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.
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HOMESERVICE MANUALSCHEVROLET2008CHEVY EXPRESS G2500, VAN CARGO, 6.6 6REPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1616 (DATA COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC U0074CIRCUIT/SYSTEM TESTINGNOTES
2008 Chevrolet Chevy Express G2500, Van Cargo, 6.6 6
Circuit/System Testing: Notes
2008 Chevrolet Chevy Express G2500, Van Cargo, 6.6 6SECTION Notes
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
Each control module may need to be disconnected to isolate a circuit fault. Use the schematic to identify the following:
- Control modules the vehicle is equipped with
- Powertrain High Speed GMLAN serial data circuit terminating resistors
- Control module locations on the Powertrain High Speed GMLAN serial data circuits
- Each control module's Powertrain High Speed GMLAN serial data circuit terminals
- Attempt to communicate with all control modules on the Powertrain High Speed GMLAN serial data circuit, refer to. Communications should not be available with two or more control modules on the Powertrain High Speed GMLAN serial data circuit.
- If only one control module is not communicating, diagnose that control module only; refer to.
- Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
- Ignition ON, test for less than 4.5 V between the serial data circuits listed below and ground:
- Terminal 3
- Terminal 11
- 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 VoltageΒ .
- Ignition OFF for 60 seconds, test for greater than 100 Ξ© between the serial data circuit listed below and ground:
- Terminal 3
- Terminal 11
- If not the specified value, test the serial data circuit for a short to ground; refer to Testing the Serial Data Circuits for a Short to GroundΒ .
- Test for 50-70 Ξ© between the serial data circuit terminal 3 and terminal 11.
- If less than 35 Ξ©, test for a short between the serial data circuits; refer to Testing the Serial Data Circuits for a Short between the CircuitsΒ .
- If 35-50 Ξ© 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 Ξ© 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 ResistanceΒ .
- If infinite, test the serial data circuits between the DLC and the first connection to the serial data circuit for an open/high resistance.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- β’ You smell fuel, burning insulation, or see smoke.
- β’ Brakes feel soft, pull hard to one side, or make grinding noises.
- β’ The engine overheats, stalls repeatedly, or misfires under load.
- β’ You are missing required tools, torque specs, or safe lifting equipment.
- β’ You are not confident in the next step or safety outcome.