Circuit Description
Modules connected to the GMLAN serial data circuit monitor for serial data communications on the GMLAN network during normal vehicle operation. Operating information and commands are exchanged among the modules. The body control module (BCM) maintains a transmit error counter (TEC) and a receive error counter (REC). The counter values increase with detected errors and will decrease with error-free messages. If the TEC value exceeds 255 the BCM removes itself from the high speed GMLAN network and a DTC U2100 will be set.
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC U2100 High Speed CAN Communication Bus
Conditions for Running the DTCs
| • | Voltage supplied to the module is in the normal operating voltage range. |
| • | The vehicle power mode requires serial data communication to occur. |
Conditions for Setting the DTC
A certain number of no valid transmitted messages on the GMLAN serial data circuits are detected by the module. 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 |
Action Taken When the DTCs Sets
| • | The module suspends all message transmission. |
| • | The module uses default values for all parameters received on the high speed GMLAN serial data circuits. |
| • | The module inhibits the setting of all other high speed GMLAN communication DTCs. |
Conditions for Clearing the DTC
| • | A current DTC clears when the malfunction is no longer present. |
| • | A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction. |
Diagnostic Aids
The BCM can set this DTC as current because it is the gateway between the class 2 and high speed GMLAN serial data circuits, and the BCM diagnostics is performed on class 2 data link. Refer to Data Link Communications Description and Operation for more information about data link communication.
The high speed GMLAN serial data bus 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 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.
Step | Action | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List | |||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||||
2 |
Did you record DTC U2100, as current? | Go to Step 4 | Go to Step 3 | ||||||||
3 |
Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions. Test the high speed GMLAN serial data circuits for:
Refer to the following: Did you find and correct the condition? | Go to Step 15 | Go to Diagnostic Aids | ||||||||
4 |
Does the DTC U2100 still set as current? | Go to Step 5 | Go to Step 9 | ||||||||
5 |
Does the DTC U2100 still set as current? | Go to Step 7 | Go to Step 6 | ||||||||
6 | Test the high speed GMLAN serial data circuits between the last two disconnected modules for:
Refer to the following: Did you find and correct the condition? | Go to Step 12 | Go to Step 9 | ||||||||
7 | Is the BCM the last module still connected to high speed GMLAN data circuits? | Go to Step 8 | Go to Step 5 | ||||||||
8 |
Refer to the following: Did you find and correct the condition? | Go to Step 12 | Go to Step 10 | ||||||||
9 | Test each of the high speed GMLAN circuits of the last disconnected module to the next module closer to DLC for an open. Refer to the following: Did you find and correct the condition? | Go to Step 12 | Go to Step 10 | ||||||||
10 | Inspect for poor connections at the high speed GMLAN serial data circuits of the suspected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 12 | Go to Step 11 | ||||||||
11 | Replace the suspected module. Refer to Control Module References for replacement, setup and programming. Did you complete the replacement? | Go to Step 12 | -- | ||||||||
12 |
The scan tool may require a power up reset before communication will occur due to a short on the high speed GMLAN serial data circuits. Turn OFF or disconnect the scan tool prior to performing this test. Did you record any DTCs which begin with a "U" and with a current status? | Go to Step 13 | |||||||||
13 | Did you record any DTCs which do not begin with a "U"? | Go to Step 14 | |||||||||
14 | Did you diagnose all of the DTCs? | Go to Step 15 | Go to Step 12 | ||||||||
15 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | ||||||||
