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 may allow degraded communication between the modules or no communication with a specific module.
|
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 bus 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 engine control module/powertrain control module ECM/PCM end of the link.
Diagnostic Aids
| • | Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these
can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This
condition would most likely be attributed to one of these scenarios: |
| - | A control module on the data communication circuit was disconnected while the communication circuit is awake. |
| - | Power to one or more modules was interrupted during diagnosis |
| - | A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. |
| - | Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time. |
| - | If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to
replace a part. |
| • | Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue. |
| • | Communication may be available between the BCM and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems. |
| • | An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed. |
| • | The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus. |
| • | Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool. |
| • | The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus. |
| | These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment
of one or more of the following concerns: |
| - | Vehicle will not crank. |
| - | Vehicle cranks but will not start. |
| - | Vehicle stability enhancement system warning lights and messages. |
| - | PRNDL gear indicator position errors. |
| - | Tire Pressure Monitor (TPM) system warning lights. |
Reference Information
Schematic Reference
Data Communication Schematics
Connector End View Reference
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Testing
- Ignition OFF, disconnect the harness connector of the transmission control module (TCM).
- Test for 108-132 ohms between the serial data circuit terminal 6 and terminal 7.
| ⇒ | If less than the specified range, test the serial data circuits for a short between them. If the circuits test normal, replace the 120 ohm resistor. |
| ⇒ | If greater than the specified range, test the serial data circuits for open/high resistance. If the circuits test normal, replace the 120 ohm resistor. |
- Ignition ON, test for less than 1.0 volt between the serial data circuit terminal 6 and ground, and between terminal 7 and ground.
| ⇒ | If greater than the specified range, test the serial data circuit for a short to voltage. |
- Test for infinite resistance between the serial data circuit terminal 6 and ground, and between terminal 7 and ground.
| ⇒ | If less than the specified value, test the serial data circuit for a short to ground. |
- Test for 108-132 ohms between the serial data circuit terminal 37 and terminal 38.
| ⇒ | If less than the specified range, test the serial data circuits for a short between them. If the circuits test normal, replace the ECM. |
| ⇒ | If greater than the specified range, test the serial data circuits for open/high resistance. If the circuits test normal, replace the ECM. |
- Ignition ON, test for less than 1.0 volt between the serial data circuit terminal 37 and ground, and between terminal 38 and ground.
| ⇒ | If greater than the specified range, test the serial data circuit for a short to voltage. If the circuits test normal, replace the ECM. |
- Test for infinite resistance between the serial data circuit terminal 37 and ground, and between terminal 38 and ground.
| ⇒ | If less than the specified value, test the serial data circuit for a short to ground. If the circuits test normal, replace the ECM. |
- If the circuits test normal, replace the TCM.
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the repair.
