GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 Allure 🢒 Power and Signal Distribution 🢒 Data Communications 🢒 Scan Tool Does Not Communicate with High Speed GMLAN Device

Diagnostic Instructions
    •  Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  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

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

High Speed GMLAN Serial Data Bus (+)

1

2

1

--

High Speed GMLAN Serial Data Bus (-)

1

2

1

--

Ground (DLC pin 5)

--

1

--

--

1. No communication on any high speed GMLAN module

2. An open in only one high speed GMLAN serial data circuit will allow degraded communication between the modules

Circuit/System Description

Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. 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 connected near the data link connector (DLC) end of the link and the other is internal to the ECM/PCM, end of the link. When a module detects a bus-off condition a DTC U2100 will be set. This DTC can be retrieved as history only.

Diagnostic Aids
    • Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
       - A control module on the data communication circuit was disconnected while the communication circuit is awake.
       - Power to one or more modules was interrupted during diagnosis
       - A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
       - Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
       - If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
    • Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.
    • 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.
    • An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.
    • The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
    • Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
    • The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
        These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:
       - Vehicle will not crank.
       - Vehicle cranks but will not start.
       - Vehicle stability enhancement system warning lights and messages.
       - PRNDL gear indicator position errors.
       - Tire Pressure Monitor (TPM) system warning lights.

Reference Information

Schematic Reference

Data Communication Schematics

Connector End View Reference

Component Connector End Views

Description and Operation Reference

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing
  1. Test for less than 1.0 ohm between the DLC ground circuit terminal 5 and ground.
    2. ⇒If greater than the specified range, test the ground circuit for open/high resistance.
  2. Ignition OFF, disconnect the harness connector X3 at the engine control module (ECM).
  3. Test for less than 1 ohm between each ground circuit of the ECM and ground.
    4. ⇒If greater than the specified range, repair the ground circuit for an open/high resistance.
  4. Ignition ON, test for battery voltage between each voltage input circuit of the ECM and ground.
    5. ⇒If less than specified, test each voltage input circuit of the ECM for a short to ground or open/high resistance, and each voltage output circuit of the ECM for a short to ground. If the circuits test normal, replace the ECM.
  5. Test for infinite resistance between DLC serial data circuit terminal 14 and ground.
    6. ⇒If less than the specified range, test both serial data circuits for a short to ground. If the circuits test normal, replace the transmission control module (TCM).
  6. Test for 0 volts between DLC serial data circuit terminal 14 and ground.
    7. ⇒If greater than the specified value, test both serial data circuits for a short to voltage. If the circuits test normal, replace the TCM.
  7. Ignition OFF. Test for less than 1 ohm between the DLC serial data circuit terminal 14 and the ECM serial data circuit terminal 33.
    8. ⇒If greater than the specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM.
  8. Ignition OFF. Test for less than 1 ohm between the DLC serial data circuit terminal 6 and the ECM serial data circuit terminal 53.
    9. ⇒If greater than the specified range, test the serial data circuit for open/high resistance. If the circuits test normal, replace the TCM.
  9. Ignition OFF, disconnect the harness connector at the TCM.
  10. Test for 110-130 ohms between the DLC serial data circuit terminal 6 and terminal 14.
    11. ⇒If less than the specified range, test for a short between the + and - serial data circuits. If the circuits test normal, replace the terminator resistor.
    ⇒If greater than the specified range, replace the terminator resistor.
  11. Connect the harness connector at the TCM.
  12. Test for 115-125 ohms between the DLC serial data circuit terminal 6 and terminal 14.
    13. ⇒If less than the specified range, test for a short between the + and - serial data circuits between the TCM and ECM. If the circuits test normal, replace the TCM.
  13. If the circuits test normal, replace the ECM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

    •  GMLAN Wiring Repairs
    •  Control Module References for module replacement, setup, and programming