Technical Description of OBD-II Trouble Code
High Sense Circuit for TCM Power Relay
What is the meaning of P0891?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to many OBD-II vehicles (1996-newer). This includes vehicles from Hyundai, Kia, Smart, Jeep, Dodge, Ford, Dodge, Chrysler, etc. Although it is a generic code, the exact repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a code P0891, along with a malfunction indicator lamp (MIL), it indicates that the powertrain control module (PCM) has detected a high voltage condition in the TCM power relay control circuit.
While some transmission control modules (for OBD-II equipped vehicles) are integrated into the PCM, most are stand-alone components. TCM related codes are stored as P-codes, B-codes, or even U-codes, depending on the vehicle make and model. If a P0891 code is stored, there is a good chance that there are other PCM and/or TCM codes stored as well.
The CAN is a complex system of wiring and connectors used to transmit data between the TCM and the PCM. Data, including stored codes, may also be shared with other controllers via the CAN. Transmission input and output speed (RPM), vehicle speed, and wheel speed are all shared between multiple controllers. This data is not only used for comparison in electronic traction control systems, but also in anti-lock braking systems and electronic stability control systems. This code is unique because it is usually only stored if other traction control system related codes are present. These may be codes related to mechanical or electronic issues.
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 uses 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, which lubricates and cools the transmission. The high-pressure fluid allows the clutches to separate from the sprag momentarily so that the gear ratio can be changed smoothly. One or more electronic pressure control (EPC) solenoids help regulate fluid pressure. Electronic pressure (EP) sensors provide the PCM with 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 can shift 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.
Like many other automatic transmission malfunctions, a transmission control system code can be caused by either an electrical or mechanical failure. If the PCM detects a malfunction that requires MIL illumination, a code P0891 will be stored and the MIL may be illuminated.
An example of a typical TCM transmission control module:
What is the severity of this DTC?
Battery voltage is supplied to the TCM using one or more fuses and/or contact relays. If the TCM or PCM detects insufficient voltage on the input supply (from the relay or fuse), a P0891 will be stored and the MIL will be illuminated.
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 P0865, it should be diagnosed as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0891 trouble code may include:
What are some of the common causes of the code?
Causes for this code may include:
What are some P0891 troubleshooting steps?
In order to accurately diagnose a code P0891, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information. An oscilloscope can also be helpful in diagnosing various speed sensors.
You may save time by searching for technical service bulletins (TSB) that match the stored code, vehicle (year, make, model, and engine), and exhibited symptoms. This information can be found in your vehicle information source. If you find the right TSB, it could provide a quick solution to your diagnosis.
After connecting the scanner to the vehicle diagnostic port and retrieving all stored codes and pertinent freeze frame data, write down the information in case the code is intermittent. Then, clear the codes and test drive the vehicle until one of two things happens: the code is restored or the PCM enters readiness mode.
If the PCM enters readiness mode at this point, the code may be more difficult to diagnose because it is intermittent. The condition that caused the P0891 to be stored may need to worsen before an accurate diagnosis can be made. If the code is restored, continue with the diagnosis.
You can obtain connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts for the code and vehicle in question using your source of vehicle information.
Perform a visual inspection of related wiring and connectors. Repair or replace any wiring that has been cut, burned, or damaged.
Use the DVOM to test voltage and ground circuits at the TCM and/or PCM. If no voltage is detected, check the system relay and related fuses. Replace any defective relays and/or blown (or otherwise defective) fuses as necessary.
If voltage and ground are detected at the TCM, test the corresponding circuit at the PCM connector. If no voltage is detected there, suspect an open circuit between the component in question and the PCM. If voltage is discovered there, suspect a defective TCM, PCM, or a programming error.
