Technically Speaking
- Author: Wayne Colonna, Technical Editor
It is quite a surprise for a technician to learn that a malfunctioning TCC solenoid could cause a slipping in reverse or loss of reverse with 4L30-E transmissions. Even after a hydraulic change to the 2000 and later Troopers and BMWs with the four-valve-pump EC3 converter-clutch-apply arrangement does this potential problem still exist. In Figure 1, a partial EC3 hydraulic schematic for a 4L30-E, you can easily see that the TCC signal can stroke a reverse-lockout valve.
The original design purpose for allowing TCC-signal oil to stroke a reverse-lockout valve is that should the driver select Reverse while the vehicle is moving forward at 7 miles per hour or greater, the computer has a way to prevent the engagement by simply energizing the TCC solenoid. But if the solenoid malfunctions mechanically, no reverse or delayed and slipping reverse may occur.
The same is true for the 4/5L40-E transmissions, as you can see in both figures 2 and 3. So the next time you encounter one of these units with a no-reverse condition, you can move forward on a repair quickly by inspecting the TCC solenoid and reverse-lockout valve.
