GM Convertor Lock-Up and the TCC Solenoid

transmission cutaway view
This is a GM Hydramatic transmission.

Museum of Science and Industry, Chicago / Getty Images

There is a solenoid in your GM converter that causes the the torque convertor clutch (TCC) to engage and disengage. When the TCC solenoid receives a signal from the engine control module (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 converter lock in "gear" or unlock out of "gear." If you think of it in a non-technical way, the torque converter 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. Sometimes this can be misleading, because you don't know for sure if the cause is 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 procedures outline below. If you follow the test step by step, you will be able to determine the exact cause of the problem.

Before You Test

These tests as outlined in the General Motors repair manual can be performed by a competent home mechanic, but proper care must be taken to perform the tests in a safe manner. That means the drive wheels must be raised off the ground and the engine and transmission run in gear. Support your 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.

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. Once you've done so successfully, you can proceed with the following steps:

  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.
  7. If the tester does not light, replace the solenoid.
  8. If the tester does light, but the problem persists, take your car to a mechanic.

Test #1 (Quick Method)

Check for 12 volts to terminal A at the transmission Assembly Line Diagnostic Link (ALDL) as described at the beginning of the Regular Method, above. Most quick methods of testing can be done at the ALDL, which 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 third gear 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.

When to Head to the Shop

While the tests outline above can be performed by the competent home mechanic, additional tests recommended by GM require that the transmission be opened and the valves be physically inspected. I do not recommend that you do this. If your vehicle passes the above tests but the problem persists, then it's time to bring it to a shop and have the internal parts checked for proper operation.