017 Buicks and Cadillacs. Covers how the system works and all the idiosyncrasies you’ll need to be familiar with.

Change seems to be a constant part of life, especially in our industry. Whether it’s driven by government regulation, product problems, consumer needs, or simply to keep up with competitors, change never seems to slow down. This makes our industry a challenge but it also makes it exciting.

Like other manufacturers, GM has introduced a “shift by wire” system for the 2017 model year. Known as ETRS — Electronic Transmission Range Select — the new system is being introduced on the 2017 Buick Lacrosse and Cadillac XT5 equipped with the AF50-8 (RPO MRC) transmission.

As with most changes today, the ETRS system uses multiple modules and communication buses to operate. Modules include the Chassis Control Module (CCM, K38), the Transmission Range Control Module (TRCM K173), and the Transmission Control Module (TCM K71).

Bus communication occurs via the CAN (GMLAN), Chassis Expansion, and LIN (Local Interconnect Network). The Chassis Control Module (CCM) is the key module with this system and receives information from the shifter (driver) regarding shifter position. The CCM also sets and stores most of the codes related to system operation.


The shifter operates similar to the shifter in some BMW applications. It has an LED display that indicates range and it returns to the center position after use. The shifter is a multi-axis design unit, meaning it can move forward and backward as well as sideways to initiate the desired shift.

Two sensors are located in the shifter to transmit the X-Y position of the shifter via four inputs to the Chassis Control Module (CCM). The orange dot in the center of the shifter and the display value in red indicate the selected gear position. The shifter also has two buttons: one on the top (park) and one on the side (shift interlock; figure 1).
ETRS shifter ranges
Look for the park button on the top of the shift lever. The park button is equipped with two sensors that monitor the position of the park button. To shift to park, press the park button with the vehicle stopped.

Pressing the park button with the vehicle moving can result in various actions depending on the vehicle speed:

  • VSS over 10 mph (16 km/h), the park request is ignored.
  • VSS below 3 mph (5 km/h), the transmission shifts to park when speed drops below 1 mph (2 km/h).
  • VSS between 3 mph (5 km/h) and 10 mph (16 km/h), the transmission prepares to shift to park. Once the road speed drops below 1 mph (2 km/h), the transmission will shift to park.

Four other park engagement routines can also occur. The TRCM will automatically shift to park whenever:

  • You shut the engine off with the car in drive or reverse.
  • You open the driver’s door while a VSS signal is present.
  • The transmission is in drive or reverse, the driver’s seat belt is unbuckled, and you remove your foot from the brake pedal.
  • The Chassis Control Module (CCM) is defective or suffers from a loss of power or ground.

To shift out of park:

  • Apply the brakes.
  • Press the shift interlock button on the side of the shift lever.
  • Select the desired gear range.

You’ll also use the shift interlock button to select reverse; you’ll need to press it to move the selector into reverse. Then move the shifter forward and to the left to select reverse.

To select neutral, simply move the shift lever in the desired direction: The neutral LED will display the selected range.

One unique feature is that, if you shift to neutral with the vehicle running and leave it there for an extended period, the Transmission Range Control Module (TRCM) will automatically shift the transmission into park without any other input.


Since the unit can automatically shift the transmission into park without any input from the customer, they added a mode to inhibit park for situations such as automatic car washes.

With the vehicle in neutral, press the engine start/stop button. This forces the system into accessory mode, allowing the transmission to remain in neutral.


The TCRM mounts to the top of the transmission. The TCRM contains a shift motor that controls the position of the manual shaft.

In addition, a “backup park actuator” is included in the TCRM. Although housed in the TCRM, the backup park actuator is electrically isolated from the TCRM to allow it to act as a safety system, allowing the vehicle to shift into park if the TCRM fails.

The chassis control module (CCM) receives the shifter position from the shifter sensors. The CCM then commands the TCRM to move the manual shaft into the appropriate position (figure 2).ETRS Motor


The park control system also interfaces with the vehicle brake control system. If you try to put the transmission in park while the vehicle is on a hill, the EBCM will apply the electronic park brakes.

This system consists of electric actuators mounted to the vehicle’s rear brake caliper pistons, effectively actuating the rear disc brakes. When you select forward or reverse, the EBCM will release the park brakes, allowing the vehicle to move.


The Chassis Control Module (CCM) is the primary controller in charge of this system’s diagnostics. The CCM stores most of the DTCs related to park operation, so your scan tool needs to be able to communicate with the CCM to diagnose the system.

Some faults result in the application of park while others will provide a warning before shifting into park. Some faults may prevent you from shifting out of park.

In addition, if the vehicle battery is dead or disconnected, you won’t be able to shift out of park unless you remove the TRCM from the transmission and manually shift the transmission into neutral.


DTCs for this system are typically stored in the Chassis Control Module (CCM). If the system sets a DTC, it’ll usually try to warn the customer that it requires service. The persistence of the system’s warning depends on the number of key cycles since the DTC set:

1–25 key cycles — The system will display a “Service Transmission” message and chime one time per key cycle.

26–37 key cycles — The system will display the message and the chime will sound at startup and every 10 minutes of operation.

38–50 key cycles — The system will display the message and the chime will sound at startup and every two minutes of operation.

50 or more key cycles — The system won’t allow the transmission to shift from park. A constant “Service Transmission” message will display on the driver information center.


Like other control modules today, the TRCM may require relearning because it’s been removed or replaced.

Before relearning, you’ll need to address and clear any DTCs in memory. An interruption in your data connection or a drop in battery voltage during the motor position learn process can damage the TRCM.

  1. Install a battery maintainer or connect a booster battery in parallel to the vehicle’s battery.
  2. Make sure the TRCM is attached to the transmission properly.
  3. Connect your scan tool.
  4. Bring the transmission temperature to between 50Fº and 104Fº (10Cº and 40Cº).
  5. Select the chassis control configuration-reset functions from the CCM scan tool menu.
  6. Select the TRCM motor position learn procedure and follow the instructions.
  7. Clear any DTCs that set during the procedure.

As you can see, change is constant, and it’s always a good idea to understand how systems are supposed to function. In addition, it enhances your customer’s respect if you can explain the what, why, and how his vehicle operates.