Verifying the Serial Data Circuits using Digital Multimeter

Ignition ON.
Verify two or more devices are not communicating on the high speed GMLAN serial data circuit. Refer to Data Link References to determine how many devices should be communicating on the high speed GMLAN bus.
- If only one device is not communicating
  Refer to Testing the Device Circuits .
- If two or more devices are not communicating
Ignition 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. Disconnect the scan tool from the X84 Data Link Connector. The following tests will be done at the X84 Data Link Connector.
Test for less than 10 Ω between the ground circuit terminal 5 and ground.
- If 10 Ω or greater
1. Ignition OFF.
2. 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.
- If less than 10 Ω
Ignition ON.
Test for less than 4.5 V between the serial data circuits listed below and ground.
- Terminal 6
- Terminal 14
- If 4.5 V or greater
  Refer to Testing the Serial Data Circuits for a Short to Voltage .
- If less than 4.5 V
Ignition 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 the serial data circuits listed below and ground.
- Terminal 6
- Terminal 14
- If 100 Ω or less
  Refer to Testing the Serial Data Circuits for a Short to Ground .
- If greater than 100 Ω
Test for 50-70 Ω between the serial data circuit terminals 6 and 14.
- If less than 35 Ω
  Refer to Testing the Serial Data Circuits for a Short between the Circuits .
- If between 35-50 Ω
  There may be a third terminating resistor between the serial data circuits. This can happen if the incorrect device is installed. Some devices are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness. Refer to Testing the Serial Data Circuits for a Short between the Circuits .
- If greater than 70 Ω but less than infinite
  Refer to Testing the Serial Data Circuits for an Open/High Resistance (Digital Multimeter) .
- If infinite resistance
  Repair the open/high resistance in the circuit between the X84 Data Link Connector and the first splice/device in the serial data circuit.
- If between 50-70 Ω
Refer to Testing the Device Circuits .