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P0789 is a diagnostic trouble code (DTC) for "Shift/Timing Solenoid Intermittent". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
When the Powertrain Control Module (PCM) detects a problem in the shift/timing solenoid, a P0789 trouble code is stored. Some codes that could accompany a P0789 trouble code include P0785, P0786, P0787, and P0788.
Using calculations based on the position of the throttle, vehicle speed, engine speed, and engine load, the PCM determines the appropriate gear ratio. If the desired gear ratio does not correspond with the actual gear ratio, a P0789 trouble code is stored and a Check Engine Light illuminates. In some makes and models, a Check Engine Light is not automatically illuminated, requiring multiple failure first.
The cause of a P0789 trouble code usually stems from a faulty shift solenoid, though other causes include blockage inside the transmission fluid passages or a mechanical failure of the transmission. Low fluid levels or dirty transmission fluid can also cause the storage of this code, in addition to a faulty PCM or Transmission Control Module (TCM).
The mechanic might find a variety of symptoms associated with this trouble code. These include no detectable symptoms, a failure of the transmission to shift into one or more gears, slippage of the transmission, and overheating of the transmission. Some final symptoms are a decrease in the vehicle's fuel efficiency and the TCM placing the transmission into Limp-in mode.
To diagnose a P0789 trouble code, the mechanic needs an advanced scanner, digital volt/ohmmeter, manual pressure gauge, and a hydraulic pressure diagram for the vehicle. To diagnose the trouble code, the mechanic needs to perform the following:
Check all of the wiring, connectors, and components for damage. When replacing wiring, connectors, and components, the mechanic might need to replace the full internal transmission harness.
Pull the dipstick out and check the level and condition of the transmission fluid.
If the fluid looks abnormal or smells burnt, drop the transmission pan and look for evidence of debris. Debris can indicate a more far reaching problem with the transmission itself.
After making all of the necessary repairs to the internal transmission, the mechanic should fill the transmission with enough fluid to repair any leaks.
Once the leaks are repaired, the mechanic should fill the transmission fluid to a normal level, start the vehicle, and allow the engine to reach a normal operating temperature.
The mechanic should clear the code and take the vehicle for a test drive.
If the code returns, connect a manual pressure gauge to the transmission to check the hydraulic pressure pump. The mechanic should use a hydraulic pressure diagram when doing so.
The mechanic should then compare their findings with the manufacturer's specifications to find any discrepancies.
If the code persists, the mechanic can use the digital volt/ohmmeter to check the reference voltage and ground signal of the shift solenoid.
If the reference voltage or ground signal check out as open, the mechanic should then check for continuity and resistance in all of the related circuits. The mechanic should make sure to disconnect any control modules in the circuit to avoid damage.
If everything else checks out, inspect the PCM to see if it is at fault.
Finally, After making all of the necessary repairs, the mechanic should clear the code and retest the system to see if it returns.
Mechanics commonly assume a faulty shift solenoid circuit is at the root of the problem. This leads to the replacement of one or more shift solenoids in error, and failure to ultimately clear the code.
A P0789 trouble code can be an indicator of more serious problems and if left undiagnosed and unfixed can ultimately lead to transmission failure. The mechanic needs to clear trouble codes in the order they were stored in order to successfully determine the cause of a P0789 trouble code.
Repairing and clearing a P0789 trouble code requires a mechanic to perform the following steps:
Replace any defective or otherwise damaged wiring, connectors, or components. This usually requires the mechanic to replace the full internal transmission harness.
Rebuild the transmission if internal transmission damage is detected. The mechanic should also install a replacement torque converter when completing this repair.
Repair any leaks before filling the transmission fluid to an appropriate level to test drive the vehicle.
Replace the hydraulic pump if the pump pressure reads low. Mechanics should keep in mind that when replacing the hydraulic pump they need to remove the transmission and partially disassemble it.
Replace any shift solenoids, or solenoid packs that test as at fault during a diagnosis of the wiring connectors and other components. The mechanic should disconnect any related control modules before performing this repair to avoid damage and control module memory loss.
Replace the PCM if it is detected as faulty.
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