|Trouble Code||Fault Location||Probable Cause|
|P0864||Transmission control module (TCM) communication circuit -range/performance problem||Wiring, poor connection, TCM|
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What Does Code P0864 Mean?
Modern vehicles have dozens of modules onboard. The transmission control module (TCM) is one such module. The TCM takes inputs from various sensors and uses that information to control transmission output devices. In a vehicle equipped with a communication area network, the TCM shares this information with other modules over a communication bus.
An example of a controller area network (CAN)
All modules on the bus have at least one other module they report their state of health to. If a module fails to send a state of health message, the companion module will set a DTC for the module that did not respond. Code P0864 stands for Transmission control module (TCM) communication circuit -range/performance problem. This code indicates the TCM did not respond when addressed by its companion module.
What are the common causes of code P0864 ?
To sum things up, the common causes for code P0864 are as follows:
- Problem with the TCM
- Problem in the TCM circuit
- Problem with the network bus
What are the symptoms of code P0864 ?
Code P0864 may be accompanied by several different symptoms. These include: a transmission that shifts poorly, a transmission that is stuck in “limp mode” and an illuminated check engine light.
How do you troubleshoot code P0864 ?
- Clear the code
Sometimes this code is set in error. The first thing to do is clear the code, drive the vehicle and see if it returns. If it does not, the problem is solved.
- Perform a visual inspection of the TCM and connector
Many problems can easily be found in the harness and connectors. So, begin your diagnosis by visually inspecting the TCM and its connection.
- Verify the TCM is inactive on the network
An OEM capable scan tool can be used to determine whether or not the TCM is the only module inactive on the network. To do this, connect a scan tool and perform a roll call of the modules. If the TCM is the only module not present, the problem is with the TCM or its circuit.
Terminating resistors in a CAN network
If multiple modules are offline, the problem is with the CAN bus. In this case, you’ll want to start by checking the network terminating resistors. Usually, two 120 ohm resistors are connected in parallel at each end. To do this, set your digital multimeter to the ohms position. Connect a lead to each side of the resistor. Since the resistors are connected in parallel, each should measure approximately 60 ohms.
Another test is to unplug modules from the network. Depending on the network design, a single failed module can wreak havoc on the rest of the system. Unplug the modules one at a time to see if system operation is restored while doing so.
- Check the TCM for power and ground
The next thing to do is confirm the TCM has proper power and ground. Start by consulting the factory repair information to determine the correct pins. The TCM will typically have many wires going to it, so it’s important to make sure you are testing the correct wire.
To check for power, disconnect the TCM connector and touch one end of a test light to the B+ pin. Connect the test light clamp to the battery negative post. The test light should illuminate. If it does not, there is problem in the power supply circuit and you will need to consult the factory wiring diagram.
Next, check that the TCM has proper ground. Disconnect the TCM connector and touch the test light to B- pin. Connect the test light clamp to the battery positive post. If there is proper ground, the test light should illuminate. If it does not, there is problem on the ground side of the circuit and you will need to consult the factory wiring diagram.
- Check the data line
An example of a CAN bus data waveform pattern
The final step is to check the TCM data communication lines. CAN systems often have a high speed and low speed side, and the TCM typically communicates on both. CAN high has a voltage range of 2.5 to 3.5 volts, whereas CAN low has a voltage range or 2.5 to 1.5 volts.
Start by consulting the factory repair information to determine which pins on the TCM connector are used for data communication. Using a back probe test lead, connect the leads of the scope to CAN high, CAN low and ground. You should see a healthy digital waveform on your instrument.
This test can also be performed with a digital multimeter, although a scope is best. Using a back probe test lead, connect the leads of the multimeter to CAN high and ground. You should see a variable voltage indicating messages are being sent and received. Perform the same operation on CAN low.
If you do not see a variable voltage when performing this test, there is problem with the data line wiring and you will need to consult the factory wiring diagram. A signal that is zero all the time indicates a short to ground. A voltage that is high all the time indicates a data line that is shorted to voltage.
If everything checks out to this point, there is a problem with the TCM. However, before replacing the TCM check technical service bulletins to see if there are any re-flashes available.