Diagnostic Instructions
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
| --
|
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 high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other
than OFF. The high speed GMLAN serial data circuits must be operational for the vehicle to start due to body control module (BCM) and engine/powertrain control module (ECM/PCM) communications. The vehicle theft deterrent (VTD) module and ECM/PCM
exchange information using the BCM as the gateway module allowing communication between the high and low speed serial data busses. The low speed GMLAN serial data circuit must also be operational for vehicle starting. 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. The DLC
terminating resistor is not internal to a module.
Diagnostic Aids
| • | 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 will 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 |
Reference Information
Schematic Reference
Connector End View Reference
Master Electrical Component List
Electrical Information Reference
Circuit/System Testing
- Ignition OFF, disconnect the harness connector of the BCM.
- Ignition ON, test for battery voltage between each voltage input circuit of the BCM and ground.
| ⇒ | If less than battery voltage, test each voltage output circuit of the BCM for a short to ground, and each voltage input circuit of the BCM for a short to ground or and open/high resistance. If the circuits test normal, replace the faulty
module. |
- Test for less than 1 ohm of resistance between each ground circuit of the BCM and ground.
| ⇒ | If greater than 1 ohm, repair the ground circuit for an open/high resistance. |
- Disconnect the vehicle communication interface module (VCIM) connector if equipped with UE1, or terminator resistor connector if not equipped with UE1.
- Test for less than 1.0 volt between the high speed GMLAN serial data circuits of the DLC and ground.
| ⇒ | If greater than 1.0 volt, repair the serial data circuit for a short to voltage. |
- Test for infinite resistance between the high speed GMLAN serial data circuit of the DLC and ground.
| ⇒ | If less than infinite resistance, repair the serial data circuit for a short to ground. |
- Test for infinite resistance between the high speed GMLAN serial data (+) and (-) circuits of the DLC.
| ⇒ | If less than infinite resistance, repair the serial data circuits for a short between them. |
- Reconnect the BCM harness connector.
- Attempt to communicate with the BCM. Communication should not be available.
| ⇒ | If equipped with UE1, test between the high speed GMLAN serial data (+) and (-) circuits of the terminator resistor for 120 ohm resistance . If the circuit tests normal, replace the VCIM. |
| ⇒ | If not equipped with UE1, replace the terminator resistor. |
- Beginning with the farthest module from the DLC that is connected to high speed GMLAN serial data circuits, disconnect each module one at a time, attempting to communicate with the BCM after each disconnect. Communications should not be available.
| ⇒ | If furthest module was the last module disconnected, replace the faulty module. |
| ⇒ | If more than one module was disconnected, test the high speed GMLAN serial data (+) and (-) circuits between the last 2 disconnected modules for a short to voltage, short to ground or a short between them. If the circuits test normal,
replace the last disconnected module. |
- Test the high speed GMLAN serial data (+) and (-) circuits between the BCM and the last disconnected module for a short to voltage, short to ground or a short between them. If the circuits test normal, replace the BCM.
Repair Instructions
Perform the
Diagnostic Repair Verification
after completing the repair.
