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. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module.
Diagnostic Aids
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 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 are connected to 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 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 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 5 | |||||||||
3 | Did you record DTC U0101 or U2105? | Go to DTC U0100-U0299 or 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.
Refer to the following in Wiring Systems: Did you find and correct the condition? | Go to Step 14 | Go to Diagnostic Aids | ||||||||
Did you find and correct the condition? | Go to Step 12 | Go to Step 6 | |||||||||
6 |
Does the scan tool communicate with the ECM? | Go to Step 7 | Go to Step 10 | ||||||||
7 |
Refer to the following in Wiring Systems: Did you find and correct the condition? | Go to Step 12 | Go to Step 8 | ||||||||
8 | Inspect for poor connections at the high speed GMLAN serial data circuits of the TCM. 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 12 | Go to Step 9 | ||||||||
9 | Replace the TCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 12 | -- | ||||||||
10 |
Refer to the following in Wiring Systems: Did you find and correct the condition? | Go to Step 12 | Go to Step 11 | ||||||||
11 | Replace the ECM. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | Go to Step 12 | -- | ||||||||
12 |
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 Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information | Go to Step 13 | ||||||||
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 14 | |||||||||
14 | Use the scan tool in order to clear the history DTCs. Did you complete the action? | System OK | -- | ||||||||
