Cobalt L4-2.4L (2007)
Step 9 - Step 13
The numbers below refer to the step numbers on the diagnostic table.
2. A partial loss or intermittent loss of communication in the class 2 serial data circuit uses a different procedure than a total loss of communication
of the class 2 serial data circuit.
10. If there are no current DTCs that begin with the letter "U", the communication concern has been repaired.
11. The communication concern may have prevented diagnosis of the customer complaint.
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 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 body control module
(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 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 power moding.
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
