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 bus 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 instrument panel module (IPM) and the other is at the engine control module (ECM). When testing for a short between high speed GMLAN (+) and (-) a reading of 60 ohms is normal. If the high speed GMLAN serial data is open testing the resistance between high speed GMLAN (+) and (-) will read about 120 ohms.
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 |
| • | Any of the high speed GMLAN serial data circuits open |
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. The scan tool communicates with the following modules on the high speed GMLAN serial data circuits:
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Data link connector (DLC) 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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If only one of the high speed GMLAN circuits is open all communication on the high speed bus will be disrupted.
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The communication malfunction may have prevented diagnosis of the customer complaint.
| • | Distance sensing cruise control (DSCC) module |
| • | Instrument panel module (IPM) |
| • | Electronic brake control module (EBCM) |
| • | Electronic suspension control (ESC) module |
| • | Engine control module (ECM) |
| • | Transmission control module (TCM) |
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Data Communication Schematics Connector End View Reference: Data Communication Connector End Views | |||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||||
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Important:: Make sure the CANdi module works fine. When functioning properly, CANdi's LED flashes. In the event of a problem, the LED will be constantly illuminated.
Does the scan tool communicate with any module on the GMLAN serial data circuits? | Go to Step 3 | Go to Step 6 | |||||||||
3 | Did you record any DTCs in the range of U2105-U2199? | Go to Step 4 | |||||||||
4 | Did you record any DTCs in the range of U0100-U0299? | 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.
Refer to the following procedures: Did you find and correct the condition? | Go to Step 21 | Go to Diagnostic Aids | ||||||||
6 |
Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 7 | ||||||||
7 | Test the ground circuits of the IPM for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 8 | ||||||||
Inspect for poor connections at the DLC terminals 6 and 14. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 9 | |||||||||
9 | Test the high speed GMLAN serial data circuits between the IPM and DLC for:
Refer to the following procedures: Did you find and correct the condition? | Go to Step 18 | Go to Step 10 | ||||||||
10 |
Does the scan tool communicate with the IPM? | Go to Step 15 | Go to Step 11 | ||||||||
11 |
Does the scan tool communicate with the IPM? | Go to Step 12 | Go to Step 13 | ||||||||
12 | Test the high speed GMLAN serial data circuits between the last 2 disconnected modules for:
Refer to the following procedures: Did you find and correct the condition? | Go to Step 18 | Go to Step 15 | ||||||||
13 | Is the IPM the last module still connected to high speed GMLAN data circuits? | Go to Step 14 | Go to Step 11 | ||||||||
14 |
Refer to the following procedures: Did you find and correct the condition? | Go to Step 18 | Go to Step 16 | ||||||||
Test each of the high speed GMLAN circuits of the last disconnected module to the next module closer to DLC for an open. Refer to the following procedures: Did you find and correct the condition? | Go to Step 18 | Go to Step 16 | |||||||||
16 | Inspect for poor connections at the high speed GMLAN serial data circuits of the suspected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 17 | ||||||||
17 | Replace the suspected module. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 18 | -- | ||||||||
18 |
Did you record any DTCs which begin with a "U" and with a current status? | Go to Step 19 | |||||||||
Did you record any DTCs which do not begin with a "U"? | Go to Step 20 | ||||||||||
20 | Did you diagnose all of the DTCs? | Go to Step 21 | Go to Step 18 | ||||||||
21 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | ||||||||
