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Code P0743. Causes, Symptoms and How to Fix It

The code P0743 It is defined as the electrical circuit of the torque converter clutch (TCC) solenoid. This means that the PCM is detecting a fault in the TCC, which is located inside the transmission.

The job of your car’s torque converter clutch, or TCC, is to establish the “mechanical lock” between the transmission and the engine.

It does this by increasing the 1:1 rotation per minute or RPM ratio between the transmission input shaft and the rotational speed of the torque converter.

Torque Converter Operation

Most automatic transmissions used for passenger vehicles have a torque converter as a coupling device.

The main functions of the torque converter are: to decouple the engine from the transmission, when the vehicle is stationary, and to transmit torque to the transmission when the engine increases speed.

The torque converter Allows the engine to idle when the vehicle is stopped, even if the transmission is in gear.

As the name suggests, the torque converter converts (amplifies) the engine’s input torque into higher output torque. This particular feature of the torque converter is not possible with a clutch, which can transmit maximum engine torque and no more than that.

The torque converter is mounted between the internal combustion engine and the automatic transmission, in the same place where a clutch would have been in the case of a manual transmission.

The torque converter is filled with automatic transmission fluid (ATF), which is a type of transmission oil. The impeller is connected to the crankshaft and the turbine is connected (splined) to the transmission input shaft.

The impeller, stator, and turbine have curved vanes that cause fluid to flow inside the torque converter.

The torque converter clutch (TCC), has the function of mechanically connecting the impeller with the turbine to limit power losses.

When the speed difference between the impeller and the turbine is not too large, the torque converter clutch is closed and the connection from the engine to the transmission is direct, with no losses in the torque converter.

What is Code P0743?

When it is established the code P0743 is established, it is usually because the PCM reads a difference greater than 200 RPM between the rotational speed torque converter and the transmission input shaft.

This is not good, because the ratio between converter RPM and input RPM must be 1:1 when the torque converter lockup clutch is engaged.

The PCM detects the anomaly, activates the P0743 code, and turns on the “Check Engine” indicator light.

Common Symptoms of Code P0743

As with other error codes, the code P0743 activates the Check Engine light, which is one of its most obvious and main symptoms. Other symptoms include:

  • Check Engine light is activated.
  • Car shuddering when starting or stopping.
  • The engine shuts down when stopping after cruising speeds.
  • The vehicle does not shift into the highest gear at highway speed.
  • Decreased fuel economy.
  • The transmission can shift, but abruptly
  • The engine may turn off when you stop the car.
  • The transmission shows signs of overheating.

In some cases, the vehicle may not be in unusual or adverse conditions.

In other cases, some vehicles may show drivability problems, such as misfire-like symptoms or the engine shutting off when the car is stopped.

You may also be interested in Error Code P0746: Pressure Control Solenoid Perf/Off

Possible Causes of Code P0743

Among the possible causes of the appearance of this fault code, we can name the following:

  • TCC solenoid failed.
  • Failed TCC assembly on torque converter.
  • Short circuit or ground problem in circuits or cables.
  • Torque converter lockup solenoid defect.
  • Engine coolant temperature sensor failure.
  • Torque converter clutch defect.
  • Transmission control valve body defect.
  • Contaminated transmission fluid restricting hydraulic lines.

How to Diagnose Code P0743

Let’s see the steps you must take to diagnose correctly fault code P0743.

1. Scan and take the test drive

As with other codes, diagnosing this error code requires recording the freeze frame data and then performing a test drive.

The conditions (engine load, speed, throttle position, and RPM) when the code was set must be duplicated through a test drive to find out if the code returns.

2. Check the condition of the transmission fluid

Perform a check of the general condition of the fluid and verify if the levels are correct. Also, check to see if the transmission shifts correctly and if the engine is running properly.

In most cases, transmission problems are misdiagnosed due to poor engine operating conditions. Remember that the flow of motion starts in the engine and goes through the transmission and then out to the drivetrain.

3. Check the status of the electrical system in general

Next, check the external connectors and transmission harness. Also, check the fuses and make sure the transmission case is grounded at both the body and the battery.

