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 speeds equalize and create a “stall speed” where the difference between actual engine speed and 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 Transmission Control Module (TCM) direct hydraulic fluid and engage the torque converter clutch to create a solid coupling and increase efficiency.
The Transmission Control Module detected a malfunction in the circuit that operates the torque converter clutch solenoid.
Note: This code is similar to codes P0740, P0741, 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 P0742, an electrical failure is likely.
Symptoms
Symptoms of a P0742 trouble code may include:
Malfunction Indicator Light (MIL) is illuminated (also called check engine light)
If the torque converter clutch is actually locked, the engine may stall at idle. This is a common problem for some GM vehicle models, when driving at highway speed, then the engine stops, with the torque converter clutch locked. Usually, the engine restarts and resumes normal operation.
Potential Causes
Causes
of this DTC may include:
Wiring harness to the transmission, damaged, pinched, or shorted to power
Torque Converter Clutch (TCC) solenoid failure
Transmission Control Module (TCM)
Diagnostic Steps
for P0742
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.
Test the control circuit wiring between the TCM and the wiring harness connector in the transmission case using the DVOM set to the volts scale – it may be necessary to remove the pin from each side of the harness connector to isolate it from the harness while it is still plugged into the TCM and transmission case. With the positive lead at each end of the wiring and the negative lead to a known good ground, check for battery voltage with the key on/engine off. If voltage is present, suspect a short to power and determine the source of the short in the wiring harness.
Torque Converter Clutch (TCC) Solenoid – Check the resistance of the TCC solenoid and internal transmission wiring to 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 an ohms scale with the positive and negative leads on the TCC power and control circuit pins. The resistance should meet manufacturer specifications; if it is very low, it may be necessary to remove the transmission oil pan to inspect or replace the solenoid inside the transmission.
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 more comfortable 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 cycle remains at 0% or 100% depending on the circuit used, recheck the connections and if all wiring/solenoid is OK, the TCM may be faulty.