LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Cadillac >> 2003 >> Escalade EXT >> Repair and Diagnosis >> External Pages >> Different car >> Section 617 (Data Link Communication System) >> Diagnostic Information and Procedures >> Scan Tool Does Not Communicate with High Speed GMLAN Device >> Test Description
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Test Description

WARNING: This page is about a different car, the 2004 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.

The numbers below refer to the step numbers on the diagnostic table.

  1. 2:  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:
    • Engine control module (ECM)
    • Transmission control module (TCM)
  2. 6:  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.
Scan Tool Does Not Communicate with High Speed GMLAN Device

Step Action Yes No
Schematic Reference: Data Link Connector (DLC) Schematics 
Connector End View Reference: Master Electrical Component List in Wiring Systems
1 Does the scan tool power up? Go to Step 2  Go to Scan Tool Does Not Power Up 
2 
IMPORTANT: Make sure the CANdi module works properly. When functioning properly, the CANdi's LED flashes. In the event of a problem, the LED will be constantly illuminated.
Turn ON the ignition, with the engine OFF.Does the scan tool communicate with any module on the GMLAN serial data circuits?		 Go to Step 3  Go to Step 6 
3
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.
Test the high speed GMLAN serial data circuits for:
  • An intermittent short to ground
  • An intermittent short to voltage
  • An intermittent short between high speed GMLAN (+) and high speed GMLAN (-) circuits
Refer to the following in Wiring Systems:Did you find and correct the condition?		 Go to Step 13  Go to Diagnostic Aids
4 Test the following circuits of the transmission control module (TCM) for an open or short to ground:
  • The battery positive voltage input circuits
  • The battery positive voltage output circuits
  • The ignition mode switch input circuits
  • The switched battery positive voltage circuits

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 12  Go to Step 5 
5 Test the ground circuits of the TCM for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?		 Go to Step 12  Go to Step 6 
6 
  1. Turn OFF the ignition.
  2. Disconnect the scan tool from the data link connector (DLC).
  3. Inspect for poor connections at the DLC terminals 6 and 14. 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 10  Go to Step 7 
7
  1. Disconnect the engine control module (ECM) from the high speed GMLAN serial data circuits. Refer to Data Link Connector (DLC) Schematics .
  2. Turn ON the ignition, with the engine OFF.
  3. Attempt to communicate with the TCM.
Does the scan tool communicate with the TCM?		 Go to Step 10  Go to Step 8 
8
  1. Turn OFF the ignition.
  2. Disconnect the high speed GMLAN serial data circuits from the TCM that are between the TCM and ECM. Refer to Data Link Connector (DLC) Schematics .
  3. Turn ON the ignition, with the engine OFF.
  4. Attempt to communicate with the TCM.
Does the scan tool communicate with the TCM?		 Go to Step 9  Go to Step 10 
9 Test the high speed GMLAN serial data circuits between the TCM and ECM for:
  • A short to ground
  • A short to battery
  • A short between high speed GMLAN (+) and high speed GMLAN (-)

Did you find and correct the condition?		 Go to Step 12  Go to Step 10 
10 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 in Wiring Systems.
Did you find and correct the condition?		 Go to Step 12  Go to Step 11 
11
IMPORTANT: Perform the module setup procedure if required.
Replace the last disconnected module. Refer to Control Module References for the appropriate repair instructions.Did you complete the replacement?		 Go to Step 12  -
12
  1. Reconnect all of the disconnected modules.
  2. Install a scan tool.
  3. Turn ON the ignition, with the engine OFF.
  4. Wait for 10 seconds.

    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.

  5. Select the Display DTCs function for each module.
  6. Record all of the displayed DTCs and the DTC status.
Did you record any DTCs which begin with a "U" and with a current status?		 Go to Diagnostic Trouble Code (DTC) List  Go to Step 13 
13 Use the scan tool in order to clear the DTCs.
Did you complete the action?		 System OK -
scientia non olet · About LEMON Manuals