QuestionMy 97 Buick LeSabre runs fine. When the engine get to operating temperature, the torque converter will not lock up.
AnswerTorque converters contain a one-way clutch. If the clutch jams and locks the stator (which normally keeps the stator tuning only one way), the converter can’t circulate the fluid properly between the impeller and turbine. The resulting turbulence inside the unit can cause a lack of power at highway speeds and may cause the engine to overheat during cruise conditions. A locked one-way clutch will also cause the fluid to run dangerously hot, which can damage the transmission and lead to transmission failure. If the one-way clutch fails to hold its position and freewheels in both directions, the torque converter can’t multiply torque normally and the vehicle will accelerate slowly (like starting out in 2nd gear). The only cure for either condition is to replace the torque converter.
A "stall test" can be used on many older transmissions to check its ability to hold torque and the operation of the converter one-way clutch. Some manufacturers do not recommend using a stall test because it stresses the transmission, so if you use this procedure, do not do it for more than five seconds at a time. Do not perform this test on vehicles that are equipped with a traction control and/or ABS systems. The inputs of a partially opened throttle, brakes applied, and no detectable movement by the wheel speed sensors will at best set a code, and at worst cause damage to the vehicle. Also, on electronically controlled transmissions that use vehicle speed sensor to determine shift points be prepared for a code to be set if you perform a stall test.
Before conducting a stall test, check the fluid level and condition. Chock the wheels and set the parking brake. Start the engine and place the transmission in drive while holding the brake pedal under firm pressure so the vehicle doesn’t move. Then push the accelerator to the floor while holding the brakes on. Note the maximum rpm the engine reaches. This is the stall speed. If it is lower than specifications, the torque converter one-way clutch is slipping. If the stall speed is higher than specifications, the transmission is slipping. Possible causes include a low fluid level, restricted fluid filter, a sticking pressure regulator valve, slipping clutches, bands, shaft splines or one-way clutch.
Most late-model automatics have some type of lockup torque converter to improve fuel economy. If the lockup fails to engage, there will be some slippage and fuel economy will drop. Causes here include a bad lockup solenoid, incorrect sensor input information to the transmission controller or PCM (typically a speed sensor) and hydraulic control problems. If the lockup fails to release, the engine may shudder and die when coming to a stop. Causes here include a faulty lockup solenoid, a sticking lockup valve, grounded lockup solenoid wiring or a missing lockup solenoid spacer plate screen.
