What Is a Lockup Solenoid?

Lockup solenoids are electronic components found in some automatic transmissions. These solenoids are designed to lock a torque converter in place under certain circumstances, such as highway speeds. This is done for a variety of reasons, including to keep the transmission from overheating and to improve gas mileage. When a lockup solenoid fails, the torque converter may fail to lock into place at highway speeds, or can stay locked up even after the vehicle slows down. This can result in poor gas mileage, the stalling of an engine, and other problems.

Automatic transmissions use a variety of different fluid-based systems to transfer power from the engine of a vehicle to the wheels. The main component is a torque converter, which functions as a fluid coupling between the engine and transmission. Similar to the operation of a mechanical clutch, a torque converter is able to lock up and transfer power to the transmission, or spin freely when the vehicle is not in motion. Other components, including planetary gears, clutch packs, and valve bodies are used in conjunction with the torque converter to automatically change through a range of gear ratios as needed. Some transmissions are also equipped with a lockup solenoid, which can bypass all of the other components, and lock a torque converter to the rotation of an engine when the situation calls for it.

A lockup solenoid is sometimes referred to as a torque converter clutch (TCC) solenoid, since it is essentially an electric valve that a transmission control unit (TCU), or engine control module (ECM), can use to directly control the torque converter. Some of these solenoids can only lock a torque converter fully, while others have a wider range of control. The most common use of a lockup solenoid is to force a torque converter to rotate at the same speed as the engine when the vehicle is moving very quickly, which can result in improved gas mileage. Some TCC solenoids can also activate partially when a transmission is in a lower gear, which can be beneficial to fuel economy as well.

When a lockup solenoid fails, a number of different issues can be encountered. If the solenoid fails to engage, then there will typically be a reduction in fuel economy. In some cases, the transmission fluid may become too hot at highway speeds as well, which can lead to the premature failure of various internal parts. If one of these solenoids fails when locked, it can cause an engine to stall out while idling. This is due to the fact that idling engines are typically unable to handle the extra load of rotating a torque converter.