Scan Tool Does Not Communicate with High Speed GMLAN Device

Doc ID 1340290


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 buss uses two 120 ohms terminating resistors that are in parallel with the high speed GMLAN (+) and (-) circuits. One of the resistors is connected at the data link connector (DLC) end of the link and the other is at the engine management module, engine control module (ECM) or powertrain control module (PCM), end of the link. The DLC terminating resistor resides internally in the BCM. When testing for a short between high speed GMLAN (+) and (-) with the engine management module removed, a reading off 120 ohms is normal. However, to completely analyze the buss the body control module (BCM) should be temporally removed when testing for shorts between the high speed GMLAN (+) and (-) while also making sure that all modules and the scan tool are disconnected from the buss. A normal reading across the high speed GMLAN buss with the terminating resistor at the DLC remove and all module and scan tool disconnected is infinite ohms. Make certain to reconnect the BCM after testing, it is necessary for proper vehicle powermoding.

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

Test Description
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. Refer to Data Link References to identify which modules communicate on the high speed GMLAN serial data circuits.

3. The DTC U2100 may be retrieved with a history status, but is not the cause of the present condition.

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.

17. The communication malfunction may have prevented diagnosis of the customer complaint.