Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications on the high speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition a DTC U0001 or U2100 will be set. These DTCs can be retrieved as history only.
Diagnostic Aids
The high speed GMLAN serial data buss uses two 120 ohms terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the data link connector (DLC) end of the link and the other is at the engine management module, engine control module (ECM) or powertrain control module (PCM), end of the link. The DLC terminating resistor resides internally in the BCM. When testing for a short between high speed GMLAN (+) and (-) with the engine management module removed, a reading off 120 ohms is normal. However, to completely analyze the buss the body control module (BCM) should be temporally removed when testing for shorts between the high speed GMLAN (+) and (-) while also making sure that all modules and the scan tool are disconnected from the buss. A normal reading across the high speed GMLAN buss with the terminating resistor at the DLC remove and all module and scan tool disconnected is infinite ohms. Make certain to reconnect the BCM after testing, it is necessary for proper vehicle powermoding.
The engine will not start when there is a total malfunction of the high speed GMLAN serial data circuits while the engine is not running. 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 |
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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A partial malfunction in the high speed GMLAN serial data circuits uses a different procedure from a total malfunction of the high speed GMLAN data circuits. Refer to Data Link References to identify which modules communicate on the high 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 terminals 6 and 14 provide the connection to the GMLAN serial data high 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 | |||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||||
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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 GMLAN serial data circuits? | Go to Step 3 | Go to Step 6 | |||||||||
Did you record any DTCs in the range of U2105 to U2177? | Go to DTC U2105-U2199 | Go to Step 4 | |||||||||
4 | Did you record DTC U0001? | Go to DTC U0001 | Go to Step 5 | ||||||||
5 |
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 high speed GMLAN serial data circuits for:
Refer to the following: Did you find and correct the condition? | Go to Step 19 | Go to Diagnostic Aids | ||||||||
6 | Test the following circuits of the body control module (BCM) for an open or short to ground:
Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 16 | Go to Step 7 | ||||||||
7 | Test the ground circuits of the BCM for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 16 | Go to Step 8 | ||||||||
Did you find and correct the condition? | Go to Step 9 | ||||||||||
9 |
Does the scan tool communicate with the BCM? | Go to Step 14 | Go to Step 10 | ||||||||
10 |
Does the scan tool communicate with the BCM? | Go to Step 11 | Go to Step 12 | ||||||||
11 |
Important: There are two 120 ohms terminating resistors that connect between the (+) and (-) high speed GMLAN buss. Refer to Diagnostic Aids. Test the high speed GMLAN (+) and (-) serial data circuits between the last 2 disconnected modules for:
Did you find and correct the condition? | Go to Step 16 | Go to Step 14 | ||||||||
12 | Is the BCM the last module still connected to high speed GMLAN data circuits? | Go to Step 13 | Go to Step 10 | ||||||||
13 |
Did you find and correct the condition? | Go to Step 16 | Go to Step 14 | ||||||||
14 | Inspect for poor connections at the high speed GMLAN serial data circuits of the last disconnected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 16 | Go to Step 15 | ||||||||
15 |
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 16 | -- | ||||||||
16 |
The scan tool may require a power up reset before communication will occur due to a short on the high 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 Step 17 | |||||||||
Did you record any DTCs which do not begin with a "U"? | Go to Step 18 | ||||||||||
18 | Did you diagnose all of the DTCs? | Go to Step 19 | Go to Step 16 | ||||||||
19 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | ||||||||
