Diagnosing GM Converter Lock-Up Problems

   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)

   Check For 12 Volts To Terminal A At Transmission

1. Raise the vehicle on the lift 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. 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

Note: ALDL quick methods, when given, are a way to perform many of the tests at the Assembly Line Diagnostic Link (ALDL). 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. Go to Test # 6.
5. If the tester does not light, then check for 12 volts by the regular method.

Test #2

   Checking For 12 Volts Across Fuse

1. Check for 12 volts at both sides of the fuse.
2. Locate the fuse box and the fuse marked "gauges" (most models).
3. Connect the alligator clip of your test light to ground. Turn the ignition on.
4. Place the tip of your test light on one side of the fuse and the tester should light.
5. Place the tip of your test light on the other side of the fuse and the tester should again light.

Test #3

   Checking For 12 Volts Across Brake Switch

IMPORTANT: Either of these switches can be used for lock-up. To avoid mis-diagnosis, check them both. If the upper switch with the vacuum hose is used, check the two wires at that switch. On the four wire lower switch, check the two wires farthest from the plunger.

1. Check for 12 volts at both sides of the brake switch. Some GM vehicles have two electric switches at the brake pedal. One switch will have four wires and the other switch will have two wires and a vacuum hose.
2. Connect the alligator clip of your test light to ground.
3. Do not depress the brake pedal.
4. Turn the ignition "on".
5. Push the tip of your tester into one wire and the tester should light.
6. Now test the other wire and again the tester should light.
7. Depress the brake pedal and re-test. Only one wire should now be hot.

Test #4

   Adjusting/Replacing the Brake Switch

1. Remove the brake switch from its bracket.
2. Reconnect the wires to the brake switch.
3. Re-test as stated in test # 2, but push and release the plunger with your finger or thumb.
4. If it now passes the test, the brake switch is good, but needs adjusting.
5. If it still doesn't pass, replace the brake switch.

Test #5

   Checking Wires For Shorts and Opens

IMPORTANT: Make sure the ignition switch is "off" for the following tests.

   Shorts:

1. Set your ohmmeter to ohms times one (Rx1).
2. Connect one lead of your ohmmeter to one end of the suspect wire.
3. Connect the other lead of your ohmmeter to a good ground.
4. If the meter reads ANYTHING other than infinity, you have a short to ground in that wire.

   Opens:

1. If a suspect wire has no voltage through it, and its connection at both ends is good, and it's not shorted to ground, the wire has an open in it.
2. Replace the wire.

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