P077A Output speed sensor circuit - direction signal loss

P077A Output speed sensor circuit - direction signal loss

OBD-II DTC Datasheet

Output Speed ​​Sensor Circuit - Loss of Heading Signal

What does this mean?

This Generic Powertrain Diagnostic Trouble Code (DTC) is commonly applied to many OBD-II vehicles. This may include but is not limited to Chevrolet, Ford, Toyota, Dodge, Honda, etc.

When your vehicle has stored a code P077A, it means the powertrain control module (PCM) has detected a loss of heading signal from the output speed sensor.

Output speed sensors are usually electromagnetic. They use some type of toothed reaction ring or gear that is permanently attached to the transmission output shaft. As the output shaft rotates, the reactor ring rotates. The bulging teeth of the reactor ring complete the output speed sensor circuit as they pass in close proximity to the stationary electromagnetic sensor. When the reactor passes the electromagnetic tip of the sensor, the notches between the teeth of the ring of the reactor create discontinuities in the sensor circuit. This combination of rung terminations and interrupts is received by the PCM (and other controllers) as waveform patterns that represent the output baud rate.

The sensor is either screwed directly into the transmission housing or held in place with a bolt. An O-ring is used to prevent fluid from leaking from the sensor bore.

The PCM compares the input and output speeds of the transmission to determine if the transmission shifts properly and is operating efficiently.

If P077A is stored, the PCM has detected an input voltage signal from the output speed sensor indicating that the reactor ring is not moving. When the output speed sensor voltage signal does not fluctuate, the PCM assumes that the reactor ring has suddenly stopped moving. The PCM receives vehicle speed inputs and wheel speed inputs in addition to output speed sensor data. By comparing these signals, the PCM can determine if the reactor ring is moving enough (according to the signal from the output speed sensor). A stationary output speed sensor signal can be caused by either an electrical problem or a mechanical problem.

Here is an example of a transmission speed sensor:

What is the severity of this DTC?

Conditions that contribute to the persistence of the P077A code may or may be the result of a catastrophic transmission failure and should be corrected urgently.

What are some of the symptoms of the code?

Symptoms of a P077A engine code may include:

• Intermittent operation of the speedometer / odometer
• Abnormal gear shifting patterns
• Transmission slippage or delayed engagement
• Activation / deactivation of traction control (if applicable)
• Other transmission codes and / or ABS can be stored

What are some of the common causes for the code?

Reasons for this code may include:

• Defective output speed sensor
• Metal debris on the output speed sensor
• Open or short circuit in circuits or connectors (especially near the output speed sensor)
• Damaged or worn reactor ring
• Failure of a mechanical transmission

What are some steps to troubleshoot the P077A?

I usually like to start diagnosing a P077A with a visual inspection of the system wiring and connectors. I would remove the output speed sensor and remove excess metal debris from the magnetic tip. Be careful when removing the sensor as hot transmission fluid may leak out of the sensor bore. Repair open or short circuit in circuits and connectors if necessary.

After removing the sensor for inspection, check the reactor ring. If the reactor ring is damaged, cracked, or if any teeth are missing (or worn out), you've most likely found your problem.

Check the automatic transmission fluid if other transmission-related symptoms appear. The liquid should appear relatively clean and not smell burnt. If the level of the transmission fluid is less than one quart, fill it with a suitable fluid and check for leaks. The transmission must be filled with the correct fluid and in good mechanical condition prior to diagnostics.

I will need a diagnostic scanner with a built-in oscilloscope, digital volt / ohmmeter (DVOM) and a reliable source of vehicle information to diagnose the P077A code.

I like to connect the scanner to the vehicle diagnostic port and then retrieve all stored DTCs and freeze frame data. I would write this information down before clearing any codes, as it may prove useful as my diagnosis progresses.

Find the relevant 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 quick and accurate diagnosis.

Use the scanner data stream to monitor the output speed during test driving of the vehicle. Narrowing down the data stream to display only the relevant fields will increase the speed and accuracy of data delivery. Inconsistent or inconsistent signals from input or output speed sensors can cause problems with the wiring, electrical connector, or sensor.

Disconnect the output speed sensor and use the DVOM to test resistance. Your source of vehicle information should include wiring diagrams, connector types, connector pinouts, and manufacturer's recommended testing procedures / specifications. If the output speed sensor is out of specification, it should be considered defective.

Real-time data from the output speed sensor can be obtained using an oscilloscope. Check the output speed sensor signal wire and the sensor ground wire. You may need to jack or lift the vehicle to complete this type of test. After the drive wheels are safely off the ground and the vehicle is securely anchored, start the transmission by observing the waveform diagram on an oscilloscope. You are looking for glitches or inconsistencies in the waveform generated by the output speed sensor signal.

• Disconnect the connectors from linked controllers when performing circuit resistance and continuity tests with the DVOM. Failure to do so could damage the controller.

Related DTC discussions

• There are currently no related topics in our forums. Post a new topic on the forum now.

Need more help with the P077A code?

If you still need help regarding DTC P077A, post a question in the comments below this article.

NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.