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OBD Codes
Your OBD-II Trouble Codes Repair Site

P0888 TCM Power Relay Sense Circuit

OBD-II Trouble Code Technical Description

Article by
Stephen Darby
Stephen Darby
ASE Certified Technician

TCM Power Relay Sense Circuit

What does P0888 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 Hyundai, Kia, Smart, Jeep, Dodge, Ford, Dodge, Chrysler, etc. Although generic, the exact repair steps may vary depending on year, make, model and powertrain configuration.


 

If your vehicle has stored a code P0888, accompanied by a malfunction indicator lamp (MIL), it means that the powertrain control module (PCM) has detected an unspecific 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总裁俱乐部app, depending on vehicle make and model. If a P0888 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 that is utilized 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 anti-lock braking systems and electronic stability control systems as well. This code is unique in that it is usually only stored if other traction control system related codes are present. These may be codes related to mechanical or electronic issues.

总裁俱乐部appElectronic 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.

总裁俱乐部appAs 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 P0888 will be stored and a MIL may be illuminated.

An example of a typical TCM transmission control module:
P0888 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 detect insufficient voltage on the input supply (from the relay or fuse) a P0888 will be stored and a MIL illuminated.

总裁俱乐部appIf 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?

总裁俱乐部appSymptoms of a P0888 trouble code may include:

What are some of the common causes of the code?

总裁俱乐部appCauses for this code may include:

What are some P0888 troubleshooting steps?

总裁俱乐部appYou will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of reliable vehicle information in order to accurately diagnose a code P0888. An oscilloscope can also prove very helpful in diagnosing various speed sensors.

总裁俱乐部appYou may save yourself time by searching for technical service bulletins (TSB) that replicate the code stored, vehicle (year, make, model, and engine), and symptoms exhibited. This information may be found in your vehicle information source. If you find the right TSB, it could yield a speedy solution to your diagnosis.

After you connect the scanner to the vehicle diagnostic port and retrieve all stored codes and pertinent freeze frame data, write the information down (in case the code proves to be an intermittent one). After that, clear the codes and test drive the vehicle until one of two things happens; the code is restored or the PCM enters readiness mode.

总裁俱乐部appThe code may be more difficult to diagnose if the PCM enters readiness mode at this point, because the code is intermittent. The condition which caused the P0888 to be stored may need to worsen before an accurate diagnosis can be made. If the code is restored, continue with the diagnosis.

总裁俱乐部appYou may obtain connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts (pertaining to the code and vehicle in question) using your source of vehicle information.

Perform a visual inspection of related wiring and connectors. Repair or replace wiring that has been cut, burned, or damaged.

总裁俱乐部appUse 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 defective relays and/or blown (or otherwise defective) fuses as required.

总裁俱乐部appIf 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.

Related DTC Discussions

Need more help with a P0888 code?

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NOTE: This information is presented for information purposes only. It is not intended as repair advice and we are not responsible for any actions you take on any vehicle. All information on this site is copyright protected.

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