Circuit/System Verification

Perform Circuit/System Verification using one of the following two methods:

Verifying the Serial Data Circuits using Data Bus Diagnostic Tool

1. Using schematics and vehicle build information, identify all low speed GMLAN devices the tested vehicle is equipped with, and each device location on the low speed GMLAN serial data bus.
2. Ignition ON/Vehicle In Service Mode.
3. From Data Bus Diagnostic Tool main screen, click on the green Start/Stop button at lower left corner. Select DLC Terminal 1 Detected on the popup window and click on the green check button to test the low speed GMLAN serial data bus.
4. Verify that all applicable low speed GMLAN devices are displayed under the Control Module column of the Detected State tab.
   • If all low speed GMLAN devices are not responding 

     Refer to step 3 of Circuit/System Verification - Verifying the Serial Data Circuits using Digital Multimeter to test for a short to voltage or short to ground on the low speed GMLAN serial data circuit, or an open/high resistance between the X84 Data Link Connector and the first low speed GMLAN device or splice pack.

   • If one or more low speed GMLAN devices are not responding 

     Refer to Circuit/System Testing - Testing the Device Circuits to test the non-communicating low speed GMLAN device closest to the X84 Data Link Connector.

   • If all low speed GMLAN devices are responding 
5. All OK.

Verifying the Serial Data Circuits using Digital Multimeter

1. Ignition ON/Vehicle In Service Mode.
2. Test for less than 4.5 V between the X84 Data Link Connector low speed GMLAN serial data circuit terminal 1 and ground.
   • If 4.5 V or greater 

     Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Voltage.

   • If less than 4.5 V 
3. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, 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.
4. Test for greater than 100 Ω between the X84 Data Link Connector low speed GMLAN serial data circuit terminal 1 and ground.
   • If 100 Ω or less 

     Refer to Circuit/System Testing - Testing the Serial Data Circuits for a Short to Ground.

   • If greater than 100 Ω 
5. Ignition ON/Vehicle In Service Mode, attempt to communicate with all devices on the low speed GMLAN serial data circuit. Refer to Data Link References  to determine how many devices should be communicating on the low speed GMLAN bus.
6. Verify which devices are communicating on the low speed GMLAN serial data circuit.
   • If only one device is not communicating 

     Diagnose that device only. Refer to Circuit/System Testing - Testing the Device Circuits.

   • If one or more devices are communicating but not all 

     Refer to Circuit/System Testing - Testing the Serial Data Circuit for an Open/High Resistance.

   • If none of the devices are communicating 
7. Ignition/Vehicle OFF, all access doors closed, all vehicle systems OFF, all keys at least 3 m (9.8 ft) away from vehicle. Disconnect the scan tool from the X84 Data Link Connector. It may take up to 2 min for all vehicle systems to power down.
8. Test for less than 10 Ω between the X84 Data Link Connector ground circuit terminal 5 and ground.
   • If 10 Ω or greater 
   1. Ignition/Vehicle 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 
9. Repair the open/high resistance in the low speed GMLAN serial data circuit between the X84 Data Link Connector and the closest splice.