Technical Description of OBD-II Trouble Code
Intermittent Issue with Shift Timing Solenoid A
What is the meaning of this?
This is a general diagnostic trouble code (DTC) for transmission and is usually applicable to OBD-II vehicles with automatic transmission. 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 the many components that enable smooth shifting in an automatic transmission 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 internal flow of hydraulic fluid (ATF: Automatic Transmission Fluid) to facilitate smooth shifting of the transmission. Essentially, these solenoids function as electro-mechanical “pistons”. In most cases, there are multiple shift timing solenoids in 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) triggers the MIL (Malfunction Indicator Lamp) and displays the code P0789, along with associated codes (P0785, P0786, P0787, and P0788), 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 several potential causes, including mechanical and/or electrical factors, depending on the specific code and the presence of any other codes. Automatic transmissions are complex systems, and it is advisable for non-experts or average DIYers to seek professional assistance, especially for internal transmission work.
The code P0789 indicates an intermittent fault in the shift solenoid A circuit, as detected by the ECM (Engine Control Module) and/or the TCM (Transmission Control Module).
How severe is 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 financial implications.
What are some symptoms of this code?
Some symptoms associated with the P0789 trouble code may include:
What are some common causes of this code?
Possible causes for this P0789 transmission code may include:
What are some troubleshooting steps for P0789?
Prior to initiating the troubleshooting process for any malfunction, it is advisable to consult the Technical Service Bulletins (TSB’s) specific to your 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, this may not only be the cause of the problem but could have been prevented by following the recommended maintenance schedule. It’s worth considering! Old fluid is dirty fluid, and dirty fluid can cause blockages, which in turn can trigger this code. Therefore, it is important 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, depending on your specific model, you may be able 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. 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 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 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, thereby verifying its electrical integrity to some extent. Another method involves manually applying voltage to the solenoid to assess its mechanical operation. The manufacturer’s service manual or technical literature should specify the appropriate testing procedure.
Basic Step #4
Test the circuit from the TCM. You may be able 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.
