Scan Tool Does Not Communicate with High Speed GMLAN Device
Scan Tool Does Not Communicate with High Speed GMLAN Device
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.
Diagnostic Fault Information
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 circuits must be operational for the vehicle to start due to body control module (BCM) and engine control module (ECM) communications. The vehicle theft deterrent (VTD) module and ECM exchange information using the BCM as the gateway module allowing communication between the high and low speed serial data busses. The low speed GMLAN serial data circuit must also be operational for vehicle starting. The high speed GMLAN serial data buss uses two 120 ohm terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is in the rear fuse block and the other is internal to the ECM.
Diagnostic Aids
* Communication may be available between the BCM and the scan tool with the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
* 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 one module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 for diagnostics.
* An open in the DLC ground circuit terminal 5 will allow the scan tool to operate to set up the vehicle on the tool and then not communicate with the vehicle. When the scan tool is to the point of communicating with the vehicle, a message on the scan tool will indicate "no CANdi module detected" and will not communicate.
* The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
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
Circuit/System Verification
Attempt to communicate with all modules on the high speed GMLAN bus.
If any high speed module communicates and any DTC beginning with a U exists, refer to Diagnostic Trouble Code (DTC) List - Vehicle. Diagnostic Trouble Code Descriptions
If any high speed module communicates and no DTC beginning with a U exists, refer to DTC U0100-U0299. U0100
Circuit/System Testing
High Speed GMLAN Initial Test
1. Test for less than 1.0 ohm between the DLC ground circuit terminal 5 and ground.
• If greater than the specified range, test the ground circuit for open/high resistance.
2. Ignition OFF, disconnect the harness connector (splice pack comb) at JX250.
3. Install 3 amp fused jumper wires between the following JX 250 terminals.
* Terminal K and terminal A
* Terminal C and terminal M
4. Attempt to communicate with the BCM.
5. If communication is established refer to DIAGNOSTIC 1.
6. If communication is not established, refer to DIAGNOSTIC 2.
Diagnostic 1
1. Ignition OFF, test for 108-132 ohms between terminals B and L of JX250.
• If greater than the specified range, test the serial data circuits between the ECM and JX250 for an open/high resistance. If the circuits test normal, replace the ECM.
• If less than the specified range, test the serial data circuits between the ECM and JX250 for a short together. If the circuits test normal, replace the ECM.
2. Verify a test lamp does not illuminate between B+ and the following JX250 terminals.
* A
* M
• If the test lamp illuminates, test the serial data circuit for a short to ground. If the circuits test normal, replace the ECM.
3. Ignition OFF, disconnect following harness connector of the ECM:
* RPO LU3 - X1
* RPO LMF/L42/L46U3 - X1
* RPO LMM - X1
4. Ignition ON, test for less than 1 volt between ground and JX 250 terminals as follows:
* A
* M
• If greater than the specified range, test the serial data circuit for a short to voltage.
Diagnostic 2
1. Ignition OFF, disconnect the X3 harness connector at the BCM.
2. Ignition OFF, test for less than 2 ohms between the DLC serial data circuit terminal 6 and BCM serial data circuit terminal 8.
• If greater than the specified range, test the serial data circuit for open/high resistance.
3. Ignition OFF, test for infinite resistance between the following serial data circuits of the DLC and ground.
* Terminal 6
* Terminal 14
• If less than the specified value, test the serial data circuits for a short to ground.
4. Ignition ON, test for less than 1 volt between the following serial data circuits of the DLC and ground.
* Terminal 6
* Terminal 14
• If greater than the specified range, test the serial data circuit for a short to voltage.
5. Ignition OFF, test for infinite resistance between the DLC serial data circuit terminal 6 and terminal 14.
• If less than the specified value, test for a short between the serial data circuits.
6. Ignition OFF, connect the harness connector at the BCM, ECM and JX250.
7. Disconnect the harness connector of the TCM.
8. Attempt to communicate with the BCM. Communication should not be available.
• If communication is established, Replace the TCM.
9. Disconnect the harness connector of the fuel system control module (FSCM).
10. Attempt to communicate with the BCM. Communication should not be available.
• If communication is established, test the serial data circuits for a short between the circuits, short to voltage, or a short to ground between the FSCM and the TCM including the jumper in the TCM harness to the data link resistor. If the circuits test normal, replace the FSCM.
11. Disconnect the harness connector of the EBCM.
12. Attempt to communicate with the BCM. Communication should not be available.
• If communication is established, test the serial data circuits for a short between the circuits, short to voltage, or a short to ground between the EBCM and the last module disconnected (including the jumper in the TCM harness to the data link resistor if the TCM was the last module). If the circuits test normal, replace the EBCM.
13. Disconnect the X3 harness connector of the BCM. Test the serial data circuits for a short between the circuits, short to voltage, or a short to ground between the BCM and the EBCM.
• If the circuits test normal, replace the BCM.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the repair. Verification Tests
* GMLAN Wiring Repairs