P0740 Code: Torque Converter Clutch Circuit Malfunction

What the P0740 code means

The torque converter passes engine power to the transmission through fluid, which always involves some slip and therefore some lost efficiency. To avoid that at cruising speed, it has a clutch that locks it mechanically.

The computer controls that lockup with a solenoid and checks the result by comparing engine speed with the transmission output speed.

The P0740 code is stored when the lockup does not happen, happens when it should not, or the solenoid electrical circuit is out of range. It is one of the few transmission codes where the fluid level and condition explain a very high share of the cases.

It helps to tell this shudder apart from similar ones, because they are easy to confuse. The torque converter clutch shudder shows up at steady speed, in a high gear, with the engine warm and a light load, and it disappears as soon as you press the gas pedal a little or touch the brake, because either action unlocks the clutch.

A vibration from unbalanced wheels, on the other hand, depends on road speed and does not change when you lightly accelerate or brake. That simple difference identifies the source without taking anything apart and keeps you from starting in the wrong place.

P0740 code symptoms

• Check engine light on and, on many vehicles, the transmission warning light too.
• Vibration or shudder at a steady speed, between 60 and 90 kilometers per hour (about 37 to 56 mph).
• Engine rpm climbing without a gain in road speed.
• A clear increase in highway fuel use.
• Engine stalling when you come to a stop, if the clutch stays locked.
• Higher transmission fluid temperature.
• The shudder disappears when you lightly press the gas pedal or the brake.
• It shows up only with the engine warm and in a high gear.

Likely causes of the P0740 code

1. Low level or degraded transmission fluid. Always the first thing to check.
2. Failed torque converter clutch solenoid.
3. Worn or clogged valve body.
4. Torque converter clutch worn out internally.
5. Damaged transmission wiring or connector.
6. Input or output speed sensors giving wrong readings.
7. Wrong fluid, with friction properties different from the specification.
8. Transmission module with outdated software.
9. Clogged transmission cooler, which raises the fluid temperature and breaks down the clutch friction material.
10. Fluid whose friction modifier additives are used up because of an overlong change interval.
11. Clutch debris circulating through the hydraulic circuit, which jams the solenoid.

How to diagnose the P0740 code step by step

1. Check the fluid level and condition. Dark fluid, a burnt smell or particles point to internal wear.
2. Confirm the fluid is the one specified. A generic fluid in a transmission that calls for a specific one can set this code all by itself.
3. Watch engine rpm and output speed in live data. With the clutch locked, the two should hold a fixed relationship.
4. Compare the lockup state the scan tool reports with what the vehicle is actually doing.
5. Measure the solenoid resistance and compare it with the specification.
6. Check the wiring and the transmission connector, which are exposed to heat and moisture.
7. Check the fluid temperature while driving.
8. See whether the shudder goes away when you lightly press the brake. If it does, it is the torque converter clutch. If nothing changes, look at the wheels or the driveline.
9. Watch the rpm drop at lockup. With a healthy clutch the engine speed falls clearly while road speed stays the same.
10. Inspect the transmission cooler and watch the fluid temperature during sustained driving.

How to fix the P0740 code

• Change the transmission fluid and filter using the exact manufacturer specification.
• Replace the torque converter clutch solenoid if its resistance is out of range.
• Clean or replace the valve body when the diagnosis points to it.
• Repair the damaged wiring and connector.
• Update the module software if the manufacturer released a revision.
• Replace the torque converter if the clutch is worn out internally.
• Clean or replace the transmission cooler if it is clogged, because high temperature breaks down the clutch again and again.
• Keep to the fluid change interval: the friction modifier additives wear out, and they are what allow a smooth lockup.
• Replace the solenoid and the filter together if the fluid is carrying debris, so the new part does not jam the same way.

Technical reference data

• Typical clutch solenoid resistance: 10 to 30 ohms.
• Normal fluid temperature: 70 to 100 °C (158 to 212 °F). Above 120 °C (248 °F) it breaks down quickly.
• Lockup usually starts above 50 or 60 kilometers per hour (about 31 to 37 mph), with a steady load and a warm engine.
• With the clutch locked, engine speed drops by 150 to 400 rpm while road speed does not change.
• Fluid temperature in normal driving: 70 to 100 °C (158 to 212 °F).
• Lockup releases when you press the gas pedal or the brake, and that behavior is what identifies the source of the shudder.

Brand-specific notes

The shudder at steady speed that this code produces is often mistaken for unbalanced wheels or a bigger transmission problem. Checking the fluid and the solenoid before you take anything apart saves a lot of money.

The transmission cooler is a component that almost nobody inspects, yet it sets how long the torque converter clutch lives. If this code was already repaired once on the same vehicle, check the cooler before repeating the job.

Frequently asked questions about P0740