Technical Description of OBD-II Trouble Code
Electrical Circuit Issue with Torque Converter Clutch (TCC) Solenoid
This diagnostic trouble code (DTC) is a standard powertrain code that applies to vehicles equipped with OBD-II. While it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
What does that mean?
This diagnostic trouble code is triggered when the Engine Management Computer (EMC), also known as the Powertrain Control Module (PCM) or Engine Control Module (ECM), detects a problem with the electrical circuit of the torque converter clutch (TCC) solenoid, which is located inside the transmission.
An automatic transmission is not 100% efficient because it is a fluid coupling that connects the engine to the internal transmission components. The torque converter transfers power using hydraulic force and is located within the transmission assembly. However, this transfer of power generates excessive heat. The lockup torque converter was designed to improve transmission efficiency at highway speeds and regulate heat. The torque converter clutch is responsible for locking up the converter. Excessive heat can damage transmissions, which is why stop-and-go traffic is known to be hard on automatic transmissions.
At low speeds, the converter clutch does not lock up, resulting in the generation of heat. This clutch is controlled by an electrical solenoid. The torque converter clutch is locked, changing the fluid coupling to a mechanical one. This solenoid uses power and ground sent through a coil of wires to create a magnetic field that pulls or pushes a central metal rod. When the solenoid is energized or de-energized, it directs fluid flow through the valve body of the transmission.
Symptoms
Symptoms of a P0743 DTC may include:
Malfunction indicator illumination (check engine light or service engine soon light)
Stalling when coming to a stop after cruising speeds
Shudder on take off
Shudder when coming to a stop
Causes
Potential causes may include:
Low or dirty transmission fluid
Wire/circuit issue shorted to power or ground
Failed torque converter clutch (TCC) solenoid
Failed TCC assembly in the torque converter
Failed PCM/ECM/EMC
Note: This may be an intermittent condition and may not occur again, so clear the code and continue monitoring.
Possible Solutions
First, gather the necessary tools or borrow them (although mechanics rarely lend out their tools as they are essential for their work):
On Board Diagnostic (OBD-II) scan tool – to retrieve the code; available at many parts stores, online, or some parts stores offer free code retrieval
Multimeter with a resistance (or OHM) reading
Self-powered test light/probe (a Power Probe III is highly effective here). If a Power Probe is not available, a simple set of fused jumper wires and a test light will suffice
Wire diagrams will be very helpful – these can be found in a Haynes manual or obtained from the dealer or local mechanic
TIP: Remember that in most cases, there is an internal transmission harness that connects the TCC solenoid and shift solenoids. Therefore, the wire fault may only occur when the transmission is hot or cold, or when the vehicle is driven over bumps or during turns. Make every effort to recreate the issue!
TIP: Before addressing any transmission issue, ensure that the engine is running properly. Address any other sensor codes or lean/rich codes first. Many sensors are interconnected and affect transmission shift scheduling. If one sensor is “offline/faulty,” the ECM/PCM/EMC may throw this code based on the fault detected by another sensor.
TIP: Often, a simple fluid and filter change can resolve minor transmission issues. There is no harm in trying a fluid and filter change. Note that I recommend a change, not a flush, for a high mileage transmission, as a flush may cause further damage. Additionally, for a slipping transmission, I recommend using Lucas Oil Transmission Fix. I am not endorsing the product, nor am I being paid to mention it, but it has proven effective!
First, verify the condition/concern. In most cases, the TCC solenoid engages in overdrive when commanded by the PCM. Locking the TCC helps improve fuel economy and lowers transmission fluid temperature by locking the converter (for a more detailed explanation of TCC operation, refer to research on TCC theory and operation, as it can be quite technical). In my case, my vehicle stumbled and almost stalled when stopping at a light. Once the condition is verified, diagnosis can begin.
Perform an overall check of the fluid condition and level. Also, ensure that the transmission shifts correctly and that the engine is running properly. Many transmission problems are misdiagnosed due to a poorly running engine. Remember, power flow starts at the engine, goes through the transmission, and then reaches the drivetrain.
Next, check the external transmission harness and connectors. Also, check the fuses and ensure that the transmission case is properly grounded to the body and the battery.
Proceed to identify the appropriate wires corresponding to the torque converter clutch. In most cases, there will be two wires: one for ground and one for power. Apply power and ground and listen for the solenoid to click.
If the solenoid does not click, proceed to check continuity across the solenoid and continuity to and from the corresponding wires connected to the sensor. The solenoid should have a low ohm (.02 to .05) reading, although this may vary depending on the manufacturer, so verify the reading with a factory service manual or equivalent. The wires should also have a low ohm reading. Additionally, ensure that neither wire is shorted to ground or power.
If the wires check out and the solenoid clicks, there may be a PCM issue. However, I recommend pulling the transmission pan and closely examining the TCC solenoid for debris. Even a small amount of debris can cause the solenoid to “short” and trick the computer into thinking it is engaged. If the debris is metallic, it can attach itself to the solenoid and prevent disengagement. This was the issue in my case.
Additionally, I advise obtaining the latest calibration for your vehicle’s computer (PCM) from the dealer. Manufacturers frequently release new and improved software, and there may be a revised calibration to address this code, especially if it is intermittent.
Finally, if all checks out OK and you decide to replace the PCM, remember that it needs to be programmed to the vehicle. In many cases, there is an anti-theft device in the key, so the keys will also need to be programmed. Consult a dealer or mechanic for assistance.
If all else fails and you are at a loss, there may be an issue with the clutch assembly in the torque converter. This will likely be evident if you feel slipping when the transmission goes into lockup in overdrive. Additionally, the fluid may have a burnt smell or be completely black. Consider either a rebuilt transmission or a replacement.
Good luck! In my case, as a Ford trained technician with experience, I simply pulled the transmission pan and immediately identified the solenoid as the problem. This is a common issue with higher mileage transmissions.
