For 1990-2009 cars

### Circuit Description

The provision for communicating with the powertrain control module (PCM) is the data link connector (DLC). The DLC is located under the instrument panel (IP). The DLC is used to link a scan tool to the PCM serial data stream. Some common uses of the scan tool are listed below:

• | Identifying the stored diagnostic trouble codes (DTCs)

• | Reading the serial data stream

• | Clearing the DTCs

• | Storing the Freeze Frame data

### Diagnostic Aids

Some scan tools may require an external power supply. Ensure the scan tool is correctly connected to the power feed.

Ensure that the correct application (model year, carline, VIN code) has been selected on the scan tool. If communications are still not established, try the scan tool on another vehicle in order to ensure that the scan tool or the connecting cables are not the cause of the malfunction. Always try the scan tool on a vehicle that uses the same DLC terminal for serial data communication.

An intermittent malfunction may be caused by a fault in the Heated Oxygen Sensor (HO2S) circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions in Engine Controls.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step checks whether the scan tool can communicate with other control modules on the vehicle. If the scan tool can communicate with one of the other modules then the DLC connector, the shared circuits, and the scan tool are OK.

2. The modules supply a low current (about 1.85 mA) voltage on the serial data circuit. The normal circuit low current will NOT illuminate a test lamp. If the circuit is shorted to battery positive the higher current will illuminate the test lamp.

3. This step checks whether the PCM is receiving ignition positive voltage from the EFI relay when the ignition is ON. The PCM cannot provide serial data without ignition positive voltage from the EFI relay.

4. If the engine starts and runs, the EFI relay is OK.

5. The PCM cannot provide serial data without the correct power and grounds.

6. The scan tool may be damaged by high current flow on the serial data circuit at DLC terminal 7. Inspect the scan tool for proper operation on a known good vehicle that uses terminal 7 of the DLC for serial data communication.

7. A shorted sensor reference voltage circuit can cause a loss of scan tool communication with the PCM.

Step | Action | Value | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Data Link Connector Schematics Engine Controls Schematics
1 | Did you perform the Diagnostic System Check - Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls in Engine Controls
2 |

1. Turn ON the ignition, leaving the engine OFF.
2. Attempt to establish communication with the PCM.

Does the scan tool display the PCM data?

--

System OK

Go to Step 3

3 | Attempt to establish communication with the Inflatable Restraint Sensing and Diagnostic Module (SDM). Does the scan tool display communicate with the SDM? | -- | Go to Step 6 | Go to Step 4
4 |

1. Disconnect the scan tool.
2. Measure the voltage from the DLC connector terminal 7 to ground, using a DMM.

Is the voltage more than or equal to the specified value?

11 V

Go to Step 5

Go to Step 10

5 | Connect a test lamp between the DLC connector terminal 7 and ground. Does the test lamp illuminate? | -- | Go to Step 13 | Go to Step 14
6 |

1. Turn OFF the ignition.
2. Disconnect the PCM electrical connector containing the serial data circuit.
3. Turn ON the ignition, leaving the engine OFF.
4. Measure the voltage of the serial data circuit, on the harness side, with a DMM.

Is the voltage more than or equal to the specified value?

11 V

Go to Step 7

Go to Step 12

7 |

1. Turn OFF the ignition.
2. Connect the PCM electrical connector.
3. Turn ON the ignition, leaving the engine OFF.
4. Check for system voltage to the PCM from the EFI relay.

Is the voltage more than or equal to the specified value?

11 V

Go to Step 15

Go to Step 8

8 | Start the engine. Does the engine start and run? | -- | Go to Step 9 | Go to Ignition Relay Diagnosis in Engine Controls
9 |

1. Test for an open in the following circuits:

• | The ignition positive voltage circuit between the PCM and the EFI relay.

• | The ground circuits between the PCM and G106.

2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 2

--

10 |

1. Test for an open in the serial data circuit between the DLC connector terminal 7 and the splice pack SP255.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 2

Go to Step 11

11 |

1. Locate the short to ground in the serial data circuit.
2. Disconnect each circuit at the splice pack one at a time in order to determine which module or serial data circuit is shorted.
3. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 2

--

12 | Repair the open in the serial data circuit between the PCM and the splice pack SP255. Refer to Wiring Repairs in Wiring Systems. Is the repair complete? | -- | Go to Step 2 | --
13 | Repair the short to battery positive voltage in the serial data circuit. Refer to Wiring Repairs in Wiring Systems. Scan tool damage may have occurred. Is the repair complete? | -- | Go to Step 2 | --
14 |

1. Verify that all of the following scan tool selections are correct:

• | Model year

• | Carline or Truckline

• | Engine option (VIN)

• | Transmission option

2. If all the selections are correct and the scan tool does not display any engine data, replace the defective scan tool.

Is the a repair complete?

--

Go to Step 2

--

15 |

1. Test the fuel tank pressure (FTP) sensor and the throttle position (TP) sensor reference voltage circuits for a short to ground. Verify that the sensors are not internally shorted.
2. Repair any faults in the circuits as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 2

Go to Step 16

16 | Inspect for poor connections at the harness connector of the PCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 2 | Go to Step 17
17 | Replace the PCM. Refer to Powertrain Control Module Replacement in Engine Controls. Is the replacement complete? | -- | Go to Step 2 | --