4. Test the torque converter clutch

Next, find the correct cables for the torque converter clutch. In many cases, it will be two cables; one for ground and one for power. Apply power and ground and observe the clicking sound of the solenoid.

If you don’t hear a click, proceed to check continuity through the solenoid, to and from the sensor wires. The solenoid should have a low ohm reading (0.02 to 0.05).

Obviously, this varies from manufacturer to manufacturer, so check the service manual. The cables should also have a low ohms reading. Make sure both wires are NOT shorted to ground or power.

If the solenoid clicks, then the problem must be with the PCM. However, you should still examine the TCC solenoid for debris.

5. Check the TCC solenoid

Any small residue can cause a short circuit and trick the computer into thinking it applies. Or, it may stick to the solenoid and prevent it from disengaging. This is very common.

6. Check the PCM

Additionally, it is helpful to obtain the most recent calibration for your car’s PCM. Vehicle manufacturers are constantly releasing new versions of the software and there may be a revised calibration to address the code.

If everything seems fine, that means you may need to replace your PCM. Don’t forget that it needs to be programmed for your car. And in many, it comes with an anti-theft device on the key, which also has to be programmed.

7. Check the torque converter

If all else fails, then your torque converter problem must be with the clutch assembly. This may be evident if it slips when the transmission locks in overdrive.

Additionally, the liquid may smell burnt or look completely black. Look for a replacement or rebuilt transmission.

How to Repair Code P0743

Obviously, repairs for this error code depend on your diagnosis. Common solutions for this error code include:

  1. Engine coolant temperature sensor: When this sensor fails, the temperature of the entire system increases, including the transmission fluid.
  2. Transmission fluid: Transmission oil change will be mandatory when it is dirty or in poor condition.
  3. Torque Converter Clutch Solenoid: If this solenoid is damaged when performing your diagnosis, you must replace it.
  4. Torque Converter Clutch Solenoid Valve Harness: In this case, you can repair or replace the entire harness, depending on the severity of the fault.
  5. Transmission valve body: A failure that involves skilled labor. In this case, you should leave it in the hands of qualified personnel.
  6. Transmission control module: Although this failure is not common, it can happen. Remember that changing this module implies reprogramming. An expensive repair.

Before you begin addressing transmission problems, make sure the engine is in good working condition. If other sensor codes are present, or lean/rich codes, they must be corrected first.

In many cases, a simple transmission fluid and filter change can fix the problem.

Common Errors When Diagnosing Code P0743

Unjustified replacement of lockup solenoids and torque converters, when the problem is malfunctioning cables affecting the torque converter.

These types of errors happen when a proper diagnosis is not made or no diagnosis is made at all. Working blindly is something we do not recommend, the correct diagnosis is necessary to find the fault and carry out the repair.

TCC Torque Converter Clutch Operation

As we already mentioned, the work of the TCC is to increase the ratio of 1 to 1 RPM between the transmission input shaft and the rotational speed of the torque converter, so that the “mechanical lock” similar to a manual transmission is established.

In this way, the power loss that can occur with fluid and/or hydraulic lockout in the conventional Torque Converter is eliminated.

Additionally, since the engine is running at a reduced speed, both emissions output and overall fuel consumption are reduced.

However, this energy transfer results in excessive heat. The lockup torque converter works by making the transmission more efficient at highway speeds and controlling heat.

The TCC is what blocks the converter. Heat greatly affects the transmission, which is why we usually hear that stop-and-go traffic is hard on the transmission.

In the case of error code, P0743the PCM reads a difference greater than 200 RPM between the rotational speed torque converter and the transmission input shaft.

When the torque converter lockup clutch is activated, the ratio between the converter RPM and the input shaft RPM should be 1:1.

Conclusions

You may also be interested in Error Code P0760: Shift Solenoid C Malfunction

In electronically controlled transmissions, the operation of the torque converter clutch solenoid is monitored by the powertrain control module (PCM).

This can set a diagnostic trouble code (DTC) if a fault is detected.

The PCM/TCM will not set a DTC unless there is a problem with the electrical circuit that controls the torque converter clutch solenoid, or an internal malfunction of the TCC.

If you want to know other articles similar to Code P0743. Causes, Symptoms, and How to Fix It you can visit the category Fault Codes.

 

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