Diagnosing GM Converter Lock-Up Problems

Torque Converter
LuK USA LLC / Michael Poehler / Wikimedia Commons / CC BY 3.0

A common problem on many General Motors cars is the Torque Converter Clutch fails to release and causes the car to stall when it comes to a stop. Most of the time it is a stuck Torque Converter Clutch (TCC) solenoid, but this is not the only cause of this problem. General Motors has issued a few Technical Service Bulletins (TSBs) relating to this problem. There is also a specific diagnostic procedure to determine the exact cause of the TCC problem.

   Before I get into the diagnostic procedure, let's talk about the components, what they are and what they do.

The Torque Converter:

   By definition: "The torque converter converts hydraulic pressure within the transmission to mechanical torque, which drives the drive shafts and ultimately, the wheels."

   When the car is in low, second and reverse gears the converter operates in hydraulic or soft drive. In hydraulic drive, the converter functions as an automatic clutch that keeps the car from stalling when at a stop.

   The power flow:

  • The engine drives the impeller mechanically.
  • The impeller drives the turbine hydraulically.
  • The turbine drives the tube input shaft for input to the gear train.

   The impeller puts the transmission fluid in motion. Inside the impeller housing are many curved vanes, along with an inner ring that form passages for the fluid to flow through. The rotating impeller acts as a centrifugal pump.

Fluid is supplied by the hydraulic control system and flows into the passages between the vanes. When the impeller turns, the vanes accelerate the fluid and centrifugal force pushes the fluid outward so that it is discharged from openings around the inner ring. The curvature of the impeller vanes directs the fluid toward the turbine, and in the same direction as impeller rotation.

   The turbine vanes in the turbine are curved opposite to the impeller. The impact of the moving fluid on the turbine vanes exerts a force that tends to turn the turbine in the same direction as the impeller rotation. When this force creates a great enough torque on the transmission turbine output shaft to overcome the resistance of motion, the turbine begins to rotate.

   Now the impeller and turbine are acting as a simple fluid coupling, but we have no torque multiplication yet. To get torque multiplication, we must return the fluid from the turbine to the impeller and accelerate the fluid again to increase its force on the turbine.

   To get maximum force on the turbine vanes when the moving fluid strikes them, the vanes are curved to reverse the direction of flow. Less force would be obtained if the turbine deflected the fluid instead of reversing it. At any stall condition, with the transmission in gear and the engine running but the turbine standing still, the fluid is reversed by the turbine vanes and pointed back to the impeller. Without the stator, any momentum left in the fluid after it leaves the turbine would resist the rotation of the impeller.

The TCC:

   The purpose of the Transmission Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise mode.

The TCC System uses a solenoid-operated valve to couple the engine flywheel to the output shaft of the transmission through the torque converter. Lockup reduces slippage in the converter increasing fuel economy. For the converter clutch to apply, two conditions must be met:

  • Internal transmission fluid pressure must be correct.
  • The ECM must complete a ground circuit to energize the TCC solenoid that moves a check ball in the fluid line. This allows the converter clutch to apply, when hydraulic pressure is correct.

   The TCC is very similar to the clutch in a manual transmission. When engaged, it makes a direct physical connection between the engine and transmission. Generally the TCC will engage at about 50 mph and disengage at about 45 mph.

The TCC Solenoid:

   The TCC solenoid is what actually causes the TCC 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. 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.

   Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

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.
  1. Do not depress the brake pedal.
  2. Computer controlled vehicles: turn on the ignition and the tester should light.
  3. All other vehicles: start the engine and bring to normal operating temperature.
  4. Raise RPM to 1500 and the tester should light. If tester lights continue with Regular Method.
  5. 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.
    1. Reconnect the wires to the brake switch.
    2. Re-test as stated in test # 2, but push and release the plunger with your finger or thumb.
    3. If it now passes the test, the brake switch is good, but needs adjusting.
    4. 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.

       Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

    Diagnosing GM Converter Lock-Up Problems

    Test #6 (Regular Method)

       Check for ground at terminal D at the transmission.

