Circuit/System Testing: Notes

NOTE:

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. Each device may need to be disconnected to isolate a circuit fault. Use the schematics and connector end views to identify the following:

Powertrain high speed GM LAN devices and terminating resistors the vehicle is equipped with
Device locations on the powertrain high speed GM LAN serial data circuits
Each device's ground, B+, ignition, and powertrain high speed GM LAN serial data circuit terminals

Ignition/Vehicle OFF, disconnect the harness connectors with the powertrain high speed GM LAN serial data circuits at an easily accessible device that is not communicating. Refer to Data Link References to determine which devices are on the powertrain expansion bus.
Ignition ON/Vehicle In Service Mode.
Test for less than 4.5 V between each powertrain high speed GM LAN serial data circuit at the device that was just disconnected and ground.
- If 4.5 V or greater
  Refer to Testing the Serial Data Circuits for a Short to Voltage .
- Go to next step: If less than 4.5 V
Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. It may take up to 10 min for all vehicle systems to power down.
Test for greater than 100 Ω between each powertrain high speed GM LAN serial data circuit at the device that was just disconnected and ground.
- If 100 Ω or less
  Refer to Testing the Serial Data Circuits for a Short to Ground .
- Go to next step: If greater than 100 Ω
Test for 110 - 130 Ω between each pair of the powertrain high speed GM LAN serial data circuits at the device that was just disconnected:
- If less than 110 Ω
  Refer to Testing the Serial Data Circuits for a Short between the Circuits .
- If greater than 130 Ω
  Refer to Testing the Serial Data Circuits for an Open/High Resistance .
- Go to next step: If between 110 - 130 Ω
Refer to Testing the Device Circuits .