High Speed GMLAN Serial Data Line (Diesel only)

The data link connector (DLC) allows a scan tool to communicate with the high speed GMLAN serial data circuit through the CANdi module. The serial data is transmitted on two twisted wires that allow speed up to 500 Kbps. The twisted pair is terminated with two 120 ohm resistors, one is internal to the transmission control module (TCM) and the other is internal to the engine control module (ECM). 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. 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 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.

The engine control module (ECM) is functionally communicating on both class 2 and high speed GMLAN data links. However the diagnostic communication of the above modules with the scan tool is performed on class 2 data link only. All DTCs set by the high speed GMLAN modules, are reported by the ECM on class 2. Data displaying, output controls and programming of the control modules connected to high speed GMLAN only, is performed through high speed GMLAN link.

Several of the GMLAN serial data modules are in a linear configuration from SP250. The engine control module is separated from the linear configuration at SP250. If not equipped with an auxiliary heater control module (FOH) (w/K08, coolant heater) a jumper harness is in its place. Refer to Master Electrical Component List for SP250 location.

The following modules are connected to the link in a linear configuration:

• The auxiliary heater control module (FOH) (w/K08, coolant heater)
• The glow plug control module (GPCM)
• The transmission control module (TCM)