P2768 Unstable Sensor Circuit At Entry / Turbine Speed
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P2768 Unstable Sensor Circuit At Entry / Turbine Speed
Home »Codes P2700-P2799» P2768
OBD-II DTC Datasheet
Malfunction of the sensor circuit "B" speed input / turbine
What does this mean?
This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to all vehicles since 1996 (Ford, Honda, Mazda, Mercedes, VW, etc.). Although general in nature, the specific repair steps may differ depending on the brand / model.
If you receive DTC P2768, it is likely because the powertrain control module (PCM) has detected an unstable voltage input signal from the input (or turbine) speed sensor circuit labeled "B". Although the input sensors and turbine speed sensors are essentially the same and serve the same purpose, component terminology differs from manufacturer to manufacturer.
In most cases, the inlet / turbine speed sensor is a three-wire electromagnetic sensor used to monitor the gearbox inlet speed in revolutions per minute (rpm). The sensor is usually located near the rear of the bell (on the transmission input shaft) and is installed with a bolt / stud or screwed directly into the transmission case.
The main (or input) shaft of the transmission is permanently attached to either a gear reaction wheel or specially designed grooves. When a running engine is transmitting RPM to the transmission, the input shaft (or jet wheel) runs close to the end of the sensor. The steel shaft (or reactor wheel) effectively completes the electronic / electromagnetic circuit with the sensor. An electronic pattern is formed when the circuit is interrupted by grooved (or notched) sections that run past the sensor. The circuit is recognized by the PCM as a waveform, which it is programmed to interpret as transmission power input / turbine speed.
The transmission output speed, transmission input speed / turbine speed, engine speed, throttle position, engine load percentage, and other factors are compared and calculated to determine the desired input / turbine speed. A P2768 code will be stored (and the malfunction lamp may illuminate) if the input RPM / RPM or system circuit voltage cannot remain accurate within a specified degree for a specified period of time.
P2768 indicates an intermittent input circuit voltage for the input / turbine speed sensor.
symptoms
Symptoms of a P2768 code may include:
- Unstable operation of the speedometer (odometer)
- Transmission does not shift properly
- Speedometer and / or odometer don't work at all
- Transmission shift points are erratic or harsh
- Reduced fuel efficiency
reasons
Possible reasons for setting this code:
- Defective input speed sensor B
- Damaged, loose or burnt wiring and / or connectors
- PCM error or PCM programming error
- Accumulation of metal debris on the magnetic sensor
Diagnostic and repair procedures
A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.
A digital volt / ohmmeter (DVOM), manufacturer's service manual, an advanced diagnostic scanner, and possibly an oscilloscope will assist in the correct diagnosis of the P2768 code.
I usually start my diagnosis with a visual inspection of the system wiring and connectors. I would repair any clearly shorted or open circuits and / or connectors before proceeding. Be sure to inspect the battery, battery cables and cable ends at this time, and check the generator output.
Then I connected the scanner to the diagnostic port, retrieved all stored codes, and wrote them down for future use. I would also pay attention to the freeze frame data at this time.
Use the scanner data stream to determine which circuit is faulty if both input and output sensor codes are present. For the most accurate data available with the scanner, narrow your data stream to include only relevant information.
Metal debris on the magnetic contacts of the input and / or output speed sensors can cause intermittent / erratic sensor output. Remove the sensor and check for metal debris. Remove excess debris from magnetic surfaces before reinstalling. I would also inspect the break grooves and / or notches on the reactor wheel for damage or wear.
I use the DVOM to test individual sensor resistance and circuit voltage following the manufacturer's recommendations (see Service Manual or all data). I would replace sensors that do not meet manufacturer's specifications.
Controller failure can occur if all associated controllers are not shut down prior to testing resistance or continuity with the DVOM.
Suspect a defective PCM or PCM programming error if a P2768 code is stored and all system circuits and sensors are in proper operating condition and meet manufacturer specifications.
Additional diagnostic notes:
- Excessive metal debris (attracted to the electromagnetic sensor) can cause erroneous I / O speed sensor readings.
- The gap between the sensor and the reactor is critical. Make sure the mounting surfaces / threaded holes are free of debris and obstructions.
- If it is necessary to remove the input and / or output speed sensors from the transmission, use caution. Hot transmission fluid may leak from the hole.
- Look for transmission fluid in the area of the input speed sensor connector, as some sensors are prone to internal leakage.
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