Circuit/System Testing

1. Refer to Data Link References  to determine if the control module setting DTC U0074 is on the mid speed or chassis high speed serial data circuits. Only diagnose the affected serial data circuits.
2. Attempt to communicate with all control modules on the mid speed or chassis high speed serial data circuit. Refer to Data Link References . Communications should not be available with two or more control modules on the mid speed or chassis high speed serial data circuit.
   • If only one control module is not communicating, diagnose that control module only. Refer to DTC U0100-U02FF .
3. Disconnect the scan tool from the X84 DLC. The following tests will be done at the X84 DLC connector.
4. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 10 Ω between the ground circuit terminal 5 and ground.
   • If greater than the specified range, test the ground circuit for an open/high resistance.
5. Ignition ON, test for less than 4.5 V between each serial data circuit listed below and ground:
   • Mid speed GMLAN serial data terminal 3
   • Mid speed GMLAN serial data terminal 11
   • Chassis high speed GMLAN serial data terminal 12
   • Chassis high speed GMLAN serial data terminal 13
   • If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage.
6. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for greater than 100 Ω between each serial data circuit listed below and ground:
   • Mid speed GMLAN serial data terminal 3
   • Mid speed GMLAN serial data terminal 11
   • Chassis high speed GMLAN serial data terminal 12
   • Chassis high speed GMLAN serial data terminal 13
   • If less than the specified range, test the serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to Ground below.
7. Test for 50-70 Ω between the serial data circuits listed below:
   • Mid speed GMLAN serial data circuits terminal 3 and terminal 11
   • Chassis high speed GMLAN serial data circuits terminal 12 and terminal 13
   • If less than 35 Ω, test for a short between the serial data circuits. Refer to Testing the Serial Data Circuits for a Short between the Circuits below.
   • If 35-50 Ω there may be a third terminating resistor between the serial data circuits. This can happen if the incorrect control module is installed. Some control modules are available with and without the terminating resistors installed to reduce the need of terminating resistors in the wiring harness.
   • If greater than 70 Ω but less than infinite, test the serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance below.
   • If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.
8. If the circuits test normal, refer to Testing the Control Module Circuits below.

Testing the Serial Data Circuits for a Short to Voltage

1. Ignition OFF, disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V between each serial data circuit and ground. Verify that one or more serial data circuits are greater than 4.5 V.
   • If all serial data circuits are less than the specified range, replace the control module that was disconnected.
3. Ignition OFF, disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuit shorted to voltage.
4. Ignition ON, test for greater than 4.5 V between each serial data circuit and ground. Verify that one or more serial data circuits are greater than 4.5 V.
   • If all serial data circuits are less than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated:
   • A short to voltage on the serial data circuit between 2 control modules
   • A short to voltage on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short to Ground

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module.
2. Test for less than 100 Ω between each serial data circuit and ground. Verify that one or more serial data circuits are less than 100 Ω.
   • If all serial data circuits are greater than the specified range, replace the control module that was disconnected.
3. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Test for less than 100 Ω between each serial data circuit and ground. Verify that one or more serial data circuits are less than 100 Ω.
   • If all serial data circuits are greater than the specified range, replace the control module that was just disconnected.
5. Repeat step 3 until one of the follow conditions are isolated:
   • A short to ground on the serial data circuit between 2 control modules
   • A short to ground on the serial data circuit between a control module and a terminating resistor

Testing the Serial Data Circuits for a Short between the Circuits

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module, that is not communicating.
2. Test for less than 110 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 Ω.
   • If each pair of serial data circuits is greater than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the circuits shorted together.
5. Test for less than 110 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are less than 110 Ω.
   • If each pair of serial data circuits is greater than the specified range, replace the control module that was just disconnected.
6. Repeat step 3 until one of the following conditions are isolated:
   • Serial data circuits shorted together between 2 control modules
   • Serial data circuits shorted together between a control module and a terminating resistor
   • A shorted terminating resistor

Testing the Serial Data Circuits for a Open/High Resistance

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at an easily accessible control module, that is not communicating.
2. Test for greater than 130 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 Ω.
   • If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
3. Connect the harness connectors at the control module that was disconnected.
4. Disconnect the harness connectors with the mid speed or chassis high speed serial data circuits at another control module, in the direction of the open circuit.
5. Test for greater than 130 Ω between each pair of serial data circuits. Verify that one pair of serial data circuits are greater than 130 Ω.
   • If each pair of serial data circuits is less than the specified range, replace the control module that was disconnected.
6. Repeat step 3 until one of the following conditions are isolated:
   • An open/high resistance on the serial data circuit between 2 control modules
   • An open/high resistance on the serial data circuit between a control module and a terminating resistor
   • An open/high resistance terminating resistor

Testing the Control Module Circuits

1. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Disconnect the harness connectors at an easily accessible control module that is not communicating.
2. Test for less than 10 Ω between each ground circuit terminal and ground, at the control module connector that was just disconnected.
   • If greater than the specified range, test the ground circuit and ground connection for an open/high resistance.
3. Ignition OFF, verify that a test lamp does not illuminate between each ignition circuit terminal and ground.
   • If the test lamp does illuminate, test the ignition circuit for a short to voltage. If the circuits test normal, replace the appropriate control module.
4. Ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground.
   • If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the appropriate control module.
5. Ignition OFF and all vehicle systems OFF, all keys at least 3 meters away from vehicle, all access doors closed. It may take up to 2 minutes for all vehicle systems to power down. Test for less than 130 Ω between each pair of serial data circuits.
   • If greater than the specified range, test the serial data circuits for an open/high resistance.
6. Connect the harness connectors at the control module that was just disconnected.
7. Repeat step 1 at another control module until a short to voltage, a short to ground, or an open/high resistance is found on the ground, ignition, or serial data circuits.