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
Use the schematics and connector end views to identify the device's ground, B+, ignition, serial data communication enable, and serial data circuit terminals.
- Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the harness connectors at the K56 Serial Data Gateway Module. It may take up to 10 min for all vehicle systems to power down.
- Test for less than 10 Ω between each ground circuit terminal and ground.
- If 10 Ω or greater
- Ignition/Vehicle OFF.
- Test for less than 2 Ω in the ground circuit end to end.
- If 2 Ω or greater, repair the open/high resistance in the circuit.
- If less than 2 Ω, repair the open/high resistance in the ground connection.
- Go to next step: If less than 10 Ω
- If equipped, verify a test lamp illuminates between each B+ circuit terminal and ground.
- If the test lamp does not illuminate and the circuit fuse is good
- Ignition/Vehicle OFF, remove the test lamp.
- Test for less than 2 Ω in the B+ circuit end to end.
- If 2 Ω or greater, repair the open/high resistance in the circuit.
- If less than 2 Ω, verify the fuse is not open and there is voltage at the fuse.
- If the test lamp does not illuminate and the circuit fuse is open
- Ignition/Vehicle OFF, remove the test lamp.
- Test for infinite resistance between the B+ circuit and ground.
- If less than infinite resistance, repair the short to ground on the circuit.
- If infinite resistance, replace the K56 Serial Data Gateway Module.
- Go to next step: If the test lamp illuminates
- Ignition ON/Vehicle In Service Mode.
- If equipped, verify a test lamp illuminates between each ignition circuit terminal, which has a fuse in the circuit, and ground.
- If the test lamp does not illuminate and the circuit fuse is good
- Ignition/Vehicle OFF, remove the test lamp.
- Test for less than 2 Ω in the ignition circuit end to end.
- If 2 Ω or greater, repair the open/high resistance in the circuit.
- If less than 2 Ω, verify the fuse is OK and there is voltage at the fuse.
- If the test lamp does not illuminate and the circuit fuse is open
- Ignition/Vehicle OFF, remove the test lamp.
- Test for infinite resistance between the ignition circuit and ground.
- If less than infinite resistance, repair the short to ground on the circuit.
- If infinite resistance, replace the K56 Serial Data Gateway Module.
- Go to next step: If the test lamp illuminates
- Verify a test lamp illuminates between the serial data communication enable circuit terminal and ground.
- If the test lamp does not illuminate
- Ignition/Vehicle OFF, remove the test lamp, disconnect the harness connectors at the K9 Body Control Module.
- Test for less than 2 Ω in the serial data communication enable circuit end to end.
- If 2 Ω or greater, repair the open/high resistance in the circuit.
- If less than 2 Ω, replace the K9 Body Control Module.
- Go to next step: If the test lamp illuminates
- Test for less than 4.5 V between each GMLAN serial data circuit terminal listed below and ground:
- Terminal 13 X1
- Terminal 14 X1
- Terminal 15 X1
- Terminal 16 X1
- Terminal 17 X1
- Terminal 18 X1
- Terminal 26 X1
- If 4.5 V or greater
Repair the short to voltage in the serial data circuit.
- 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 infinite resistance between each GMLAN serial data circuit terminal listed below and ground:
- Terminal 13 X1
- Terminal 14 X1
- Terminal 15 X1
- Terminal 16 X1
- Terminal 17 X1
- Terminal 18 X1
- Terminal 26 X1
- If less than infinite resistance
Repair the short to ground in the serial data circuit.
- Go to next step: If infinite resistance
- Test for infinite resistance between each pair of the GMLAN serial data circuit terminals listed below and ground:
- Terminals 13 X1 and 14 X1
- Terminals 15 X1 and 16 X1
- Terminals 17 X1 and 18 X1
- If less than infinite resistance
Repair the short together in the serial data circuits.
- Go to next step: If infinite resistance
- Test for less than 2 Ω in each GMLAN serial data circuit end to end between the K56 Serial Data Gateway Module and the X84 Data Link Connector.
- If 2 Ω or greater
Repair the open/high resistance in the serial data circuit.
- Go to next step: If less than 2 Ω
- If 2 Ω or greater
- Replace the K56 Serial Data Gateway Module.
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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.