Torque Converter Clutch: Testing and Inspection
Chart C-8A - Torque Converter Clutch:
Wiring Diagram for Torque Converter Clutch:
CHART C-8A TORQUE CONVERTER CLUTCH (TCC) DIAGNOSIS
Circuit Description:
The purpose of the automatic transmission torque converter clutch feature is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise condition. This allows the convenience of the automatic transmission and the fuel economy of a manual transmission. The heart of the system is a solenoid located inside the automatic transmission which is controlled by the ECM.
When the solenoid coil is activated ("ON") the torque converter clutch is applied which results in straight through mechanical coupling from the engine to transmission. When the transmission solenoid is deactivated, the torque converter clutch is released which allows the torque converter to operate in the conventional manner (fluid coupling between engine and transmission).
The ECM turns "ON" the TCC when coolant temperature is above 65 degrees C (149 degrees F), TPS not changing, and vehicle speed above a specified value.
A 4th gear switch (mounted in the transmision) opens when the transmission shifts into 4th gear, and this switch is used by the ECM to modify TCC lock and unlock points, when in a 4-3 downshift maneuver.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. When a test light is connected from ALDL terminal "F" to ground, a test light "ON" indicates battery voltage is OK and the TCC solenoid is disengaged.
2. When the diagnostic terminal is grounded, the ECM should energize the TCC solenoid and the test light should go out.
Diagnostic Aids:
A "Scan" tool only indicates when the ECM has turned "ON" the TCC driver (grounded CKT 422) but this does not confirm that the TCC has engaged. To determine if TCC is functioning properly, engine rpm should decrease when the "Scan" indicates the TCC driver has turned "ON." To determine if the 4th gear switch is functioning properly, perform the checks in CHART C-8A (Part 2 of 2). The switches will not prevent TCC from functioning but will affect TCC lock and unlock points. If the 4th gear switch circuit is always open the TCC may engage as soon as sufficient oil pressure is reached.
Chart C-8A Torque Converter Clutch:
CHART C-8A TORQUE CONVERTER CLUTCH (TCC) DIAGNOSIS
Circuit Description:
A 4th gear switch (mounted in the transmision) opens when the transmission shifts into 4th gear, and this switch is used by the ECM to modify TCC lock and unlock points, when in a 4-3 downshift maneuver.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
3. Unless the switch or CKT 446 is open "Scan" should display "NO," indicating the trans. is not in 4th gear. The 4th gear switch should only be open while in 4th gear.
4. This step determines if the ECM and wiring are OK. Grounding CKT 446 should cause the "Scan" to display "NO," indicating the trans. is not in 4th gear.
5. Checks the operation of the 4th gear switch. When the trans. shifts into 4th gear the switch should open and the "Scan" should display "YES."
6. Disconnecting the TCC connector simulates an open switch to determine if CKT 446 is shorted to ground or the problem is in the transmission.
Diagnostic Aids:
A road test may be necessary to verify the customer complaint. If the "Scan" indicates TCC is turning "ON" and "OFF" erratically, check the state of the 4th gear switch to be sure it is not changing states under a steady throttle position. If the switch is changing states, check connections and wire routing carefully. Also if the 4th gear switch is always open the TCC may engage as soon as sufficient oil pressure is reached.