How to Test a Torque Converter

An antique illustration of an adaptor transmission gearbox. — Testing a Torque Converter requires a certain amount of skill. ilbusca / Getty Images

Your car's transmission is having some problems, but you're not sure what's wrong. It's possible that your transmission is fine, but your torque converter needs to be replaced. You can diagnose problems with your torque converters on many older transmissions by conducting a stall-speed test. Here's how to test your torque converter:

Take the following precautions Check with your car or transmission manufacturer to make sure it's safe to run the test. Most newer transmissions, as well as some older ones, can actually be ruined by a stall-speed test. Don't run a stall-speed test for more than five seconds at a time. Don't run this test on vehicles that have traction control or anti-lock brake systems. On some electronically controlled transmissions, a stall-speed test will set off your check engine light.
Prepare your car Before testing your torque converter, make sure all your fluids are in good condition. Also, chock your wheels and set the parking brake. If your car doesn't have a tachometer, install one that can be seen from the driver's seat [source: ASE Test Prep].
Start your engine Press the brake pedal all the way to the floor and start your engine. Shift your transmission into drive. Don't let go of the brake.
Put the pedal to the metal While pressing on the brake pedal, press the accelerator to the floor for two to three seconds. Don't exceed five seconds, or you risk blowing out the transmission. The RPM the engine maxes out at is the stall speed.
Interpreting the stall-speed test result If the RPM reading is lower than the specifications for your particular torque converter and engine, it means the torque converter is failing and needs to be repaired or replaced. If the RPM reading is too high, then your transmission is slipping and you'll need to investigate the problem [source: ASE Test Prep].

Originally Published: May 11, 2011

Test Torque Converter FAQs

How can you tell if a torque converter is bad?

When your car has a bad torque converter, it shudders unusually and may even show signs of slipping while you overdrive. At low speeds, cars don’t vibrate but with a malfunctioned torque converter, this happens more frequently. Also, chances are high that you might experience driving lags along with whirring and clicking.

What would cause a torque converter to lock up?

When the temperature of the coolant reaches a cold engine temperature, around 120 °F, it may cause a torque converter to lock up. Also, when the overdrive unit locks out, it also makes the torque converter to lock up as well.

Can a bad torque converter damage transmission?

Undoubtedly, a bad torque converter can severely damage the transmission. Besides, it may also cause overheating, friction, knocking and ruin transmission fluid. If overlooked for a longer period of time, these problems may even damage the engine.

How much does it cost to replace a torque converter?

If you know how to replace a torque converter yourself, the replacement will cost you around $150 to $500. On the other hand, a professional car mechanic may be costly; you might be charged around $600 to $1,000 for a complete service.

How do I know if my torque converter is locking up?

Notice your RPM reading at your dashboard. If you see that your RPM reading rises constantly even though you’re not cruising your vehicle, that’s the sign of a locking-up torque converter. Another sign can be the stalling engine even when your vehicle is at a complete halt.