Torque converter lockup is when a clutch inside the torque converter locks the engine directly to the transmission input, usually at steady cruising speeds. A converter normally transfers power through fluid, which allows some slip; locking it up removes that slip, which lowers engine rpm, reduces heat and improves fuel economy. The engine computer controls lockup through a solenoid, and it releases lockup when you brake, slow down or press hard on the throttle.
How lockup works
- At low speed, the converter slips through its fluid coupling, which lets the car idle in gear and multiplies torque.
- At cruise, the computer energizes the torque converter clutch (TCC) solenoid.
- Fluid pressure applies the clutch, locking the converter, and rpm drops slightly.
- Braking or heavy throttle releases it.
See what does a torque converter do?
Lockup problems
| Symptom | Likely cause |
|---|---|
| No lockup: higher cruise rpm, more heat | TCC solenoid, wiring, brake switch input or worn clutch; codes P0740, P0741 |
| Shudder when lockup engages | Worn converter clutch or degraded fluid; see torque converter shudder |
| Engine stalls when coming to a stop | Lockup stuck applied; codes such as P0742 |
| Lockup cycling on and off | Sensor input problems or solenoid faults |
Testing for lockup
On a warm transmission at a steady 45 to 55 mph in top gear, lightly tap the brake pedal without slowing much. With working lockup, rpm rises slightly as lockup releases, then drops again when it reapplies. A scan tool that shows converter slip speed gives a clearer answer.
Torque converter lockup solenoid
The TCC solenoid sits in the valve body or solenoid pack. When it fails, lockup stops working or will not release. Check the wiring and codes before replacing it; see transmission solenoid symptoms and transmission solenoid pack.
Lockup switch and lockup kits
Older lockup transmissions swapped into vehicles without the matching computer need a way to control lockup, which is where aftermarket lockup controllers and kits come in; see 700R4 vs 4L60E. Electronic transmissions in swaps need a compatible controller.
Lockup and fuel economy
Because an unlocked converter always slips a little, a transmission that never locks up runs a few hundred rpm higher at highway speed and uses more fuel. A sudden drop in highway fuel economy together with higher cruise rpm is often the first sign of a lockup problem.
Why lockup matters for towing
A converter that slips at highway speed creates a great deal of heat. Working lockup keeps fluid temperatures down; see transmission overheating and tow/haul mode.
Stall converters
Performance stall converters change how much the converter slips before it couples. Many still include a lockup clutch; see what is a stall converter? and bad torque converter symptoms.
More guides: transmission or torque converter? · how long a torque converter lasts.
Trouble codes: P0703 Brake Switch B Input · P0719 Brake Switch B Circuit Low · P0724 Brake Switch B Circuit High · all OBD2 codes.
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