For 1990-2009 cars

### Circuit Description

The communication among modules is performed through the low speed and high speed GMLAN serial data circuits. The modules that need real time communication are attached to the high speed GMLAN network. The instrument panel module (IPM) is the gateway between the networks. The purpose of the gateway is to transfer information from one network to another. The gateway will interact with each network according to that network's transmission model. Refer to Body Control System Description and Operation for more information about the gateway.

GMLAN network management supports selective start up and is based on virtual networks. A virtual network is a collection of signals started in response to a vehicle event. The starting of a virtual network signifies that a particular aspect of the vehicles's functionality has been requested. A virtual network is supported by virtual devices which represents a collection of signals owned by a single physical device. So, any physical device can have one or more virtual devices. The signal supervision is the process of determining whether an expected signal is being received or not. Failsofting is the ability to substitute a signal with a default value or a default algorithm, in the absence of a valid signal. Some messages are also interpreted as a "heartbeat" of a virtual device. If such a signal is lost, the application will set a no communication code against the respective virtual device. This code is mapped on the Tech 2 screen as a code against the physical device. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.

### High Speed GMLAN Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit. The serial data is transmitted on 2 twisted wires that allow speed up to 500 Kbps. The twisted pair is terminated with two 120 ohms resistors, one is internal to the engine control module (ECM) and the other is internal to the instrument panel module (IPM). The resistors are used to reduce noise on the high speed GMLAN bus during normal vehicle operation. The high speed GMLAN is a differential bus. The high speed GMLAN serial data bus (+) and high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level. The idle level which is approximately 2.5 volts is considered a recessive transmitted data and is interpreted as a logic 1. Driving the lines to their extremes adds 1 volt to high speed GMLAN serial data bus (+) and subtracts 1 volt from high speed GMLAN serial data bus (-) wire. This dominant state is interpreted as a logic 0.

Two different outputs are used by IPM to activate the high speed GMLAN bus:

• | Accessory wake up, active in all power modes when IPM is awake except Crank and in a remote start request

• | High speed communication enable, active in all power modes when IPM is awake and in a remote start request

The accessory wake up signal wakes up the ECM and the transmission control module (TCM) only. This output is disabled in Crank power mode for internal fail enable of the ECM and TCM. The high speed communication enable signal wakes up all the other modules on the high speed GMLAN bus.

### Low Speed GMLAN Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for GMLAN low speed is up to 83.33 Kbps. This high speed is used for programming purposes only. Under normal vehicle operating conditions the speed of the bus is 33.33 Kbps. This protocol produces a simple pulse train sent out over the low speed GMLAN serial data bus. To wake up the control modules connected to the low speed GMLAN serial data bus, a wake-up pulse of 10 volts is sent out over the bus.

### Data Link Connector (DLC)

The data link connector (DLC) is a standardized 16-cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

• | Scan tool power battery positive voltage at terminal 16

• | Scan tool power ground at terminal 4

• | Common signal ground at terminal 5

• | Low speed GMLAN serial data bus at terminal 1

• | High speed GMLAN serial data bus (+) at terminal 6

• | High speed GMLAN serial data bus (-) at terminal 14

### High Speed GMLAN Serial Data Line

The GMLAN serial data communications circuit on this vehicle is in a linear topology. The following modules are connected to the link, in order from data link connector (DLC) to the end of the linear configuration:

• | Instrument Panel Module (IPM)

• | Electronic Suspension Control (ESC) Module, W/F85

• | Dynamic Headlamp Leveling (DHL) Device, w/TR7

• | Electronic Braking Control Module (EBCM)

• | Engine Control Module (ECM)

### Low Speed GMLAN Serial Data Line

The low speed GMLAN serial data communication circuit on this vehicle is in a star configuration. The star has 3 splice packs, located as follows:

• | The splice pack SP201 is located under the left side of the instrument panel (I/P), near the data link connector (DLC).

• | The splice pack SP300 is located next by the left B-pillar.

• | The splice pack SP303 is located next by the right B-pillar.

The following modules communicate on the low speed GMLAN serial data circuit:

• | The splice pack SP201 attaches to:

- | Instrument Panel Module (IPM)

- | Heater-Ventilation-Air Conditioning Control

- | Instrument Panel Cluster (IPC)

- | Radio

• | The splice pack SP300 attaches to:

- | Audio Amplifier

- | Driver Door Module (DDM)

- | The Driver Door Switch (DDS) is attached to DDM

- | Driver Position Module (DPM),W/A45

- | SIR System Sensing and Diagnostic Module (SDM)

- | Left Rear Door Module (LRDM)

- | Rear Integration Module (RIM)

- | Remote Control Door Lock Receiver (RCDLR)

- | The Television (TV) module, with YK6/YK7

• | The splice pack SP303 attaches to:

- | The inflatable restraint SDM

- | Passenger Door Module (PDM)

- | Right Rear Door Module (RRDM)

Refer to Data Communication Schematics.