For 1990-2009 cars

### Circuit Description

Modules connected to the low speed GMLAN serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the data link connector (DLC) allows communication with the modules for diagnostic purposes.

### Test Description

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

1. A current DTC U2100 71 or U2100 00 may be retrieved only from instrument panel module (IPM).

2. A partial malfunction in the low speed GMLAN serial data circuit uses a different procedure from a total malfunction of the low speed GMLAN serial data circuit. The scan tool communicates with the following modules on the low speed GMLAN serial data circuit:

• | Audio amplifier

• | Digital radio receiver (DRR)

• | Driver door module (DDM)

• | Driver door switch (DDS)

• | Driver position module (DPM)

• | HVAC control module

• | SIR system sensing and diagnostic module (SDM)

• | Instrument panel cluster (IPC)

• | Left rear door module (LRDM)

• | Passenger door module (PDM)

• | Radio

• | Rear integration module (RIM)

• | Remote control door lock receiver (RCDLR)

• | Right rear door module (RRDM)

• | Television module

3. An open in the low speed GMLAN serial data circuit between the DLC and instrument panel module (IPM) will prevent the scan tool from communicating with any module on the low speed GMLAN bus. This condition will not set a DTC.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Important: Make sure the CANdi module works fine. CANdi's LED flashes when it is functioning properly. In the event of a problem, the LED will be illuminated continuously. Refer to the Test Description for the list of modules that communicate with scan tool on low speed GMLAN bus.

Did the scan tool communicate with any module on the low speed GMLAN serial data circuit?

Go to Step 5

Go to Step 3

3 | Does the scan tool communicate with IPM? | Go to Step 4 | Go to Scan Tool Does Not Communicate with High Speed GMLAN Device
4 | Is U2100 71 and/or U2100 00 retrieved as current? | Go to DTC U2100 | Go to Step 7
5 | Is any DTC set among U2100-2199? | Go to DTC U2105-U2199 | Go to Step 6
6 | Diagnose the non-communicating module by using the procedures for DTC U2100-2199. Refer to DTC U2105-U2199 . Is the operation completed? | Go to Diagnostic System Check - Vehicle | --
7 |

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 1 and 5. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 8

8 | Test the DLC ground circuit at terminal 5 for an open. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 9
9 |

1. Disconnect the instrument panel module (IPM).
2. Inspect for poor connections at the low speed GMLAN serial data circuits of the IPM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 10

10 | Repair the low speed GMLAN serial data circuit between the DLC and the IPM for an open. Refer to Circuit Testing and GMLAN Wiring Repairs . Is the operation completed? | System OK | --