To understand the relationship between a malfunctioning lockup torque converter and reduced fuel economy.

The student will identify common, standard non-lockup torque converter and common lockup torque converter malfunctions that can result in reduced mpg.

Theory of Operation.
All automatic transmissions use a torque converter to couple the engine and transmission. Torque converters are not 100% efficient. Some energy is lost between the input (the impeller) and the output (the turbine) sections. (See Figure 1 for the torque converter part relationship.) Internally, torque converters use a one-way clutch device to multiply engine torque at low engine RPMs.

Figure 1

This action aids an automatic transmission vehicle's acceleration capacity. Once turbine speed is approximately 90% of impeller speed, the one-way clutch race spins on its own axis. At this point, the converter is said to be "coupled hydraulically." Under these conditions, up to 10% of the engine's output power could be lost to the torque converter's internal slippage. This accounts for most of the mpg difference between a standard and automatic transmission vehicle. This energy loss is transmitted to the vehicle's radiator and shed as heat.

Recently developed torque converters have had a "lockup" feature added to reduce this energy loss and improve mpg. This lockup feature will not engage until the vehicle reaches approximately 40 mph. Other factors may also prevent this type converter from locking up. Besides the speed sensing indicator, a converter may not lockup for any of the following reasons:

1. Engine temperature too cold - most converters will not lockup until the coolant reaches about 120°F.

2. Overdrive unit locked out - when an automatic overdrive is "locked-out" the torque converter lockup feature will also be locked out. Overdrive lockout would normally only be used when pulling heavy loads, thus the converter is logically "locked out."

3. Under low engine vacuum, heavy part-throttle acceleration is an additional power demand which could result in the engine lugging. To prevent engine lugging, which could result in serious engine damage, a sensor determines if low manifold vacuum exists.

Typical lockup converters connect these sensors in a series creating a "string" arrangement. That is, if one sensor gives a "no" signal then the converter will not lockup. Therefore, for most converters to lockup, the following must be present:

a. Coolant must be at or above minimum temperature.
b. Transmission selector must be in drive if a non-overdrive vehicle or in "O" or "OD" overdrive if an overdrive vehicle.
c. Vehicle must be at or above minimum lockup speed.
d. Vehicle must be in a cruise or near cruise condition.
e. Transmission must have shifted into the top gear.

Some manufacturers add additional parameters to converter lockup. If any one sensor fails to complete the string, the converter will not lockup.

A converter may fail internally. Internal failure will reduce mpg and affect vehicle performance. If the one-way torque converter clutch slips, then the vehicle will characteristically have very poor acceleration. Mpg will be significantly reduced since engine torque is not being multiplied during acceleration. A vehicle with a slipping one-way torque converter clutch will experience a 20 to 30% decreased fuel economy.

A one-way torque converter clutch that does not release is a second type of torque converter failure. Vehicles with a non releasing one way clutch will accelerate normally but at a decreased top speed. The failure results in a 30 to 50% reduction in mpg. Top cruising speeds are lowered to 40 or 50 mph. The transmission may shift normally even when the one-way torque converter clutch slips or fails to release properly!

Lockup torque converters may exhibit additional fuel-eating malfunctions. Sensors - temperature, speed, selector position, engine vacuum - may fail; wiring and connectors could fail or become disconnected. Internally, the lockup device itself could fail or wear out. Whatever the malfunction, mpg will be decreased.

Complete the torque converter activity sheet. Adhere to the manufacturer's recommended specifications for testing. After completing the activity sheet, complete the information check and report the findings to the instructor.


Name:_____________________________________ Date Completed:________________

Type of vehicle tested:



Transmission Type:____________________________________

Lockup Converter? _____Yes _____No


1. Make sure brakes can prevent vehicle from moving during the stall check.
2. Do not exceed recommended engine RPMs.
3. Do not run test for more than 5 SECONDS! Severe engine and/or transmission damage could occur.

Note: Obtain specifications from manufacturer's shop manual

1. Install a tachometer.

2. Fully set parking brake. Block drive wheels in the event of brake failure.

3. Test brake pedal for firm, positive, non-fading action.

4. If brakes hold, perform the converter stall test. Note maximum RPMs in "Data Report" below.


5. To cool transmission fluid after this test, place transmission in park and run the engine at a fast idle for two to three minutes.

6. Remove wheel blocks.


7. Road test vehicle for minimum mph that torque converter locks up. Note in "Data Report" below.

8. Lightly accelerate vehicle with converter locked up. Did the converter remain locked up? Record in "Data Report" below.

9. Return vehicle to shop. Draw some conclusions about this transmission and make (a) recommendation(s).

Maximum RPMs recorded in stall test: _______________________

Minimum MPH that lock up occurred: ____________________

Did converter maintain lock up condition under light acceleration? ______yes ______no

Conclusion and Recommendation(s):






Directions: Indicate whether the statements below are true or false. If the statement is false, explain why it is false.
1. _______ Lockup torque converters do not have a one way clutch.

2. _______ Any torque converter clutch sensor malfunction will prevent a torque converter from locking up.

3. _______ When a torque converter couples, the one way clutch free spins.

4. _______ A sudden drop in mpg in addition to a decrease in top speed could indicate an internal torque converter failure.

5. _______ Which faulty sensor could cause a lockup torque converter to malfunction?

Torque converters are very reliable devices. Occasionally one fails internally. Tune up technicians usually hear the complaints of "poor performance" and "low MPG". They need to be able to quickly identify a malfunctioning torque converter or a lockup torque converter that fails to lockup. This guide can assist these students in this diagnosis.

1. False. All torque converters use a one way clutch.

2. True.

3. True.

4. True.

5. E

Automatic Transmissions, pages 39 -62

Automechanics, pages 362 - 366.

Ellinger, Herbert. Automechanics, 4th edition. Prentice Hall, Englewood Cliffs, NJ. 1988.

Brejcha, Mathias S. Automatic Drive Transmissions, Second Edition. Prentice Hall, Englewood, NJ. 1982.

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