P0866 TCM Communication Circuit High
OBD-II Trouble Code Technical Description
TCM Communication Circuit High
What does P0866 mean?
This diagnostic trouble code (DTC) is a generic powertrain code and applies to many OBD-II vehicles (1996-newer). That may include but is not limited to vehicles from Dodge, Honda, Volkswagen, Ford, etc. Although generic, the exact repair steps may vary depending on year, make, model and powertrain configuration.
If your vehicle has stored a code P0866, accompanied by a malfunction indicator lamp (MIL), it means that the powertrain control module (PCM) has detected a high voltage condition in the transmission control module (TCM) communication circuit.
While some transmission control modules (for OBD-II equipped vehicles) are integrated into the PCM, most are stand-alone components. TCM related codes may also be stored as P-codes, B-codes, or even U-codes, depending on vehicle make and model. If a P0866 code is stored, there is a good chance that there are other PCM and/or TCM codes stored as well. Antilock braking system (ABS) codes may be stored, too.
Electronic transmission control systems in OBD-II equipped vehicles are controlled using a network of computers (called control modules). This involves constant communication between the various control modules via the controller area network (CAN).
The PCM utilizes input signals from multiple engine and transmission sensors to calculate automatic transmission shift strategy. A high-pressure pump (inside the transmission) forces fluid through the valve body and into the sprag assembly; in addition to lubricating and cooling the transmission. The high-pressure fluid allows the clutches to separate from the sprag momentarily so that the gear ratio may be changed smoothly. One or more electronic pressure control (EPC) solenoids help to regulate fluid pressure.
Electronic pressure (EP) sensors provide the PCM with pertinent data regarding fluid pressure at various points in the transmission.
Electronic shift solenoids are used to interrupt the flow of high-pressure fluid so that the transmission shifts gears when commanded. The PCM uses voltage input signals from the transmission input speed sensor and transmission output speed sensor to determine if the transmission is shifting efficiently.
As with many other automatic transmission malfunctions, a transmission control system code may be caused by either an electrical or mechanical failure. If the PCM detects a malfunction which demands MIL illumination, a code P0866 will be stored and a MIL may be illuminated.
An example of a typical TCM transmission control module
What is the severity of this DTC?
If this code is stored, you may have noticed that your transmission shifts much more harshly than normal. If your vehicle has exhibited a code P0866 , it should be diagnosed as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0866 trouble code may include:
Erratic transmission shift patterns Failure of the transmission to shift Diminished fuel efficiency Transmission slippage Delayed transmission engagement Other transmission related codes ABS disabledWhat are some of the common causes of the code?
Causes for this code may include:
Transmission sensor failure Vehicle speed sensor failure Open or shorted circuits in the CAN harness Mechanical transmission failure Defective TCM, PCM, or programming errorWhat are some P0866 troubleshooting steps?
You will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of reliable vehicle information in order to accurately diagnose a code P0866. An oscilloscope can also prove very helpful in diagnosing various sensor outputs.
You may save yourself time by searching for technical service bulletins (TSB) that replicate the code stored, vehic
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