Technical Description of OBD-II Trouble Code
The circuit for the output speed sensor is experiencing a loss of directional signal.
What does that mean?
This generic powertrain diagnostic trouble code (DTC) is commonly found in many OBD-II vehicles, including Chevrolet, Ford, Toyota, Dodge, Honda, and others.
When your vehicle displays code P077A, it indicates that the powertrain control module (PCM) has detected a loss of directional signal from the output speed sensor.
Output speed sensors are typically electromagnetic and use a toothed reluctor ring or gear that is permanently attached to the transmission output shaft. As the output shaft rotates, the reluctor ring also rotates. The raised teeth on the reluctor ring complete the output speed sensor circuit when they pass near the stationary electromagnetic sensor. The notches between the teeth of the reluctor ring interrupt the sensor circuit as the reluctor passes by the electromagnetic tip of the sensor. The PCM and other controllers receive these circuit completions and interruptions as waveform patterns that represent the transmission output speed.
The sensor is either threaded directly into the transmission housing or held in place with a bolt. An O-ring seal 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 code P077A is stored, the PCM has detected an input voltage signal from the output speed sensor indicating that the reluctor ring is not moving. When the output speed sensor voltage signal fails to fluctuate, the PCM assumes that the reluctor ring has unexpectedly stopped moving. 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 sufficiently according to the output speed sensor signal. An immobile output speed sensor signal may be caused by 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 that contribute to a code P077A being stored could result in 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 P077A engine code may include:
What are some of the common causes of the code?
Causes for this code may include:
What are some P077A troubleshooting steps?
I typically begin my diagnosis of a P077A with a visual inspection of the system wiring and connectors. I would remove the output speed sensor and clean excessive metallic debris from the magnetic tip. Caution must be taken when removing the sensor to prevent hot transmission fluid from leaking. Open or shorted circuits and connectors should be repaired as necessary.
Check the reluctor ring when inspecting the sensor after removal. If the reluctor ring is damaged, cracked, or if any teeth are missing or worn, it is likely the cause of the problem.
If other transmission-related symptoms are present, check the automatic transmission fluid. The fluid should appear relatively clean and not have a burnt smell. If the transmission fluid is more than one quart low, refill it with the correct fluid and check for leaks. The transmission must be filled with the appropriate fluid and in good mechanical condition before proceeding with the diagnosis.
To diagnose a code P077A, I would need a diagnostic scanner with an integrated oscilloscope, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information.
I would connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and freeze-frame data. Before clearing any codes, I would write down this information as it may be helpful during the diagnosis.
Using the vehicle information source, search for applicable technical service bulletins (TSB) that match the symptoms and stored codes for the specific vehicle. This can lead to a faster and more accurate diagnosis.
Utilize the scanner data stream to observe the output speed while test driving the vehicle. Narrowing the data stream to display only relevant fields will improve the speed and accuracy of data delivery. Inconsistent or erratic signals from the input or output speed sensors may indicate wiring, electrical connector, or sensor problems.
When performing circuit resistance and continuity tests with the DVOM, disconnect connectors from related controllers to avoid damaging the controllers.
Disconnect the output speed sensor and use the DVOM to perform a resistance test on it. The vehicle information source should provide wiring diagrams, connector views, connector pin-out charts, and manufacturer’s recommended testing procedures/specifications. If the output speed sensor does not comply with the specifications, it should be considered defective.
Use the oscilloscope to obtain live data from the output speed sensor. Probe the output speed sensor signal wire and the sensor ground wire. This type of testing may require jacking or lifting the vehicle. After safely lifting the drive wheels off the ground and securing the vehicle, run up the drivetrain while observing the waveform pattern on the oscilloscope. Look for glitches or inconsistencies in the waveform pattern created by the output speed sensor signal.
