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Error Code P0740 is described as a Torque Converter Clutch (TCC) Circuit Malfunction. It is a generic trouble code to all makes and models of vehicles made since 1996. Specific steps for diagnosis and repairs may vary from on different models.
Modern automatic transmission cars or vehicles with multiple axles must use a torque converter. It is located between the engine and the transmission and works by improving the torque output of the engine and drives the rear wheels.
The engine and the transmission of the vehicle are connected together through a fluid coupling mechanism found inside the torque converter. The job of the torque converter is to multiply the torque until it equalizes the speed, making a “stall” speed where the difference is in the actual engine RPM. This results in a transmission input RPM efficiency rating of about 90%. The PCM (Powertrain Control Module) commands the TCC solenoids or the TCM (Transmission Control Module) to instruct hydraulic fluid and employ the torque converter clutch to improve efficiency and make a solid coupling.
When the Error Code P0470 appears, this means the TCM has detected a problem in the circuit that operates the TCC solenoids. In this entry, we will consider the TCM as the controller, though in some cases, it may be controlled by the PCM. To determine which is which, make sure you consult factory wiring diagrams
There are lots of different diagnostic trouble codes associated with transmission control module. If there are additional transmission related trouble codes aside from the P0740, then the problem might be electrical failure.
- Check Engine light is on
- Loose connection, open or shorted circuit, or damage in wiring harness in transmission
- Problem in Transmission Control Module
- Poor fuel economy
- Unable to shift to highest gear at freeway speed
- Performance problems such as dying at stop signs and misfire-like symptoms
- Faulty torque converter lockup solenoid
- Faulty engine coolant temperature sensor
- Faulty TCC
- Faulty valve body
- Dirty transmission (restricting smooth fluid flow in the hydraulic passages)
How to Test
Check the wiring harness of the transmission, look for any signs of damage and loose connections. Refer to your factory wiring diagram to locate the right power source and identify all the connection points between circuits. The transmission is triggered by the TCM through a power supplied by a fuse or relay. Disconnect the harness of the transmission in its connector, TCM and power source. Check the continuity inside the internal wiring harness of the transmission by locating the right + and – pins for the torque converter clutch solenoid.
Set the DVOM (digital volt ohm-meter) to ohm scale, and check the circuit for resistance using the positive and negative leads on their appropriate pins. If resistance is too high or over the limit (OL), then there must be a fault in the TCC solenoid or internal harness – some models may require the transmission oil pan to be removed for better diagnosis of the TCC solenoid.
Then, using the DVOM (again, set at ohm scale), test the wiring between the wiring harness connector and the TCM at the transmission case. Check for possible ground or short by moving the DVOMs ’negative lead to a known good ground. The resistance should be very high or OL.
Testing the TCC Solenoid
Remove the harness plug of the transmission (if applicable) and then check the resistance of the TCC solenoid, as well as the internal transmission wiring at the transmission case. Note that, some makes and models of transmission wire harness with TCC solenoid and internal harness as a single unit. Again, set the DVOM to ohm scale, with its positive and negative leads on the pins for the control circuit and TCC power. Of course, resistance should be right within the manufacturer’s specs. If it is too high or OL, then remove the transmission oil pan and check the solenoid inside the transmission (if applicable). Look for voltage in the power side circuit of the TCC solenoid or the wire harness connector at the TCM using the DVOM (this time, set at volts scale). Connect the positive lead at the wire to be tested, and the negative lead to a known good ground, with the vehicle key on and engine off. Battery voltage must be present.
Testing the TCM
Since TCM is activated only during certain driving conditions, it is important to monitor the TCM using advanced scan tool to its performance, and whether it is commanding the TCC and what is in its feedback reading. TCC is usually controlled by a duty cycle to engage more comfortable clutch management of comfortable torque converter.
To test whether the TCM is sending signal or not, you need a graphing multi-meter set to conduct duty cycle or a digital storage oscilloscope. The positive lead is connected to the wiring harness that is plugged in the TCM, while the negative lead is connected to a known good ground. Duty cycle should remain the same as what the TCM commands through the scan tool reading. If the cycle is at 0% or 100%, as intermittent, then you need to re-check the connections if all wirings are okay, then the problem must be at the TCM.
How to Repair
Transmission flush or replacement is a good start for fixing this error code. Of course, it should be accompany by a full circuit check of both the sensors and solenoids. If any of these components are defective, then they could be a start of the problem, rather than a costly cause.
Note: Extended driving with this error code can result to overheating of the transmission fluid, causing more serious transmission problems.
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