Technical Description of OBD-II Trouble Code
The code indicates a problem with the output speed sensor circuit, specifically related to the direction signal.
What does this mean?
This generic DTC is applicable to various OBD-II vehicles, including Chevrolet, Ford, Toyota, Dodge, Honda, and others.
When code P077B is stored, it means that the PCM has detected a directional error in the output speed sensor signal circuit.
Output speed sensors are typically electromagnetic and use a toothed reluctor ring or gear attached to the transmission output shaft. As the shaft spins, the reluctor ring spins as well. The raised teeth on the ring complete the output speed sensor circuit when they pass near the stationary electromagnetic sensor. The notches between the teeth create interruptions in the sensor circuit as they pass by the sensor. These interruptions and completions are received by the PCM as waveform patterns representing the transmission output speed.
The sensor is either threaded into the transmission housing or held in place with a bolt, with an O-ring seal to prevent fluid leakage.
The PCM compares the transmission input speed and output speed to determine if the transmission is shifting correctly and operating efficiently.
If code P077B is stored, it means that the PCM has detected an input voltage signal from the output speed sensor indicating that the reluctor ring is moving in an incorrect direction. A negative voltage signal suggests that the reluctor ring has unexpectedly moved in the wrong direction. The PCM receives vehicle speed input data and wheel speed input data, allowing it to compare these signals and determine if the reluctor ring is moving correctly according to the output speed sensor signal. A negative output speed sensor signal can 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 lead to a stored code P077B 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 P077B engine code may include:
What are some of the common causes of the code?
Possible causes for this code include:
What are some P077B troubleshooting steps?
To diagnose a P077B code, start with a visual inspection of the system wiring and connectors. Remove the output speed sensor and clean any excessive metallic debris from the magnetic tip. Take caution when removing the sensor to avoid leakage of hot transmission fluid. Repair any open or shorted circuits and connectors as necessary.
Inspect the reluctor ring when the sensor is removed. If the ring is damaged, cracked, or missing teeth, it is likely the cause of the problem.
If other transmission-related symptoms are present, check the automatic transmission fluid. The fluid should be relatively clean and not have a burnt smell. If the 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 P077B, a diagnostic scanner with an integrated oscilloscope, a digital volt/ohmmeter (DVOM), and reliable vehicle information are necessary. Connect the scanner to the vehicle diagnostic port and retrieve all stored trouble codes and freeze-frame data. Write down this information before clearing any codes, as it may be helpful during the diagnosis.
Search for relevant technical service bulletins (TSB) using the vehicle information source. Finding a TSB that matches the symptoms and stored codes for the specific vehicle can lead to a faster and more accurate diagnosis.
Use the scanner data stream to observe the output speed while test driving the vehicle. Narrow down the data stream to display only relevant fields for faster and more accurate data delivery. Inconsistent or erratic signals from the input or output speed sensors may indicate wiring, electrical connector, or sensor problems.
Disconnect the output speed sensor and perform a resistance test using the DVOM. Refer to the vehicle information source for 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.
Use an 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. Safely lift the drive wheels off the ground and secure the vehicle before running 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.
