Torque converters are used in automatic transmissions in lieu of a clutch assembly. The torque converter transfers the power to the transmission and drive wheels. It does this by "spooling up" and locking the engine and transmission together, similar to how a clutch assembly works, but is able to do it automatically instead of making the driver manually shift gears and engage the engine and transmission. While typically trouble-free, torque converters can--and do--malfunction, so here are some things to look for when you suspect a broken torque converter.
Start the engine and allow the vehicle to reach normal operating temperature. Make sure the vehicle is on a level or flat surface.
Shift the vehicle into "Drive."
Let your foot off the brake pedal but do not press on the accelerator. You should notice the vehicle start to "creep" forward. This is a good indication that the torque converter is working. If the vehicle does not creep, and you are on a flat road, then it may indicate that the torque converter has failed.
Press on the accelerator. If the vehicle bucks or revs up before engaging and moving forward, this may indicate a torque converter that is about to fail. It should be noted, however, that a delay in gear engagement may mean that you just need to add transmission fluid. Rough gear engagement can also be symptomatic of serious transmission problems.
Press the brake pedal and hold it while pressing on the accelerator pedal. Do not allow the vehicle to move, but note the maximum RPMs (if your vehicle has a tachometer) that your vehicle reaches. This is called the "stall test." Check the maximum RPMs against the manufacturer's specifications for stall test RPMs. This information is found in the vehicle's service manual.
