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Diagnosing GM Converter Lock-Up Problems

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Diagnosing GM Converter Lock-Up Problems

   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

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