Circuit Description
The transmission control module (TCM) sends vehicle speed data to the engine control module (ECM). The data is sent to the ECM through a communication network called the GM local area network (LAN). Two circuits are used to communicate LAN data between the ECM and TCM. A fault in the LAN will not cause DTC P0864 to set by itself. If a LAN fault occurs, other DTCs sets before DTC P0864.
If the ECM receives invalid vehicle speed data from the TCM, DTC P0864 sets.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC P0864 TCM Communication Circuit Performance
Conditions for Running the DTC
| • | The ignition is on for more than 3 seconds. |
| • | The ignition 1 voltage is between 10.5-18 volts. |
| • | No other LAN errors are present. |
| • | DTC P0864 runs continuously once the above conditions are met for more than 2.5 seconds. |
Conditions for Setting the DTC
The ECM receives no valid vehicle speed data from the TCM for more than 4 seconds.
Action Taken When the DTC Sets
| • | The control module stores the DTC information into memory when the diagnostic runs and fails. |
| • | The malfunction indicator lamp (MIL) will not illuminate. |
| • | The control module records the operating conditions at the time the diagnostic fails. The control module stores this information in the Failure Records. |
| • | The driver information center, if equipped, may display a message. |
Conditions for Clearing the DTC
| • | A current DTC Last Test Failed clears when the diagnostic runs and passes. |
| • | A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic. |
| • | Clear the DTC with a scan tool. |
Step | Action | Yes | No |
|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |
2 |
Important: Record all Failure Records before clearing the DTC. Using Clear Info erases the Failure Records from the engine control module (ECM) and the transmission control module (TCM). Did you record any TCM Freeze Frame/Failure Records? | Go to Step 3 | |
3 |
Did DTC P0864 fail this ignition? | Go to Step 4 | Go to Intermittent Conditions |
4 | Did any other TCM DTCs set? | Go to Step 5 | |
5 | Replace the ECM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 6 | -- |
6 |
Did the DTC fail this ignition? | Go to Step 7 | Go to Step 2 |
7 | Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed? | System OK |
