2010 Cadillac Truck Escalade RWD V8-6.2L Page 2764
Circuit/System Description
The serial data circuit is the means by which the control modules in the vehicle communicate with each other. Once the scan tool is connected to the
serial data circuit through the DLC, the scan tool can be used to monitor each control module for diagnostic purposes and to check for diagnostic
trouble codes (DTCs). When the ignition switch is in RUN, each control module communicating on the serial data circuit sends a state of health
(SOH) message to ensure that the control module is operating properly. When a control module stops communicating on the serial data circuit, for
example if the module loses power or ground, the SOH message it normally sends on the serial data circuit disappears. Other control modules on
the serial data circuit, which expect to receive that SOH message, detect its absence; those control modules in turn set a DTC associated with the
loss of SOH of the non-communicating control module. The DTC is unique to the control module which is not communicating and one or more
control modules may set the same exact code. A loss of serial data communications DTC does not represent a failure of the control modules that
contain the stored code.
Conditions for Running the DTC
The system voltage is between 9-16 V.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter module availability has not been received.
Action Taken When the DTC Sets
* Specific subsystems will not function.
* DTC U0100 in the TCM will cause the transmission to go into default gears.
* Both DTC U0100 in the TCM and DTC U0101 in the ECM will cause the malfunction indicator lamp (MIL) to illuminate.
Conditions for Clearing the DTC
* The ECM/TCM turns OFF the MIL after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
* A current DTC clears when the malfunction is no longer present.
* A history DTC clears when the control module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction.
Diagnostic Aids
* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no
associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are
transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically
after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A control module on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more control modules was interrupted during diagnosis.
- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above.
These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between
control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a
control module based only on a history U-code.
* Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication
issue.
* Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system
inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems.
* Use Data Link References (See: Powertrain Management/Computers and Control Systems/Information Bus/Testing and Inspection/Initial
Inspection and Diagnostic Overview) to determine if the control module uses high or low speed GMLAN serial data communications.
