Just when one thinks they have a handle on all of the weird stuff that happens in the world of transmissions, in rolls an issue you have never seen. With the number of new transmissions in the market today, the variety of issues the average shop is asked to deal with continues to grow. Many manufacturers, including GM, have introduced “shift by wire” technology into the market. While I am not a fan of the change, it is something that is here and likely here to stay. Unfortunately, this means that sooner or later, you will have to deal with one of these systems and the problems that we see as they arrive on the tow truck.
GM has used a shift-by-wire system for several years in its front-wheel-drive and rear-wheel-drive applications. The system is quite a bit more complicated than some of the other systems on the market, so one needs to understand the system and its operation to diagnose the malfunctions. Some GM systems use a conventional-looking shift lever, while others use a series of pushbuttons located on the dash to control the system. With either of these configurations, there is no shift cable or physical connection between the shifter and the transmission. GM refers to their system as ETRS or Electronic Transmission Range Select.
Four different ETRS designs are in use, GEN 1 internal, GEN 1 external, Gen 2 internal, and GEN 2 external. The primary difference between the systems evolves around the module used to control the system function and the module that stores the DTCs when one sets.
To understand system operation, let’s look at the system used in the 9T50/9T60 applications equipped with ETRS. Seven components comprise the system for the 9T applications; these include:
- Park Inhibit Solenoid: The solenoid acts as a safety system to mechanically hold the park servo in the released position if hydraulic pressure is lost to the transmission. This system prevents the park pawl from engaging as long as vehicle speed is present.
- Mode 1 ETRS Valve: This valve controls pressure to the Mode 2 valve. In Drive and Reverse, it directs pressure to the Park Release Servo Valve Plunger to shift the transmission out of Park.
- Range Select Valve Position Switch: This switch monitors the position of the Mode 1 valve.
- Park Release Servo: Mechanically controls the park release lever and park pawl actuator.
- Park Servo Position Switch: Tells the controller the position of the park release servo piston plunger.
- Mode 2 ETRS valve: Works with the Mode 1 ETRS valve to control the park release servo piston plunger.
- Mode Valve Position Switch: It is used to monitor the position of the mode valves.
The components are mounted in and on the valve body (figure 1). The switches act as position sensors and are of the hall effect design, which is controlled using magnets. The switches are fed 9 volts from the controller and output between .8 and 2 volts back to the controller, indicating valve position. The inhibit solenoid controls the park pawl via a valve body piston plunger known as the park release servo (figure 2). The sensors, switches, and the solenoid can be monitored using your scan tool to help you identify system faults.
The driver information center can display several different messages, which may explain why the system is not allowing the transmission to engage the correct range:
- Press Brake to Shift from Park
The “Press Brake to Shift from Park” message indicates the vehicle did not detect the brake pedal was depressed before attempting to shift out of Park. - Hold Shifter Button to Shift
The “Hold Shifter Button to Shift” message indicates that the driver has not pressed the shifter button before attempting to shift out of Park on those applications equipped with a shift lever. - Conditions Not Correct for Shift
The “Conditions Not Correct for Shift” message indicates that the driver has attempted a prohibited shift. - Service Shifter The “Service Shifter” message indicates an ETRS system shifter fault, which requires the operator’s attention.
- Service Driver Door Switch
The “Service Driver Door Switch” message indicates that the driver door switch or its circuit has a fault. - Service Transmission
The “Service Transmission” message indicates an ETRS system fault which can affect shifter operation. - Service Transmission Now, Unable to Shift Soon
The “Service Transmission Now, Unable to Shift Soon” message indicates that one or more ETRS system faults have occurred. It is a warning to get the vehicle into the shop for service immediately, or it may shortly leave you unable to engage the selected range.
If any system components fail or one of the valves the system controls sticks in the valve body, system operational issues will occur. System faults may or may not set a DTC. DTCs for the system include:
- P187E: Transmission Park Valve Stuck Off
- P18AA: Transmission Range Mode Control Valve 1 Position Switch Circuit Stuck On
- P18AB: Transmission Range Mode Control Valve 1 Position Switch Circuit Stuck Off
- P18AC: Transmission Range Mode Control Valve 2 Position Switch Circuit Stuck On
- P18AD: Transmission Range Mode Control Valve 2 Position Switch Circuit Stuck Off
- P18AE: Transmission Range Mode Control Enable Valve Stuck Off
- P27EC: Transmission Range Mode Control Valve 1 Position Switch Performance
- P27F0: Transmission Range Mode Control Valve 2 Position Switch Performance
The most common failure with the 9T system typically results in a P187E setting which relates to the park valve being stuck in the OFF position. The most common cause of the P187E is a broken magnet within Park Valve Position Sensor 1 or Park Valve Position Sensor 2. When disassembling the valve body, it will likely be apparent that the magnet is broken as soon as you take it apart. In addition, you need to examine the condition of the valves positioned inside the magnets. You will often find the tips of the valves positioned within the magnets have broken. Be aware that neither the magnets nor the valves are serviced separately from the valve body. Finally, inspect the valves for being stuck within the valve body bore as that will act just like a bad magnet.
Well, there you have it. When you get the call from the customer stranded at the Costco parking lot, you will now have an idea of where to look. Until next time remember, “The good Lord made us with two ends, one to sit on and one to think with. Heads you win, tails you lose.”
