What does it mean?
Modern vehicles equipped with automatic transmissions/transaxles use a torque converter between the engine and transmission to increase engine torque and drive the rear wheels.
The engine and transmission are actually connected by a fluid coupling mechanism inside the torque converter, which multiplies torque until the speeds equalize and create a “stall speed” where the difference between the actual engine speed and the transmission input speed is about 90% efficient. The Torque Converter Clutch (TCC) solenoids controlled by the Powertrain Control Module/Engine Control Module (PCM/ECM) or the Transmission Control Module (TCM) direct the hydraulic fluid and engage the torque converter clutch to create a solid coupling and increase efficiency.
The Transmission Control Module detected a failure in the circuit that operates the torque converter clutch solenoid. We will consider the TCM as the controller for the purpose of this article, although it may be controlled via the PCM/ECM; consult factory wiring diagrams to determine the system used.
Note: This code is similar to codes P0741, P0742, P0743, P0744, P2769, and P2770.
There may be other diagnostic trouble codes associated with the Transmission Control Module that are only accessible using an advanced scan tool. If additional transmission-related DTCs appear in addition to P0740, an electrical failure is likely.
Symptoms
Symptoms of a P0740 fault code may include:
Malfunction Indicator Lamp (MIL) is illuminated (i.e., Check Engine Light)
Minimal decrease in fuel economy; this will not affect engine performance
Potential Causes
Causes
of this DTC may include:
Damaged transmission wiring harness, loose connection, open or shorted circuit
Torque Converter Clutch (TCC) solenoid
Transmission Control Module (TCM)
Diagnostic Steps
for P0740
Wiring Harness – Check that the transmission wiring harness is not damaged or loose. Use a factory wiring diagram to locate the appropriate power source and all connection points between circuits. The transmission may be powered by a fuse or relay and triggered by the TCM. Disconnect the transmission harness at the transmission connector, power source, and TCM. Check continuity within the internal transmission wiring harness by locating the appropriate + and – pins for the torque converter clutch solenoid.
Using a Digital Volt Ohm Meter (DVOM) set to the ohms scale, check the resistance in the circuit with the positive and negative leads on the appropriate pins. If the resistance is extremely high or above the limit (OL), suspect a failure in the internal harness or TCC solenoid – removal of the transmission oil pan may be necessary to further diagnose the TCC solenoid.
Test the wiring between the TCM and the transmission case harness connector using the DVOM set to ohms. Check for a possible short to ground by moving the negative DVOM lead to a known good ground; the resistance should be very high or above the limit (OL).
Torque Converter Clutch (TCC) Solenoid – Check the resistance of the TCC solenoid and internal transmission wiring at the transmission case after removing the transmission harness plug (if applicable; some makes/models use a TCM bolted directly to the transmission case). Some makes/models use a transmission wiring harness with the TCC solenoid and internal harness as a single unit. The DVOM should be set to the ohms scale with the positive and negative leads on the TCC power and control circuit pins. The resistance should be within the manufacturer’s specifications; if it is very high or above the limit (OL), remove the transmission oil pan to inspect the solenoid inside the transmission if possible. Check the voltage on the power side of the TCC solenoid circuit or at the harness connector to the TCM with the DVOM set to the volts scale, the positive lead to the tested wire and the negative to a known good ground with the vehicle key on/engine off; battery voltage should be present.
Transmission Control Module (TCM) – Since the torque converter clutch is only engaged under certain driving conditions, it will be necessary to monitor the TCM with an advanced scan tool to determine if the TCM is commanding the TCC solenoid and what the actual feedback reading is to the TCM. The TCC solenoid is normally controlled by a duty cycle to achieve a smoother torque converter lockup engagement.
To test if the TCM is actually sending the signal, a graphing multimeter set to duty cycle or a digital storage oscilloscope will also be needed. The positive lead is probed into the wiring harness plugged into the TCM and the negative lead to a known good ground. The duty cycle should match what is commanded by the TCM in the advanced scan tool reading. If the duty cycle remains at 0% or 100% or is intermittent, recheck the connections, and if all wiring/solenoid is OK, the TCM may be faulty.