HomeTips & GuidesWhat Is Torque Converter Lockup: A 1:1 Engine to Gear Drive Ratio Lock

What Is Torque Converter Lockup: A 1:1 Engine to Gear Drive Ratio Lock

What is torque converter lockup? Torque converters are critical in automatic transmissions because they provide the engine-to-transmission link. 

The fluid-filled, doughnut-shaped device provides hydraulic coupling between the engine crankshaft and gearbox, enabling them to rotate at different speeds. 

This fluid coupling eliminates the need for a gear pedal and makes driving easy. 

But the indirect connection introduces some energy losses and can create excessive heat when going uphill or towing heavy trailers. 

So torque converter lockup is necessary to overcome these issues. 

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What Is a Torque Converter?

Automatic transmissions have had the slang slushboxes for a while, and this slushiness refers to the fluid coupling in torque converters.

Manual transmissions created direct connections with engines via clutch plates. 

But they require more work to drive and can be tricky to balance when starting from a hill. Also, they can create shock loading that causes jerking.

A car transmission cutaway (note the torque converter position)

A car transmission cutaway (note the torque converter position)

Automatic transmissions solved this issue but at a cost. Manual transmissions were efficient because they linked directly to the engine. 

But automatic transmissions use automatic transmission oil to transfer energy from an engine-rotated impeller to the turbine connected to the output shaft.

This fluid coupling is the function of the torque converter.

How Torque Converters Work

Torque converters simplified driving and eliminated shock jerking. But they introduced two unwanted factors: overheating and energy losses.

When the crankshaft turns, it spins the torque converter impeller, exerting pressure on the transmission fluid. 

And these two parts rotate at the same speeds because the impeller bolts to the torque converter’s case, which then bolts to the flywheel. 

The flywheel mass connects to the crankshaft to smoothen the engine power.

An image showing how torque converters work (note the centrifugal force on the oil)

An image showing how torque converters work (note the centrifugal force on the oil)

Inside, the impeller acts as a transmission oil pump because it has fins that push the oil outwards.

 This push creates a “vacuum” at the center, which forces the oil to the turbine blades and back to the middle.

The resulting oil circulation transmits energy to the turbine, which spins the transmission input shaft, then to the differentials and drive shaft. 

Torque converters also have stators to redirect the oil from the turbine before it hits the converter pump to avoid slowing the engine and wasting energy.

But some RPM slippage occurs between the turbine vanes and impeller blades, causing a disturbance in the oil. 

This disturbance heats the oil and causes losses in power transmission.

These losses and heating become more profound when driving uphill or carrying/towing heavy loads.

A dismantled torque converter

A dismantled torque converter

What Is Torque Converter Lockup?

A torque converter lockup is a mechanism that locks the torque converter case and turbine using a friction disc (internal clutch) on the converter’s front inner cover. 

Once locked, the automatic transmission functions like a manual due to the direct connection and 1:1 drive ratio.

This setup eliminates pressure from the transmission oil, meaning it won’t heat. And the car won’t have fluid coupling losses, resulting in better fuel economy.

Usually, the ECU locks up the torque converter when the vehicle attains a specific gear and speed under a low load. 

It activates a solenoid electrically, which controls a valve that allows the transmission fluid pressure to force the friction disc out. 

When pushed by the hydraulic pressure circuit, this frictional clutch contacts the transmission housing, causing it to rotate at the engine speed.

A clutch plate

A clutch plate

Some eco-modders alter these lockup parameters, enabling the lockup to occur at lower speeds/gears, when accelerating, or when going uphill to get better fuel economy. 

These lockup kits keep the engine RPMs lower to reduce fuel consumption.

Torque Converter Lockup Failures

A torque converter clutch will not engage when the engine runs at cold temperatures (when the coolant temperature is below 120°F). 

Also, it won’t kick in if the overdrive unit locks out.

But in some cases, the clutch can fail, such as by remaining locked to the front of the inner converter case. 

This situation locks the crankshaft and driveshaft even at speeds unideal for direct connection.

Old torque converters in a transmission repair workshop

Old torque converters in a transmission repair workshop

If you’ve driven a manual trans car and pressed the brake pedal while in high gear, the engine will jerk, then stall if the speed goes too low. 

Something similar to this scenario will happen if the torque converter lock fails. The motor will stall if you brake.

The clutch can also fail to lock, decreasing the fuel economy and overheating the transmission oil.

Lastly, the lockup clutch can slip after engaging, causing the engine speed to be slightly higher than the driveshaft or increase gradually.

 Slipping creates friction, generating extra heat in the transmission fluid.

How To Troubleshoot Torque Converter Lockup Failure

You can find out if your vehicle’s converter is functional when driving in two ways. The first is when accelerating. 

Pay attention to the transmission shift timing. The locking clutch can change the engine speed slightly. 

And you can use an auxiliary tachometer to check this transmission shift timing.

A torque converter disassembled from an automatic transmission

A torque converter disassembled from an automatic transmission

Secondly, observe the revs (engine speed) after releasing the throttle or when braking. 

The lockup converter clutch tends to unlock when you brake or let go of the accelerator and will change the engine speed more dramatically than when locking. 

So it is easier to notice and troubleshoot than using the first method.

Wrap Up

In conclusion, torque converter lockup gives several benefits to automatic transmissions, such as better fuel efficiency and cooler operation.

 And considering the high cost of fuel, this feature is handy for car owners, especially those with trailers. 

That’s it for this article. Contact us if you have any questions about this topic, and we’ll contact you.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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