Circuit Description
Serial data is the method of communication by components or modules to each other. The serial data communication bus consists of four components or modules: the powertrain control module (PCM), diagnostic energy reserve module (DERM), electronic brake control module (EBCM), and the electronic instrument panel cluster (IPC). The IPC is the master of this network and sends out a message through serial data (CKT 800) to the other modules. Each module in return responds back to the IPC with updated information used by the IPC, if any of the modules do not respond to the IPC; a DTC is set in each applicable module.
Conditions for Setting the DTC
| • | The PCM does not receive 25 valid responses from the IPC. |
| • | System voltage is greater than 6.8 volts. |
| • | Above conditions are met for greater than 5 seconds. |
Action Taken when the DTC Sets
| • | MIL does not illuminate |
| • | IPC illuminates the telltales the PCM controls through the serial data line: low coolant, low oil level, check oil, and volts. |
Conditions for Clearing the DTC
| • | A history PCM DTC will clear after 40 consecutive ignition cycles have occurred without a fault. |
| • | All DTC(s) can be cleared by using a scan tool. |
Diagnostic Aids
Move the PCM connectors while watching the IPC to see if serial data is lost. When serial data is lost, the IPC telltales are illuminated. This is a good indication the serial data is lost to the IPC.
An intermittent problem may be caused by a poor connection, rubbed through wire insulation, or a wire that is broken inside the insulation.
Any circuitry that is suspected as causing the intermittent complaint, should be thoroughly check for backed out terminals, improper formed or damaged terminals, poor terminal to wiring connections or physical damage to the wiring harness.
Refer to Section 8A-50 for loss of serial data diagnosis.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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When serial data is lost, the IPC telltales will illuminate. This is a good indication when serial data is lost to the IPC.
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If the scan tool cannot read serial data from the IPC, be sure that the scan tool is working properly on another vehicle.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Schematic Reference: Engine Controls Schematics | ||||
1 | Was the On-Board Diagnostic (OBD) System Check perform? | -- | Go to Step 2 | |
Are all the telltales ON and the gauges not operating properly? | -- | Go to Step 3 | Go to Diagnostic Aids | |
Can scan tool communicate with IPC? | -- | Go to Step 4 | Go to Step 8 | |
4 | Read IPC DTCs from scan tool. Is DTC 21 (PCM communication loss from IPC) stored? | -- | Go to Step 5 | Go to Diagnostic Aids |
5 |
Is resistance less than specified value? | 0.5 ohms | Go to Step 6 | Go to Step 7 |
6 | Check PCM connection and make sure it is clean and tight. If OK, replace PCM. Is action complete? | -- | -- | |
7 | Repair open in UART serial data line. Is action complete? | -- | -- | |
8 |
Can the scan tool communicate with the IPC? | -- | Go to Step 9 | Go to Section 8A-50 |
9 |
Was a condition necessary to require a repair? | -- | Go to Step 10 | |
10 | Replace the PCM. Is action complete? | -- | -- | |
