For 1990-2009 cars only
Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
High Speed GMLAN Serial Data Bus (+) | 1 | 2 | 1 | -- |
High Speed GMLAN Serial Data Bus (-) | 1 | 2 | 1 | -- |
Ground (DLC pin 5) | -- | 1 | -- | -- |
1. No communication on any high speed GMLAN module 2. An open in only one high speed GMLAN serial data circuit will allow degraded communication between the modules | ||||
Circuit/System Description
Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. The high speed GMLAN serial data buss uses two 120 ohm terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected near the data link connector (DLC) end of the link and the other is internal to the ECM/PCM, end of the link. When a module detects a bus-off condition a DTC U2100 will be set. This DTC can be retrieved as history only.
Diagnostic Aids
| • | Use the Data Link References to identify the high speed GMLAN serial data modules. |
| • | When testing for a short between the high speed GMLAN (+) and (-) circuits (ECM/PCM disconnected), a resistance reading of 120 ohms is normal. To test for shorts to voltage or ground in the high speed GMLAN (+) and (-) circuits ensure that all modules, resistors and the scan tool are disconnected from the buss. A normal resistance reading across the high speed GMLAN buss with the resistors, all module and scan tool disconnected is infinite ohms. |
| • | This test is used for a total high speed GMLAN communication failure. If only 1 module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 for diagnostics. |
| • | Use the DMM MIN/MAX function to capture/locate intermittent conditions. |
| • | The engine may not start when there is a total malfunction of the high speed GMLAN serial data bus. |
| The following conditions may cause a total loss of high speed GMLAN data communication: |
| - | A short between high speed GMLAN (+) and high speed GMLAN (-) circuits |
| - | Any of the high speed GMLAN serial data circuits shorted to ground or voltage |
| - | A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits |
| - | An inoperative BCM |
Reference Information
Schematic Reference
Connector End View Reference
Master Electrical Component List
Description and Operation Reference
Data Link Communications Description and Operation
Electrical Information Reference
Circuit/System Testing
- Test for less than 1.0 ohm of resistance between the DLC ground circuit terminal 5 and ground.
- Ignition OFF, disconnect the harness connector C 2 at the engine control module (ECM).
- Test for less than 1 ohm of resistance between each ground circuit of the ECM and ground.
- Ignition ON, test for battery voltage between each voltage input circuit of the ECM and ground.
- Test for infinite resistance between DLC serial data circuit terminal 14 and ground.
- Test for 0 volts between DLC serial data circuit terminal 14 and ground.
- Ignition OFF. Test for less than 1 ohm of resistance between the DLC serial data circuit terminal 14 and the ECM serial data circuit terminal 55.
- Ignition OFF. Test for less than 1 ohm of resistance between the DLC serial data circuit terminal 6 and the ECM serial data circuit terminal 23.
- Ignition OFF, disconnect the harness connector at the TCM.
- Test for 115-125 ohms between the DLC serial data circuit terminal 6 and terminal 14.
- Connect the harness connector at the TCM.
- Test for 115-125 ohms between the DLC serial data circuit terminal 6 and terminal 14.
- If the circuits test normal, replace the ECM.
| ⇒ | If greater than 1.0 ohm, test the ground circuit for open/high resistance. |
| ⇒ | If greater than 1 ohm, repair the ground circuit for an open/high resistance. |
| ⇒ | If less than battery voltage, test each voltage input circuit of the ECM for a short to ground or open/high resistance, and each voltage output circuit of the ECM for a short to ground. If the circuits test normal, replace the ECM. |
| ⇒ | If less than the specified range, test both serial data circuits for a short to ground. If the circuits test normal, replace the transmission control module (TCM). |
| ⇒ | If greater than the specified range, test both serial data circuits for a short to voltage. If the circuits test normal, replace the TCM. |
| ⇒ | If greater than specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM. |
| ⇒ | If greater than specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM. |
| ⇒ | If greater than specified range, replace the terminator resistor. |
| ⇒ | If less than the specified range, test for a short between the + and - serial data circuits. If the circuits test normal, replace the terminator resistor. |
| ⇒ | If less than the specified range, test for a short between the + and - serial data circuits between the TCM and ECM. If the circuits test normal, replace the TCM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the repair.
| • | Control Module References for module replacement, setup, and programming |
