What does it mean?
This generic powertrain diagnostic trouble code (DTC) generally applies to many OBD-II vehicles. This may include, but is not limited to, vehicles from Chevrolet, Ford, Toyota, Dodge, Honda, etc.
When your vehicle has stored a P077B code, it means the Powertrain Control Module (PCM) has detected an error in the direction of the output speed sensor signal circuit.
Output speed sensors are typically of the electromagnetic variety. They use a certain type of toothed reluctor ring or gear that is permanently fixed to the transmission’s output shaft. When the output shaft rotates, it spins the reluctor ring. The raised teeth on the reluctor ring complete the output speed sensor circuit as they pass near the stationary electromagnetic sensor. As the reluctor passes by the sensor’s electromagnetic tip, the notches between the reluctor ring’s teeth create breaks in the sensor circuit. This combination of circuit completions and breaks is received by the PCM (and other controllers) as waveforms that represent the transmission output speed.
The sensor screws directly into the transmission case or is held in place with a bolt. An O-ring is used to prevent fluid leakage from the sensor opening.
The PCM compares the transmission input speed and output speed to determine if the transmission is shifting correctly and operating efficiently.
If a P077B code 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 goes negative, the PCM assumes 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 (based on the output speed sensor signal). A negative output speed sensor signal can be caused by an electrical malfunction or a mechanical problem.
How severe is this DTC?
Conditions that contribute to the storage of a P077B code can lead to (or be the result of) catastrophic transmission failure, so it should be addressed urgently.
What are some of the symptoms of the code?
Symptoms of a P077B engine code may include:
Intermittent operation of the speedometer/odometer
Abnormal transmission shifting patterns
Transmission slipping or delayed engagement
Activation/deactivation of the anti-lock brake system (if applicable)
Other transmission and/or ABS codes may be stored
What are some of the common causes of the code?
Causes
of this code may include:
Faulty output speed sensor
Metal debris on the output speed sensor
Open or shorted circuits or connectors (especially near the output speed sensor)
Damaged or worn reluctor ring
Mechanical transmission failure
What are the troubleshooting steps for P077B?
I generally like to start my diagnosis of a P077B with a visual inspection of the system’s wiring and connectors. I would remove the output speed sensor and clean excessive metal debris from the magnetic tip. Be careful when removing the sensor as hot transmission fluid may leak from the sensor opening. Repair any open or shorted circuits and connectors as needed.
Inspect the reluctor ring when the sensor is removed for inspection. If the reluctor ring is damaged, cracked, or has missing (or worn) teeth, you have likely found your problem.
Check the automatic transmission fluid if other transmission-related symptoms are present. The fluid should be relatively clean and not have a burnt smell. If the transmission fluid is more than a quart low, refill it with the correct fluid and check for leaks. The transmission must be filled with the proper fluid and in good mechanical condition before proceeding with diagnosis.
I would need a diagnostic scanner with a built-in oscilloscope, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information to diagnose a P077B code.
I like to connect the scanner to the vehicle’s diagnostic port and then retrieve all stored trouble codes and freeze frame data. Before clearing the codes, I would write down this information as it may prove useful as my diagnosis progresses.
Look for applicable Technical Service Bulletins (TSBs) 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 quick and accurate diagnosis.
Use the scanner’s data stream to observe the output speed during a vehicle road test. Limiting the data stream to display only relevant fields will increase the speed and accuracy of data delivery. Inconsistent or erratic signals from the input or output speed sensor may lead you to wiring, electrical connector, or sensor problems.
Disconnect the output speed sensor and use the DVOM to perform a resistance test. Your vehicle information source should provide wiring diagrams, connector views, connector pinout charts, and the manufacturer’s recommended testing procedures/specifications. If the output speed sensor does not meet specifications, it should be considered faulty.
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 up or lift the vehicle to perform this type of test. Once the drive wheels are safely off the ground and the vehicle is secure, run the transmission while observing the waveform pattern on the oscilloscope. You are looking for faults or inconsistencies in the waveform pattern created by the output speed sensor signal.
Disconnect the associated controllers’ connectors when performing circuit resistance and continuity tests with the DVOM. Failure to do so may damage the controller.