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 U1814 00: Powertrain High Speed Communication Enable Circuit

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Accessory Wake Up Signal Serial Data | U1814 | U0100 | -- | --

### Circuit/System Description

The accessory wake up serial data circuit is a voltage wake up signal from the body control module (BCM) to the engine control module (ECM) and the transmission control module (TCM). The signal is activated in 2 ways; any time the ignition switch is in the ACC or RUN position, or anytime the BCM determines that communication is needed from the powertrain. When the ignition switch is in the ACC or RUN position, the accessory voltage circuit input to the BCM is internally connected to the accessory wake up serial data circuit output, and voltage is supplied to the modules. The BCM also supplies voltage to the accessory wake up serial data circuit when it determines that communication is necessary with the powertrain and the ignition is not in the ACC or RUN position such as during remote start.

### Conditions for Running the DTC

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

• | The vehicle power mode master requires serial data communication to occur.

### Conditions for Setting the DTC

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

### Action Taken When the DTC Sets

• | The modules use a default value for the missing parameters until the next ignition cycle.

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

• | The vehicle may not start.

### 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 may open the fuse feeding the ignition switch.

• | Short to ground may also set multiple 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

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

1. Ignition OFF, disconnect the X1 harness connector at the ECM and the TCM one at a time. After each disconnect, test for current DTCs. DTC U1814 should remain current.

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

2. Ignition OFF, disconnect the X4 harness connector at the BCM.
3. Test for infinite resistance between the signal circuit terminal 19 and ground.

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

4. If all circuits tests normal, replace the BCM.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Control Module References for module replacement, setup, and programming

• | GMLAN Wiring Repairs