Technical Description of OBD-II Trouble Code
The shift timing solenoid B is operating at a high level.
What does that mean?
This is a standard diagnostic trouble code (DTC) for transmissions and is typically found in OBD-II vehicles with automatic transmissions. It can be seen in various vehicle makes such as Chevrolet / Chevy, Volvo, GMC, Saab, Subaru, VW, BMW, Toyota, Ford, Dodge, etc.
The shift timing solenoid is a component that allows an automatic transmission to shift smoothly based on driving conditions. The TCM (Transmission Control Module) monitors and controls the functionality of the solenoid.
The solenoid’s main function is to regulate the flow of hydraulic fluid (ATF: Automatic Transmission Fluid) internally to facilitate smooth shifting of the transmission. These solenoids can be considered as electro-mechanical “pistons”. In most cases, there are multiple shift timing solenoids in the system, so the specific solenoid can be identified by referring to the letter included in the code, depending on the make and model of the vehicle.
The ECM (Engine Control Module) activates the MIL (Malfunction Indicator Lamp) and displays the codes P078D, along with associated codes (P078A, P078B, P078C, and P078E), when it detects one or more issues with the shift timing solenoid or its circuit. The circuit includes the TCM, which may be located in a volatile area depending on the model. There can be various potential causes, including mechanical and/or electrical factors, depending on the specific code and the presence of any other active codes. Automatic transmissions are complex systems, and it is advisable for non-professionals or average DIYers to seek professional assistance, especially for internal transmission work.
The code P078D Shift Timing Solenoid B High is set when the ECM and/or TCM detects a specific high electrical value in the shift timing solenoid B circuit.
What is the severity of this DTC?
I would classify this code as moderately severe. If left unresolved for an extended period, it can cause significant damage to the transmission and result in costly repairs.
What are some of the symptoms of the code?
Some symptoms associated with the P078D trouble code may include:
What are some of the common causes of the code?
Possible causes for this P078D transmission code may include:
What are some P078D troubleshooting steps?
Prior to starting the troubleshooting process for any malfunction, it is recommended to research the Technical Service Bulletins (TSB’s) specific to the vehicle’s year, model, and transmission. This step can save time and money during diagnosis and repair!
Basic Step #1
The first step is to check the ATF (Automatic Transmission Fluid). If the fluid is dirty, has a burnt smell, is overfilled, or has an abnormal color, it may be necessary to change the fluid. If the vehicle has been driven for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) without a fluid change, this could be the cause of the problem. Regular maintenance can help prevent issues, so it is important to follow the recommended maintenance schedule. Remember, old fluid can become dirty and cause blockages, which can lead to this code, so it is crucial to change the fluid!
NOTE: Follow the manufacturer’s recommended steps when checking ATF. The process may vary significantly between different makes and models.
Basic Step #2
If the fluid is in good condition but the code is still active and symptoms persist, inspect the shift solenoid and its harness/connectors for any signs of damage. Repair or replace any issues as necessary. This is also a good time to check for any leaks!
TECH TIP: If the shift solenoid is internal, it is not recommended to attempt accessing it. Instead, explore other diagnostic options. Internal transmission diagnosis and repair should be left to certified technicians.
NOTE: The TCM and harnesses involved in this circuit may be exposed to harsh road conditions, so ensure that everything is intact and functioning properly.
Basic Step #3
Test the shift timing solenoid. There may be multiple ways to test the solenoid, such as a resistance test using a multimeter to measure the resistance between the pins to assess its electrical integrity. Another method may involve applying voltage to the solenoid manually to verify its mechanical operation. The manufacturer’s service manual or technical literature should provide specific instructions.
Basic Step #4
Test the circuit from the TCM. It may be possible to verify the functionality of the sensor and its circuit by performing the same tests as mentioned above, but from the pins at the TCM and/or ECM. This can help determine the integrity of both the solenoid and the circuit.
