Technical Description of OBD-II Trouble Code
The shift timing solenoid B is operating at a low 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 found in various vehicle makes such as Chevrolet / Chevy, Volvo, GMC, Saab, Subaru, VW, BMW, Toyota, Ford, Dodge, etc.
The shift timing solenoid is one of many components that allows an automatic transmission to shift smoothly based on your driving requirements. The TCM (Transmission Control Module) monitors and controls the functionality of the solenoid.
The main purpose of the solenoid is to regulate the flow of hydraulic fluid (ATF: Automatic Transmission Fluid) internally to facilitate smooth shifting of the transmission. Essentially, these solenoids function as electro-mechanical “pistons”. In most cases, there are multiple shift timing solenoids within the system, so you can refer to the letter included in the code to identify the specific solenoid based on your vehicle’s make and model.
The ECM (Engine Control Module) activates the MIL (Malfunction Indicator Lamp) and displays the codes P078C, along with associated codes (P078A, P078B, P078D, 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 are numerous potential causes for this code, including mechanical and/or electrical issues, depending on the specific code and the presence of any other active codes. Automatic transmissions are complex systems, and it is advisable to seek professional assistance, especially for internal transmission work.
The code P078C Shift Timing Solenoid B Low is triggered when the ECM and/or TCM detects a specific low 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, not to mention the associated expenses.
What are some of the symptoms of the code?
Some symptoms of a P078C trouble code may include:
What are some of the common causes of the code?
Possible causes for this P078C transmission code may include:
What are some P078C troubleshooting steps?
Prior to initiating 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 you have driven your vehicle for 30,000 to 60,000 miles (50,000 to 100,000 kilometers) without ever changing the fluid or keeping any records of it, not only could this be the cause of the code, but it could have been prevented by following the maintenance schedule. It is important to note that old fluid is dirty fluid, and dirty fluid can lead to blockages, which can trigger this code. Therefore, it is recommended to change the fluid!
NOTE: Follow the manufacturer’s recommended steps when checking ATF. This simple step can 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 advisable to attempt to access it. Instead, explore other options. Internal transmission diagnosis and repair should be left to a certified technician.
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. One method involves conducting a resistance test using a multimeter to measure the resistance between the pins in the solenoid, which helps verify its electrical integrity to some extent. Another method involves manually applying voltage to the solenoid to verify its mechanical operation. The specific testing procedure should be outlined in the manufacturer’s service manual or technical literature.
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 this time using the pins at the TCM and/or ECM. This is a useful technique to assess not only the integrity of the solenoid but also the electrical integrity of the circuit.
