Torque Converter
Torque Converter
The torque converter sits between the engine and the transmission. It is a fluid coupling — the engine and transmission are never mechanically connected at low speeds. Instead, the engine spins a pump inside the torque converter that throws transmission fluid at a turbine. The turbine is connected to the transmission input shaft. The moving fluid makes the turbine spin, which makes the input shaft turn, which drives the transmission. Think of two fans facing each other — turn one on and the wind spins the other. That is the basic idea, but with fluid instead of air.
Torque multiplication
What makes a torque converter more than just a fluid coupling is the stator. The stator sits between the pump and turbine and redirects fluid flow to multiply torque. At low vehicle speeds when there is a big speed difference between the pump and turbine, the stator redirects the fluid so it hits the pump in a way that adds to the engine's effort. This can multiply engine torque by 2 to 2.5 times. As vehicle speed increases and the turbine catches up to the pump speed, the stator freewheels and torque multiplication drops to 1:1. This is why an automatic vehicle has good low-speed pulling power even before the transmission downshifts.
Stall speed
Stall speed is the maximum RPM the engine can reach with the transmission in gear, brakes fully applied, and throttle wide open. It tests the torque converter's ability to multiply torque. A typical stall speed is 2,000 to 2,500 RPM. If stall speed is too low, the stator one-way clutch may be seized. If stall speed is too high, the converter or transmission clutch packs are slipping and not loading the engine properly.
A stall speed test puts extreme heat into the torque converter fluid. Never hold the test for more than five seconds. Allow the fluid to cool for 30 seconds between tests. Excessive heat causes fluid breakdown and internal damage.
Lockup clutch
A fluid coupling always has some slip — the turbine never spins quite as fast as the pump. At highway speed, this slip wastes fuel. The torque converter lockup clutch solves this by mechanically connecting the pump and turbine at cruising speeds so there is zero slip. The lockup clutch is applied and released by the TCM through a solenoid in the valve body. When the lockup clutch wears, it causes a shudder — a vibration felt at light throttle between 40 and 60 mph during lockup. This is the single most common torque converter complaint. In many cases, a fluid service with the correct friction-modified fluid resolves the shudder completely.