Scan Tool Does Not Communicate With High Speed GMLAN Device

SCAN TOOL DOES NOT COMMUNICATE WITH HIGH SPEED GMLAN DEVICE

Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications on the high speed GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module detects a bus-off condition a DTC U0001 or U2100 will be set. These DTCs can be retrieved as history only.

Diagnostic Aids
The high speed GMLAN serial data bus uses two 120 ohm terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the body control module (BCM) and the other is at the engine control module (ECM). When testing for a short between high speed GMLAN (+) and (-) a reading of 60 ohms is normal. If the high speed GMLAN serial data is open testing the resistance between high speed GMLAN (+) and (-) will read about 120 ohms.

The engine will not start when there is a total malfunction of the high speed GMLAN serial data circuits while the engine is not running. The following conditions may cause a total loss of high speed GMLAN data communication:
- A short between high speed GMLAN (+) and high speed GMLAN (-) circuits
- Any of the high speed GMLAN serial data circuits shorted to ground or voltage
- A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits
- Any of the high speed GMLAN serial data circuits open

Test Description

Steps 1 - 8:

Steps 9 - 17:

Steps 18 - 21:

The numbers below refer to the step numbers on the diagnostic table.
2. A partial malfunction in the high speed GMLAN serial data circuits uses a different procedure from a total malfunction of the high speed GMLAN data circuits. The following modules are connected to the high speed GMLAN serial data circuits:
- BCM
- Communication interface module (OnStar(R))
- Distance sensing cruise control (DSCC) module
- Electronic brake control module (EBCM)
- Electronic suspension control (ESC) module
- Engine control module (ECM)
- Transmission control module (TCM)
8. Data link connector terminals 6 and 14 provide the connection to the GMLAN serial data high circuit and the GMLAN serial data low circuit respectively.
15. If only one of the high speed GMLAN circuits is open all communication on the high speed bus will be disrupted.
19. The communication malfunction may have prevented diagnosis of the customer complaint.