When fully automatic transmissions were first introduced in the 1940s, they used a fluid coupling to build the hydraulic fluid pressure that actuated shifts. These early automatic transmissions worked extremely well, however they weren’t very efficient in terms of fuel consumption and power delivery. As emissions systems, fuel injection and other automotive technologies started to become more prevalent, a new way to control shift patterns became necessary. Electronically controlled automatic transmissions begin to appear in the 1980s, but the use of a dedicated transmission control unit didn’t become widespread until the 1990’s. So let’s take a closer look at what a transmission control module is, does, and how it can impact your driving experience.
What Transmission Do I Have?
What is a transmission control module and how does it work?
In a nutshell, a transmission control module is a collection of circuit boards and microprocessors. It’s responsible for activating solenoid’s, which then direct the flow of pressurized hydraulic fluid (a.k.a. transmission fluid) down the right channel to engage a clutch or lock/unlock a planetary gearset. The transmission control unit may be located outside the transmission in-line with the wiring harness. Or it could be located inside the transmission next to the valve body. In order to decide what gear ratio is appropriate, the TCM has to interpret data from a number of sensors including;
Throttle position sensor (TPS)
The TPS sensor provides one of the most important data sources to the transmission control module. The signal coming from the throttle position sensor tells the transmission control module how much load is being placed on the engine. This information can help the TCM determine if an upshift or downshift will be necessary. The TCM can also compare this data to the vehicle speed sensor data, to determine which shift program would best suit the situation.
Vehicle speed sensor / VSS
The transmission control module uses this VSS sensor to determine how fast the vehicle is going, so it will know when it’s time to change gears. If this sensor goes bad, the TCM may select the wrong gear ratio, and the transmission may slip.
Wheel speed sensor
The WSS sensor data can help the transmission control module to understand what the vehicle is currently doing, so it can adjust the shift program accordingly. It can also help the TCM to control function on the torque converter.
Turbine speed sensor/input speed sensor
The ISS sensor helps the transmission control module to calculate the correct amount of slippage to apply to the transmission clutches, bands, and torque converter clutch.
Transmission fluid temperature sensor
The TFT sensor is often used by older transmission control units to monitor the health of the transmission. If an overheat event was detected, the transmission control unit may activate a limp mode to protect the transmission from further damage. Newer transmission control modules can use the data from the sensor to modify the operation of the torque converter lockup clutch, or the line pressure.
Brake light switch
Modern transmission control modules that are programmed with an engine braking algorithm will use the data from this sensor to decide if a downshift is needed for braking affect. The TCM may also use this information to operate the shift lock solenoid, which will prevent the driver from selecting a gear unless the brake pedal is depressed.
Cruise control module
The transmission control module can receive inputs from the cruise control system, so it can modify shifting behavior to maintain the desired speed.
Traction control system
The traction control system may signal the transmission control module to modify shifting behavior (prevent torque converter lockup, 1st gear starts etc.) if the road conditions warrant.
Once the appropriate gear ratio has been determined, the transmission control module can send signals/commands to various transmission related components. These include:
Shift solenoids
A shift solenoid is a little plunger-like device that is used to direct the flow of pressurized hydraulic fluid/ATF. If the transmission control module sends an electric charge through the wire that wraps around the plunger mechanism, it will move to block or allow fluid to flow into the appropriate passage.
Pressure control solenoids
Since an automatic transmission requires pressurized hydraulic fluid to operate, the transmission control module has to precisely control that pressure. A pressure control solenoid allows the TCM to regulate that pressure for the clutches or bands. If one of the solenoid fails, the transmission could slip, experience rough shifts or overheat.
Torque converter clutch solenoids
When the torque converter clutch is in lockup position, the engine and the transmission spin at the same speed. Torque converter clutch slip means that only a particular degree of lockup is being applied. The transmission control module is able to control the torque converter clutch function by sending a signal to the torque converter clutch solenoid/TCC.
ECU
The engine control unit/ECU and transmission control module/TCM have to work together in order for the vehicle to go down the road. Some of the many things that the TCM may talk to the ECU about include triggering the check engine light, storing error codes related to the transmission, and temporarily adjusting the engine RPM to accommodate a gear change.
Since the transmission control module is an electronic component with circuit boards and processors, it can store a number of shifting programs. These algorithms allow the TCM to compare the data sent from the various sensors against a number of preset parameters. It can then adjust the transmission shift points by commanding particular solenoids on/off in order to best suit the driving situation.
Transmission control module problems
With the exception of manufacturers design flaws, transmission control module in a well maintained vehicle can last for a very long time. That said, a number of issues can cause transmission control module problems, including:
- Excess heat/transmission or engine overheating
- Electrical short
- Voltage surge
- Water damage
Because the transmission control module is essentially a computer, it can be reprogrammed/flashed to correct for a number of transmission issues. TCM software are becoming a common fix for today’s electronically controlled transmissions. Simply uploading new TCM shift algorithms can drastically alter the way modern transmissions function, and many automakers are issuing updates in technical service bulletins/TSB’s. A vehicle owner can simply visit their local dealer, where the transmission software update will be performed free of charge (in most cases). However, software updates may not be the solution if you have TCM problems. Some of the symptoms of transmission control module problems include:
- Reduced performance
- Unusually poor fuel economy
- Erratic transmission behavior
- Failure to engage the selected gear
- Check engine/transmission light/diagnostic trouble codes
- Engine stalling
- Limp mode (transmission only works in a single gear)
- TCM will not communicate with the ECU or a scan tool
- Transmission gets stuck in or shifts into neutral unexpectedly
Is it safe to drive with a bad transmission control module?
Besides causing serious drivability issues, transmission control module problems can also cause the engine to stall unexpectedly. Needless to say, this poses a serious safety concern. On top of that, ignoring transmission control module problems for too long can result in permanent damage to the internal components of your transmission.
Should you attempt to replace the transmission control module yourself?
This short answer is, no. Unless you have extensive experience with automotive repair, along with a shop full of tools, you should actually not attempt to perform transmission control module repairs on your own. An experienced mechanic will be able to determine the root cause of the problem and repair it effectively. In some cases, TCM problems can be mistaken for other issues like clogged hydraulic passages and faulty sensors. But if the transmission control module is at fault, replacing it is a delicate procedure. Especially if it is located inside the transmission on the valve body. So do yourself in your wallet a favor, and have a professional mechanic solve all of your transmission control module problems.
Example: 6L80 / 6L90 TEHCM Applications & Compatibility
The 6L80 / 6L90 TEHCM sends and receives data from the ECU to the TCM, then distributes actionable commands to the various components like shift solenoids and pressure control solenoids. It also receives data from the various components, which it then sends back to the TCM and ECU, so it can decide which shift program to use for a given situation. All of this happens along the data bus, which is extremely important to making everything work. To facilitate this, there is a 16-pin connector on the side of the TCM, through which all of the data flows.
The 6L80 / 6L90 TEHCM is a single component that contains the following transmission parts:
6L80 / 6L90 line pressure control solenoid
6L80 / 6L90 pressure control solenoid (4)
6L80 / 6L90 shift solenoid (2)
6L80 / 6L90 TCC pressure control solenoid (torque converter clutch)
6L80 / 6L90 pressure switch (4)
6L80 / 6L90 transmission fluid temperature sensor
6L80 / 6L90 TCM
What Transmission Do I Have?
