Technical Description of OBD-II Trouble Code
Slipping of Transmission Component
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to many OBD-II vehicles (1996-newer). This includes vehicles from Chevrolet, GMC, Mazda, Mercedes Benz, Toyota, Lexus, etc. Although it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration.
If your vehicle has stored a code P0894, it means that the powertrain control module (PCM) has received a data input signal from the input and output speed sensors indicating that an internal transmission component is slipping.
The transmission control module (TCM) may be a stand-alone unit, but it is commonly integrated into a single housing with the engine control module (ECM). This combined unit is called the PCM.
The PCM uses input signals from various engine and transmission sensors to calculate the automatic transmission shift strategy. Electronic shift solenoids are utilized to interrupt the flow of high-pressure fluid, allowing the transmission to shift gears. The PCM relies on voltage input signals from the transmission input speed sensor and transmission output speed sensor to determine if the transmission is functioning efficiently and without slippage.
Similar to other automatic transmission malfunctions, a transmission slippage code can be caused by electrical or mechanical failure. When the friction material on one or more clutch discs is compromised, the transmission is unable to achieve the desired gear ratio due to slippage. This condition is often accompanied by the distinct odor of burnt friction material in the fluid.
If the PCM detects a degree of transmission slippage that exceeds the maximum allowable parameters, it may store a code P0894 and illuminate the malfunction indicator lamp (MIL).
What is the severity of this DTC?
A stored code P0894 indicates that either a serious electrical issue or a catastrophic mechanical transmission failure has occurred. It is crucial to rectify the conditions that led to the storage of this code as quickly as possible.
What are some of the symptoms of the code?
Symptoms of a P0894 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P0894 troubleshooting steps?
When diagnosing automatic transmission codes, it is important to start by ensuring that the transmission fluid is clean and at the appropriate level. If the fluid level is low, locate the source of the leak and repair it. Refill the transmission with the recommended fluid and proceed.
If the fluid is excessively dark and has a strong burnt friction material odor, it needs to be replaced. This can be done by removing the transmission pan (and draining the torque converter) or by flushing the transmission (recommended). In either case, the filter should also be replaced. When removing the pan, be careful to preserve any debris that may have accumulated. If the pan is relatively clear of debris, it can be assumed that the clutches are mechanically intact. However, if there is a significant amount of friction material in the pan (and stuck to the magnet), the transmission will require a rebuild by a qualified technician.
To diagnose a code P0894, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a source of vehicle-specific diagnostic information. An oscilloscope may also be helpful in diagnosing input and output speed sensor malfunctions.
You can consult your vehicle information source to find a technical service bulletin (TSB) that matches the year, make, and model of your vehicle, as well as the stored codes and exhibited symptoms. If you find a relevant TSB, it may provide helpful diagnostic information.
Once the transmission is filled to the appropriate level with the recommended fluid, you can proceed with the first step of the diagnosis.
Step 1
Use the scanner (connected to the vehicle diagnostic connector) to retrieve all stored codes and relevant freeze frame data. It is advisable to write down this information before clearing the codes and then test-drive the vehicle until the PCM either enters readiness mode or the code is reset.
If the PCM enters readiness mode at this time, the code is intermittent and may be more challenging to diagnose. In such cases, the conditions that led to the storage of the code may need to worsen before an accurate diagnosis can be made.
Step 2
If the code is immediately reset, the next step in your diagnosis will involve searching your vehicle information source for diagnostic flowcharts, connector pin-out charts, connector face views, and component testing procedures/specifications.
Step 3
Use the DVOM to test the voltage, ground, and signal at the transmission input and output speed sensors (with the key on and the engine off). The input and output speed sensor circuits typically consist of a reference voltage, a signal wire, and a ground.
Step 4
Use the oscilloscope (connected to the signal circuit and ground) to check for spikes and glitches in the speed sensor signal pattern.
