Technical Description of OBD-II Trouble Code
Malfunction in Shift Timing Solenoid B
What does that signify?
This is a standard diagnostic trouble code (DTC) for transmission issues and is commonly found in OBD-II vehicles with automatic transmissions. It can be applicable to various vehicle makes such as Chevrolet / Chevy, Volvo, GMC, Saab, Subaru, VW, BMW, Toyota, Ford, Dodge, etc.
The shift timing solenoid is one of the many components responsible for smooth shifting in an automatic transmission. The TCM (Transmission Control Module) monitors and controls the functionality of this solenoid.
The main purpose of the solenoid is to regulate the flow of hydraulic fluid (ATF: Automatic Transmission Fluid) internally to facilitate smooth shifting. These solenoids can be considered as electro-mechanical “pistons”. In most cases, there are multiple shift timing solenoids in the system, and the specific solenoid can be identified by referring to the letter mentioned 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 P078A, along with associated codes (P078B, P078C, 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 position 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-experts or average DIYers to seek professional help, especially for internal transmission work.
The code P078A indicates a malfunction in Shift Timing Solenoid B.
What is the severity of this DTC?
I would classify this code as moderately severe. If left unaddressed for an extended period, it can cause significant damage to the transmission, leading to costly repairs.
What are some of the symptoms of the code?
The symptoms associated with a P078A trouble code may include:
What are some of the common causes of the code?
The potential causes for this P078A transmission code may include:
What are some P078A troubleshooting steps?
Prior to initiating the troubleshooting process for any malfunction, it is recommended to refer to 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 problem, but it could have been prevented by following the maintenance schedule. It’s worth considering! Old fluid is dirty fluid, and dirty fluid can cause blockages, which in turn can trigger this code, so it is important 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, depending on the specific model, it may be possible to access the shift solenoid externally. In such cases, inspect the solenoid and its harness/connectors for any signs of damage. Repair or replace any issues as necessary. It 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 options. Diagnosing and repairing internal transmission issues should be left to a certified technician.
NOTE: The TCM and the harnesses involved in this circuit may be exposed to harsh road conditions, so ensure that everything is intact and in proper condition.
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 specified 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 from 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.