    1. On Non-computer-controlled vehicles skip this test and go directly to cooler line pressure, or surge test.
    2. Raise the vehicle on the lift so the driving wheels are off the ground.
    3. Unplug the wires at the case and connect the alligator clip of your test light to terminal A.
    4. Place the tip of your test light on terminal D.
    1. Start the engine and bring to normal operating temperature.
    2. Place the selector in Drive. (O.D. on four speed units).
    3. Accelerate slowly to 60 mph and the tester should light.
    4. If the tester does not light you have a computer system problem. Go to test # 7 (Regular Method).

    Test #6 (Quick Method)

       Check for ground at terminal D at the ALDL

    Note: First you must have passed ALDL Quick method (Test # 1. Otherwise continue with regular method Test # 6).
    1. The test light should still be connected between terminal A and F at the ALDL.
    2. With the engine at normal operating temperature, go for a road test
    3. As you start your road test the tester should be lit.

      Note: If your foot is on the brake the light will be out.

    4. Watch the test light to see if it goes out at some point during the road test
    5. If the test light goes out, you have ground at terminal D at the transmission. Go to test # 7.
    6. If the test light stays on you have a computer system problem. (See test # 13) Go test # 7.

      Test #7 (Regular Method)

         Ground the D wire at the transmission

      1. Shave a little insulation from or pierce the D wire near the transmission connector. Reseal with silicon.
      2. Connect one end of a jumper wire to the bare wire you just shaved or pierced.
      3. Connect the other end of the jumper wire to ground.
      1. Road test for lock-up (can be done on lift).
      2. If you're not sure if lock-up occurred, then hold a steady speed of 60 mph (on the lift) and lightly touch and release the brake. You should feel lock-up disengage and re-engage.

      Test #7 (Quick Method)

         Ground the D wire at the ALDL

      Note: You must first have passed ALDL Quick method (Test # 1).

      1. Connect one end of a test light or jumper wire to terminal A at the ALDL.
      2. Go for a road test. (This can also be done on the lift)
      3. At approximately 35 mph, connect the other end of the test light or jumper wire to terminal F at the ALDL. The torque converter should Lock-up.
      4. Whether the T/C locks up or not, follow the trouble-shooting tree to the next step, cooler line surge test.

      Test #8

         Checking Cooler Line Pressure or Surge

      1. Check cooler line pressure or surge.
      2. Disconnect a cooler line.
      3. Attach one end of a rubber hose to the disconnected line coming from the radiator.
      4. Insert the other end of the rubber hose into the fill tube of the transmission.
      5. With the driving wheels off the ground, start the engine. Hold the rubber hose in your hand. Have as assistant place the selector in Drive and (slowly) accelerate to 60 mph. When the lock-up valve moves, the rubber hose should jump slightly.

        Test #9

           Checking the Solenoid

        You will need an ANALOG ohmmeter and a 12-volt source for this test.

        1. Connect the Black lead of your ohmmeter to the RED wire on the solenoid.
        2. Connect the RED lead of your ohmmeter to the BLACK wire on the solenoid. If you have a one-wire solenoid then connect the RED lead of your ohmmeter to the solenoid body.
        3. With the ohmmeter set at ohms times one (Rx1), the reading should be no less then 20 ohms, but not infinite.
        4. Connect the RED lead of your ohmmeter to the RED wire on the solenoid and the Black lead to the Black wire or body (You're just switching your connections).
        5. The ohmmeter should read less than the reading in the first test.
        6. Connect the solenoid to a 12-volt source. BE SURE TO OBSERVE PROPER POLARITY, if using a car battery.
        7. With lung pressure (or very low pressure) try to blow through the solenoid. It should be sealed.
        1. Disconnect the 12-volt source and you should now be able to blow through the solenoid.

        Test #10

           Checking Electrical Switches on Transmission

        Note: If you have passed the ALDL Quick methods, the electrical switches are NOT causing the any lock-up condition. Go to test # 11.

        SWITCH TYPE: Single terminal normally open
        PART#: 8642473
        TEST: Connect one ohmmeter lead to the terminal of the switch and the other lead to the body of the switch. Ohmmeter should read infinite. Apply 60 psi of air to the switch and the ohmmeter should read 0.

        SWITCH TYPE: Signal terminal normally closed
        PART#: 8642569, 8634475
        TEST: Connect one ohmmeter lead to the terminal of the switch and the other lead to the body of the switch. Ohmmeter should read 0. Apply 60 psi of air to the switch and the ohmmeter should read infinite.

           Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved

        Diagnosing GM Converter Lock-Up Problems

        SWITCH TYPE: Two terminal normally open
        PART#: 8643710
        TEST: Connect one ohmmeter lead to one terminal of the switch and the other lead to the other lead to the other terminal. Ohmmeter should read infinite. Apply 60 psi of air to the switch and the ohmmeter should read 0.

        SWITCH TYPE: Two terminal normally closed
        PART#: 8642346
        TEST: Connect one ohmmeter lead to one terminal of the switch and the other lead to the other terminal.

        Ohmmeter should read 0. Apply 60 psi of air to the switch and the ohmmeter should read infinite.

        Test #11

        Checking Lockup Apply Valve (Requires disassembly)

        Test #12

        Checking Signal Oil Circuit (Requires disassembly)

        Test #13

        Checking the Computer System

           The purpose of the following tests is to allow the Professional Transmission Technician to locate the general area of a computer system malfunction. For a more complete test procedure, refer to the appropriate shop manual. The computer system has a self-diagnostic capability. Always begin computer system checks by accessing the computer's diagnostic circuit.

           All of the sensors that send information to the computer are assigned a two-digit trouble code. If one of these sensors malfunctions, the computer will store the sensor's trouble code in its memory and usually activate the "Check Engine" or "Service Soon" light. When the computer is in the diagnostic state, it will read out the trouble codes stored in its memory.

        You then have a place to start looking for the malfunction.

        Diagnostic Circuit Check

        1. Turn the ignition "ON" and have the engine "OFF".
        2. The check engine light should be "ON" steady. (If the check engine light is "OFF", check the bulb).
        3. If the bulb is good, or the light flashes intermittently, refer to the car's service manual for further checks.
        1. Connect a jumper between pins A and B of the 12 pin ALDL.
        2. The check engine light should flash a code 12. (If it does not flash a code 12, refer to the car's service manual for further tests).
        3. If you get a code 12, note and record any additional codes.
        4. If a 50 series code is stored, refer to the car's service manual for further tests.
        5. Clear the computer's long-term memory, and go for another road test.
        6. Retest and record codes.
        7. If no codes were present in EITHER test, the computer doesn't see any malfunctions. (This does not mean that there is no malfunction).
        8. If codes were only present in the first test, they are intermittent.

           If codes were present in BOTH tests, the computer is seeing a current malfunction. The following codes are most likely to affect transmission performance.

        1. Code 14 = Shorted Coolant Temperature Circuit
        2. Code 15 = Open Coolant Temperature Circuit
        3. Code 21 = Throttle Position Sensor Circuit
        4. Code 24 = Vehicle Speed Sensor Circuit
        5. Code 32 = Barometric Pressure Sensor Circuit
        6. Code 34 = MAP or Vacuum Sensor Circuit

        How To Read Trouble Codes:

           Trouble code 12 will show as one flash of the check engine light followed by a pause and then two more quick flashes. This will repeat two more times.

        Code 34 will show as three flashes followed by a pause and then 4 quick flashes. All codes in the computer will flash three times, starting with the lowest code, until all codes have been displayed. The computer will then start the entire sequence again beginning with code 12. If more than one trouble code is present, always start your checks with the lowest number code. Exception: A 50 series code is always checked first. An example: if a code 21 and a code 32 were present, you would diagnose code 21 first.

        How To Clear The Computer:

        1. Turn the key "off".
        2. Remove the jumper between A and B at the ALDL.
        3. Disconnect the pigtail lead on the positive battery cable or remove the ECM fuse for 10 seconds.
        4. Reconnect the pigtail or replace the fuse and the codes are erased.
        5. Drive the car at operating temperature for at least 5 minutes before re-checking for trouble codes. Go back to test # 13.

             If you followed this test procedure step by step you will have found exactly where the problem is. Now the question is: "If I have a bad TCC solenoid, how do I replace it?" Since the TCC solenoid is attached to the auxiliary valve body it is best left to a transmission expert to replace. Also there is the possibility of a physical obstruction or an auxiliary valve body cross leak. Additionally there is a modification to be made to the auxiliary valve body gasket that has to be made in certain transmissions. And finally, If you have a vehicle that is earlier than 1987, replace the TCC solenoid with #8652379. The pre-1987 type of solenoid would clog easier than the late type.

             Copyright © 2001 - 2003 Vincent T. Ciulla All Rights Reserved