Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Circuit/System Testing: Notes
Each device may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:
- Gateway expansion high speed GMLAN devices and terminating resistors the vehicle is equipped with
- Device locations on the gateway expansion high speed GMLAN serial data circuits
- Each device's ground, B+, ignition, and gateway expansion 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.
- Ignition/Vehicle OFF, disconnect the harness connectors with the gateway expansion high speed GMLAN serial data circuits at an easily accessible device that is not communicating. Refer to Data Link References to determine which devices are on the gateway expansion high speed GMLAN bus.
- Ignition ON/Vehicle In Service Mode.
- Test for less than 4.5 V between each gateway expansion high speed GMLAN 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 .
- If less than 4.5 V
- If 4.5 V or greater
- 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 2 min for all vehicle systems to power down.
- Test for greater than 100 Ω between each gateway expansion high speed GMLAN 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 .
- If greater than 100 Ω
- If 100 Ω or less
- Test for 110 - 130 Ω between each pair of the gateway expansion high speed GMLAN 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 .
- If between 110 - 130 Ω
- If less than 110 Ω
- Refer to Testing the Device Circuits .
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.