GM Convertor Lock-Up and the TCC Solenoid

transmission cutaway view
This is a GM Hydramatic transmission. Getty

The TCC solenoid is what actually causes the TCC (also known as the torque convertor clutch) to engage and disengage. When the TCC solenoid receives a signal from the ECM, it opens a passage in the valve body and hydraulic fluid applies the TCC. When the ECM signal stops, the solenoid closes the valve and pressure is vented causing the TCC to disengage. This lets the torque convertor lock in "gear" or unlock based on what you are telling the car or truck to do.

If you think of it in a very non-technical way, the torque convertor clutch does the same thing inside an automatic transmission that your standard clutch does on a manual transmission. If the TCC fails to disengage when the vehicle comes to a stop, the engine will stall.

Testing the TCC

Before attempting to diagnose converter clutch electrical problems, mechanical checks such as linkage adjustments and oil level should be performed and corrected as needed.

Generally, if you unplug the TCC solenoid at the transmission and the symptoms go away, you have found the problem. But sometimes this can be misleading because you don't know for sure if it's a bad solenoid, dirt in the valve body or a bad signal from the ECM. The only way to know for certain is to follow the diagnostic procedure as outlined by General Motors. If you follow the test step by step you will be able to determine the exact cause of the problem.

Since some of these tests require the drive wheels be raised off the ground and the engine and transmission run in gear, proper care must be taken to perform the tests in a safe manner. Support the vehicle with jack stands. NEVER run the vehicle in gear when supported only with a jack. Chock the drive wheels and apply the parking brake.

In addition, some of the tests (test #11 and 12) require the transmission be opened and the valves be physically inspected. I do not recommend that you do this. If all the other tests pass, then it's time to bring it to a shop and have the internal parts checked for proper operation.

Test #1 (Regular Method)

Before you begin this test, use a test light or multimeter to check For 12 Volts To Terminal A At the Transmission.

  1. Raise the vehicle on a lift or support it safely using strong jack stands so the driving wheels are off the ground.
  2. Connect the alligator clip of your test light to ground. Unplug the wires at the case and place the tip of your test light on the terminal marked A.
  3. Do not depress the brake pedal.
  4. Computer controlled vehicles: turn on the ignition and the tester should light.
  5. All other vehicles: start the engine and bring to normal operating temperature.
  6. Raise RPM to 1500 and the tester should light. This indicates a successful test. If tester lights continue with Regular Method.
  7. If the tester does not light go to Test # 2.

Test #1 (Quick Method)

Check For 12 Volts To Terminal A At The ALDL as described at the beginning of the Regular Method above.

Note: ALDL quick methods, when given, are a way to perform many of the tests at the Assembly Line Diagnostic Link (ALDL).

The ALDL is the plug interface that your factory-like diagnostic tool plugs into. Barring that, the info is still accessible using leads from your test light. This will allow you to do most of the electrical checks from the driver's seat and save much valuable diagnostic time. 

  1. Connect one end of a test light to terminal A at the ALDL.
  2. Connect the other end to terminal F at the ALDL.
  3. Turn on the ignition and the tester should light. Note: some transmissions, like the 125C must shift to 3rd before the tester will light.
  4. If the tester lights, you have 12 volts to terminal A at the transmission.
  5. If the tester does not light, then check for 12 volts by the regular method.