Circuit/System Testing: Notes

Each device may need to be disconnected to isolate a circuit fault.

Use the schematic to identify the following:

• Chassis high speed GMLAN devices the vehicle is equipped with
• Chassis high speed GMLAN serial data circuit terminating resistors
• Device locations on the chassis high speed GMLAN serial data circuits
• Each device's ground, B+, ignition and chassis high speed GMLAN serial data circuit terminals

Some devices with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground or open/high resistance prior to replacing the device for each of the following tests.