Torque Converter Clutch Facts
If you own a C4 with an automatic transmission, you have probably heard about something called the torque converter but you may not understand what it is or what it does.
Torque converters are devices that limit slippage in automatic transmissions since slippage equates to lost power at the rear wheels. In the C4 service manuals, the troubleshooting procedures in the transmission area refer to the TCC (Torque Converter Clutch) and the solenoid that controls it. In other portions of the book, the troubleshooting procedures refer to the sensors and the Electronic Control Module---all of these are part of the TCC system.
When the TCC is engaged, there is a straight through mechanical coupling from the engine to the transmission instead of a fluid coupling which is the normal condition.
Whether or not the TCC is engaged is dependent on a signal from the Electronic Control Module/Powertrain Control Module (ECM/PCM) to the TCC solenoid. The ECM/PCM in turn monitors several sensors to determine whether to engage the clutch.
It can engage the clutch in any gear although we are mostly aware of engagement when in 4th gear/overdrive.
TCC Operating Conditions
The normal condition for the TCC is off. It is engaged by pulling in the coil of the TCC control solenoid via a ground path provided by the ECM/PCM. The TCC can be forced into the engaged condition by grounding pin "F" on the 12 pin ALDL connector which forces the solenoid to pull in.
The conditions for engaging the TCC are based on the ECM/PCM's evaluation of inputs from the Vehicle Speed Sensor (VSS), the Throttle Position Sensor (TPS), the Coolant Temperature Sensor (CTS), an overdrive switch mounted inside under the center console trim plate and finally the brake switch (the same one that lights the brake lights)
The 4th gear switch inside the transmission is also a part of the equation. When the transmission is in 4th gear, this switch opens. The ECM/PCM can engage the TCC in any gear (and does around 30 MPH) but being in 4th gear means the TCC must be controlled more precisely so that the engine is not loaded down at low speeds.
A TCC that does not disengage can cause the car to stall much as leaving the clutch engaged in top gear on a manual transmission can cause the engine to stall if you reduce speed beyond a certain point.
If your car wants to die as you slow down but recovers if you slip it into neutral, the TCC may not be disengaging.
A TCC that does not engage to begin with can result in lower than expected average MPG since the drive train will have more slippage in the path to the rear wheels and therefore a loss of efficiency.
If you suspect the TCC is not engaging, perform this test: warm the car up to more than 149 degrees coolant temperature and hold a constant amount of throttle with a speed above 55 MPH for a minute or so.
Lightly tap the brakes (not enough to appreciably slow the car, just enough to flash the brake lights) and the engine RPM should rise 100 to 200 RPM immediately after you tap the brakes.
If it does, the TCC has disengaged which means it must have engaged to begin with. If you do not see any difference in the engine RPM, you have a problem with the TCC, the solenoid that controls it, one of the sensors that the ECM/PCM monitors, a wiring harness problem or the ECM/PCM itself.
To totally troubleshoot the system, you need a scan tool and then follow the troubleshooting tree in the service manual.
Since few DIY (Do It Yourselfers) have a way to work on the THM 700 automatic transmission, repairs will probably be limited to replacing defective sensors, adjusting or replacing the overdrive or brake switches or adjusting the throttle position sensor.
If all the switches and sensors are working correctly, the problem is either in the ECM/PCM, associated harnesses or the transmission.
Finally, many of the aftermarket "speed chips" or performance PROMS (Programmable, Read Only Memory) for the ECM have had their programming adjusted so the the TCC does not engage at the relative low speed of the stock system.
This is to eliminate the "lugging down" condition that many find objectionable in town. If you are in overdrive, the stock system will engage the TCC at around 30 MPH and under some conditions this causes shuddering and poor low speed performance. Most aftermarket chips raise this lower limit to around 40-42 MPH which eliminates the problem but does slightly lower the gas mileage.