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Scan Tool Does Not Communicate with High Speed GMLAN Device



SCAN TOOL DOES NOT COMMUNICATE WITH HIGH SPEED GMLAN DEVICE (DIESEL)

DIAGNOSTIC FAULT INFORMATION




Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview

CIRCUIT/SYSTEM DESCRIPTION
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 when the ignition switch is in any position other than OFF. 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 near the data link connector (DLC) end of the link and the other is internal to the ECM/PCM end of the link. The DLC terminating resistor is not internal to a module. Modules connected to the high speed GMLAN serial data circuits are the transmission control module (TCM), glow plug control module (GPCM), and the engine control module/powertrain control module (ECM/PCM).

DIAGNOSTIC AIDS
- Use the Data Link References to identify the high speed GMLAN serial data modules.
- This test is used for a total high speed GMLAN communication failure. If only 1 module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 (Diesel Engine) DTC U0100-U0299 (HP2) for diagnostics.
- Use the DMM MIN/MAX function to capture/locate intermittent conditions.
- 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
- An open in a high speed GMLAN serial data circuit between the DLC and the first splice. This open will cause no communication with the scan tool, and will not effect the module communication on the vehicle.

CIRCUIT/SYSTEM TESTING

Automatic Transmission
1. Ignition OFF, disconnect the harness connector of the GPCM.
2. Ignition ON, with a scan tool attempt to communicate with the ECM/PCM. Communication should not be available.
- If the ECM/PCM communicates, replace the GPCM.
3. Disconnect the ECM/PCM. Ignition ON, with a scan tool attempt to communicate with the TCM. Communication should not be available.
- If the TCM communicates, replace the ECM/PCM.
4. All 3 modules disconnected, test for less than 5 ohms of resistance between the DLC and the TCM on both high speed GMLAN serial data circuits.
- If greater than 5 ohms, repair the data circuit for open/high resistance between the DLC and the first splice.
5. All 3 modules disconnected, test for infinite resistance between all 4 of the high speed GMLAN serial data (+) and (-) circuits of the TCM.
- If less than infinite resistance, repair the short between the serial data circuits.
6. Ignition on and all 3 modules disconnected, test for less than 1.0 volt between the high speed GMLAN serial data circuits of the TCM and ground.
- If greater than 1.0 volt, repair the serial data circuit of the TCM for a short to voltage.
7. All 3 modules disconnected, test for infinite resistance between the high speed GMLAN serial data circuits of the TCM and ground.
- If less than infinite resistance, repair the serial data circuits for a short to ground.
8. If all circuits test normal, replace the TCM.

Manual Transmission
1. Ignition OFF, disconnect the harness connector of the GPCM.
2. Ignition ON, with a scan tool attempt to communicate with the ECM/PCM. Communication should not be available.
- If the ECM/PCM communicates, replace the GPCM.
3. Test for less than 5 Ohms of resistance between the DLC and the ECM/PCM on both high speed GMLAN serial data circuits.
- If greater than 5 Ohms, repair the data circuit for open/high resistance between the DLC and the first splice.
4. Both modules disconnected, test for infinite resistance between the high speed GMLAN serial data (+) and (-) circuits of the DLC.
- If less than infinite resistance, repair the short between the serial data circuits.
5. Ignition on and both modules disconnected, test for less than 1.0 volt between the high speed GMLAN serial data circuits of the DLC and ground.
- If greater than 1.0 volt, repair the serial data circuit for a short to voltage.
6. Both modules disconnected, test for infinite resistance between the high speed GMLAN serial data circuits of the DLC and ground.
- If less than infinite resistance, repair the serial data circuits for a short to ground.
7. If all circuits test normal, replace the ECM/PCM.


REPAIR INSTRUCTIONS
Perform the Diagnostic Repair Verification after completing the repair. Verification Tests
- Control Module References for module replacement, setup, and programming Programming and Relearning
- GMLAN Wiring Repairs
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