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 will be set. This 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 transmission control module (TCM) 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 following modules communicate on the high speed GMLAN serial data circuits:
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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.
| • | Engine control module (ECM) |
| • | Transmission control module (TCM) |
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Data Communication 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 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 7 | |||||||||
3 | Did you record any DTCs in the range of U0100 to U0299? | Go to DTC U0100-U0299 | Go to Step 4 | ||||||||
4 | Did you record DTC U0001, with a history status? | Go to Step 5 | Go to Step 6 | ||||||||
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 16 | Go to Diagnostic Aids | ||||||||
6 | Diagnose the non-communicating module by using the DTC U0100-U0299 procedure for the module which is not communicating. Refer to DTC U0100-U0299 . Did you complete the action? | -- | |||||||||
Did you find and correct the condition? | Go to Step 16 | Go to Step 8 | |||||||||
8 |
Does the scan tool communicate with the TCM? | Go to Step 11 | Go to Step 9 | ||||||||
9 |
Refer to the following procedures: Did you find and correct the condition? | Go to Step 13 | Go to Step 10 | ||||||||
10 | Test the high speed GMLAN serial data circuits between the TCM and DLC for the following conditions:
Refer to the following procedures: Did you find and correct the condition? | Go to Step 13 | Go to Step 11 | ||||||||
11 | 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 13 | Go to Step 12 | ||||||||
12 | Replace the last disconnected module. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 13 | -- | ||||||||
13 |
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 14 | |||||||||
Did you record any DTCs which do not begin with a U? | Go to Step 15 | ||||||||||
15 | Did you diagnose all of the DTCs? | Go to Step 16 | Go to Step 13 | ||||||||
16 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | ||||||||
