P077B Output Speed Sensor Circuit – Direction Error
OBD-II Trouble Code Technical Description
Output Speed Sensor Circuit – Direction Signal
What does that mean?
This generic powertrain diagnostic trouble code (DTC) typically applies to many OBD-II vehicles. That may include but is not limited to vehicles from Chevrolet, Ford, Toyota, Dodge, Honda, etc.
When your vehicle has stored a code P077B, it means that the powertrain control module (PCM) has detected a directional error from the output speed sensor signal circuit.
Output speed sensors are typically of the electromagnetic variety. They utilize some type of toothed reluctor ring or gear that is permanently affixed to the transmission output shaft. As the output shaft spins, so spins the reluctor ring. The raised teeth of the reluctor ring complete the output speed sensor circuit when they pass in close proximity to the stationary electromagnetic sensor. As the reluctor passes by the electromagnetic tip of the sensor, the notches between the reluctor ring teeth create interruptions in the sensor circuit. This combination of circuit completions and interruptions are received by the PCM (and other controllers) as wave form patterns which represent transmission output speed.
The sensor either threads directly into the transmission housing or is held in place with a bolt. An O-ring seal is used to prevent fluid leaking from the sensor opening.
The PCM compares transmission input speed and output speed to determine if the transmission is shifting correctly and operating efficiently.
If a code P077B is stored, the PCM has detected an input voltage signal from the output speed sensor indicating that the reluctor ring is moving (or has moved) in an incorrect direction. When the output speed sensor voltage signal inputs negative voltage, the PCM assumes that the reluctor ring has unexpectedly moved in an incorrect direction. The PCM receives vehicle speed input data and wheel speed input data in addition to the output speed sensor data. By comparing these signals, the PCM can determine if the reluctor ring is moving in the correct direction (according to the output speed sensor signal). A negative output speed sensor signal may be caused by either an electrical malfunction or a mechanical problem.
Here is an example of a transmission speed sensor:
What is the severity of this DTC?
The conditions which contribute to a code P077B being stored could result in (or be the result of) catastrophic transmission failure, it should be rectified with urgency.
What are some of the symptoms of the code?
Symptoms of a P077B engine code may include:
What are some of the common causes of the code?
Causes for this code may include:
What are some P077B troubleshooting steps?
I typically like to begin my diagnosis of a P077B with a visual inspection of system wiring and connectors. I would remove the output speed sensor and clean excessive metallic debris from the magnetic tip. Use caution when removing the sensor because hot transmission fluid may leak from the sensor opening. Repair open or shorted circuits and connectors as required.
Check the reluctor ring when the sensor is removed for inspection. If the reluctor ring is damaged, cracked, or if any teeth are missing (or worn), you have most likely found your problem.
Check automatic transmission fluid if other transmission related symptoms are exhibited. Fluid should appear relatively clean and not smell burnt. If transmission fluid is more than one-quart low, refill it with the correct fluid and check for leaks. The transmission must be full of the appropriate fluid and in good mechanical condition before proceeding with the diagnosis.
I would need a diagnostic scanner with an integrated oscilloscope, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information to diagnose a code P077B.
I like to connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and freeze-frame data next. Prior to clearing any codes, I would write this information down as it may prove helpful as my diagnosis unfolds.
Search for applicable technical service bulletins (TSB) using your vehicle information source. Finding a TSB that matches the symptoms and stored codes (for the vehicle in question) will likely lead to a speedy and accurate diagnosis.
Utilize the scanner data stream to observe output speed while test driving the vehicle. Narrowing the data stream to display only pertinent fields will increase the speed and accuracy of data delivery. Inconsistent or erratic signals from input or output speed sensors may lead you to wiring, electrical connector, or sensor problems.
Disconnect the output speed sensor and use the DVOM to perform a resistance test it. Your vehicle information source should yield wiring diagrams, connector views, connector pin-out charts and manufacturer’s recommended testing procedures/specifications. If the output speed sensor does not comply with specifications, it should be considered defective.
Live data from the output speed sensor can be obtained using the oscilloscope. Probe the output speed sensor signal wire and the sensor ground wire. You may need to jack or lift the vehicle in order to perform this type of testing. After the drive wheels are safely lifted off the ground and the vehicle is secure, run up the drivetrain while observing the waveform pattern on the oscilloscope. You are looking for glitches or inconsistencies in the waveform pattern created by the output speed sensor signal.
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"Delete Error Code Permanently, Disable DTC Error Code Function for p077b in the ECU"
In extreme cases, we even have the option to permanently remove the specific DTC p077b and disable the function of the specific error code in the ECU. All other fault code functions will then continue to work to protect the engine. This is different from clearing an error code with a diagnostic device. We have the option to permanently remove the function of the specific error code, so that it never comes back. Also emergency running that occurs on the specific error code can be solved in most cases.
Of course we prefer to first try to solve the cause of the error code, but sometimes the only solution is to permanently disable the DTC function of p077b.
