The torque converter is hydraulic coupling that trades slippage for torque multiplication, effectively providing an additional spread of gear ratios for drive-off, acceleration, and passing.
The torque converter clutch locks all of the elements of the torque converter together, preventing slippage and losses in the torque converter when isn't needed.
This improves fuel economy, reduces engine speed, and builds less heat in the transmission.
Here are the basic rules used for controlling the torque converter clutch or "TCC":
The most important considerations for deciding when to lock and unlock the TCC are vehicle speed and engine load / throttle.
At lower speeds and higher loads, the torque converter should be active, so the clutch should be unlocked to allow torque multiplication.
At higher speeds and lower loads, the torque converter isn't needed, so the clutch should be locked to improve efficiency.
The torque converter helps absorb shift shock, so the torque converter clutch should be unlocked during shifting.
The torque converter clutch shouldn't lock too soon after a shift completes.
For simplicity sake, the torque converter clutch is only engaged in 4th gear.
The parameters below take these factors into account and allow you to tune for your engine and vehicle.
The drawing below illustrates the relationship between throttle, speed, and the torque converter clutch control parameters.
Torque Converter Clutch Lock Throttle Position
This parameter is the lowest throttle position for which the torque converter clutch will lock.
Usually this can be left set to 0%, but if there is shudder during deceleration with the torque converter clutch locked, this parameter can be set to a very small value (just a couple of %) to allow the TCC to unlock when the throttle is closed.
Torque Converter Clutch Unlock Throttle Position
This parameter is the highest throttle position for which the torque converter clutch will stay locked. Or, put another way, this is the throttle position at which the torque converter clutch will unlock and allow the torque converter to multiply torque.
This should be set somewhat above the throttle percentage used for ordinary driving and cruising in 4th gear.
Torque Converter Clutch Lock Speed
The is the slowest speed at which the torque converter clutch will lock. This should usually be toward the lower end of the 4th gear operating range.
Torque Converter Clutch Unlock Speed
This is the speed at which the torque converter clutch will automatically unlock if previously locked. This gives some buffer room after the lock occurs (based on the Torque Converter Clutch Lock Speed).
Adjusting this parameter properly can help prevent a "hunting" or cycling situation where the TCC turns on and off.
In most circumstances, this parameter will be set to a speed below the minimum 4th gear speed and thus won't be used because the 4-3 shift will always unlock the TCC.
Torque Converter Clutch Lock Delay
This parameter adjusts how long after a 3-4 shift until the torque converter clutch will be allowed to lock.
After the transmission shifts into 4th gear, a timer countdown begins. Once the timer reaches zero, the clutch can lock, subject to the other parameter settings.
The main function of this parameter is to prevent a 3-4 shift from being immediately followed by a torque converter clutch lock. Depending on the vehicle, and especially the torque converter stall speed, this rapid sequence of shift and lock events can be unpleasant.