For 1990-2009 cars

### 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.

### DTC Descriptor

DTC U2099 00: High Speed Communication Enable Circuit

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
High Speed GMLAN Serial Data Wake Up | U2099, U0100-U0299¹ | U0100-U0299 | U0100-U0299² | --
¹ Vehicle will not start. ² History only. If cleared the DTC will return after several ignition cycles.

### Circuit/System Description

The body control module (BCM) applies voltage to the data communication enable circuit when the ignition key is in ACC, ON, or START to enable several modules for bus communication.

Depending on option content, modules that may receive the signal are as follows:

• | Transfer case control module (TCCM)

• | Electronic brake control module (EBCM)

### Conditions for Running the DTC

• | The system voltage is between 9-16 volts.

• | The BCM attempts to enable the modules connected to the high speed GMLAN serial data wake up circuit.

### Conditions for Setting the DTC

The BCM senses a short to ground on the serial data wake up circuit.

### Action Taken When the DTC Sets

• | The output command is turned off while the malfunction is present.

• | The modules use a default value for the missing parameters.

• | The modules are never signaled, therefore the specific subsystems will not function.

• | The vehicle will not start while the circuit is shorted to ground.

### Conditions for Clearing the DTC

• | A current DTC clears when the malfunction is no longer present.

• | A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.

### Diagnostic Aids

Short to ground will also set multiple history no communication DTCs for GMLAN high speed modules.

### Reference Information

#### Schematic Reference

• | Body Control System Schematics

• | Data Communication Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Data Link Communications Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the following modules one at a time. After each disconnect, clear the DTC, cycle the ignition key 3 times, then test for the DTC. DTC U2099 should remain current.

• | EBCM

• | Transfer case control module

⇒ | If the DTC becomes history, replace the last module disconnected when the DTC became history.

2. Ignition OFF, disconnect the X3 harness connector at the BCM.
3. With the modules still disconnected, test for infinite resistance between BCM X3 terminal 19, and ground.

⇒ | If less than the specified value, test the circuit for a short to ground.

4. If all circuits test normal and the DTC is set as current, replace the BCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup, and programming