Modules connected to the low speed GMLAN serial data circuits monitor for serial data communications on the low speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition, DTC U2100 will be set. These DTCs can be retrieved as history only.
Diagnostic Aids
The engine will not start when there is a total malfunction of the low speed GMLAN serial data circuits while the engine is not running. The following conditions may cause a total loss of low speed GMLAN data communication:
| • | Any of the low speed GMLAN serial data circuits shorted to ground or voltage |
| • | A module internal malfunction that causes a short to voltage or ground on the low speed GMLAN circuits |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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A partial malfunction in the low speed GMLAN serial data circuits uses a different procedure from a total malfunction of the low speed GMLAN data circuits. Refer to Data Link References to identify which modules communicate on the low speed GMLAN serial data circuits.
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The DTC U2100 may be retrieved with a history status, but is not the cause of the present condition.
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Data link connector terminal 1 provides the connection to the GMLAN serial data low circuit and the GMLAN serial data low circuit respectively.
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The communication malfunction may have prevented diagnosis of the customer complaint.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Data Link Connector Schematics Connector End View Reference: Master Electrical Component List in Wiring Systems | |||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | ||||||||
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Important:: Make sure the CANdi module is not malfunctioning. When functioning properly, the CANdi module LED will be flashing. In the event of a problem, the LED will be continually illuminated.
Does the scan tool communicate with any module on the low speed GMLAN serial data circuit? | Go to Step 3 | Go to Step 5 | |||||||||
Did you record any DTCs in the range of U2105 to U2177? | Go to DTC U2105-U2199 | Go to Step 4 | |||||||||
4 |
Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions. Test the low speed GMLAN serial data circuits for:
Refer to the following procedures in Wiring Systems: Did you find and correct the condition? | Go to Step 18 | Go to Diagnostic Aids | ||||||||
5 | Test the following circuits of the body control module (BCM) for an open or short to ground:
Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 6 | ||||||||
6 | Test the ground circuits of the BCM for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 15 | Go to Step 7 | ||||||||
Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information | Go to Step 8 | |||||||||
8 |
Does the scan tool communicate with the BCM? | Go to Step 13 | Go to Step 9 | ||||||||
9 |
Does the scan tool communicate with the BCM? | Go to Step 10 | Go to Step 11 | ||||||||
10 | Test the low speed GMLAN serial data circuit between the last two disconnected modules for:
Did you find and correct the condition? | Go to Step 15 | Go to Step 13 | ||||||||
11 | Is the BCM the last module still connected to low speed GMLAN data circuit? | Go to Step 12 | Go to Step 9 | ||||||||
12 |
Did you find and correct the condition? | Go to Step 15 | Go to Step 13 | ||||||||
13 | Inspect for poor connections at the low speed GMLAN serial data circuit of the last disconnected module. 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 15 | Go to Step 14 | ||||||||
14 |
Important: Perform the module setup procedure if required. Replace the last disconnected module. Refer to Control Module References for the appropriate repair instructions. Did you complete the replacement? | Go to Step 15 | -- | ||||||||
15 |
The scan tool may require a power up reset before communication will occur due to a short on the low speed GMLAN serial data circuits. Turn OFF or disconnect the scan tool prior to performing this test. Did you record any DTCs which begin with a "U" and with a current status? | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 16 | ||||||||
Did you record any DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 17 | |||||||||
17 | Did you diagnose all of the DTCs? | Go to Step 18 | Go to Step 15 | ||||||||
18 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | ||||||||
