Scan Tool Does Not Communicate with Low Speed GMLAN Device
Diagnostic Instructions
Diagnostic Fault Information
Circuit
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Short to Ground
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Open/High Resistance
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Short to Voltage
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Signal Performance
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Low Speed GMLAN Serial Data
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1
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U0100-U0299*
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1
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-
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Ground (DLC, terminal 5)
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-
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1*
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-
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-
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* An open between the data link connector serial data circuit
terminal 1 or ground circuit terminal 5 and the BCM circuit will
only effect the communication with the scan tool.
1. No communication on any low speed GMLAN module
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Circuit/System Description
The serial data is transmitted over a single wire to the
appropriate control modules. The transmission speed for GMLAN low
speed is up to 83.33 Kb/s. Under normal vehicle operating
conditions, the speed of the buss is 33.33 Kb/s. The control
modules toggle the serial data circuit between 0-5 V during normal
communications. To wake the control modules connected to the GMLAN
low speed serial data circuit, a voltage wake up pulse of 10 V is
sent out. If serial data is lost, control modules will set a no
communication code against the non-communicating control module. A
loss of serial data communications DTC does not represent a failure
of the module that set it.
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: |
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- |
A control module
on the data communication circuit was disconnected while the
communication circuit is awake. |
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Power to one or
more modules was interrupted during diagnosis |
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A low battery
condition was present, so some control modules stop communicating
when battery voltage drops below a certain threshold. |
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Battery power was
restored to the vehicle and control modules on the communication
circuit did not all re-initialize at the same time. |
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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. |
• |
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.
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• |
Communication may be available
between the body control module (BCM) and the scan tool with the
low speed GMLAN serial data system inoperative. This condition is
due to the BCM using both the high and low speed GMLAN
systems. |
• |
An open in the low speed GMLAN
serial data circuit between the splice pack and a module will only
affect that specific module. This type of failure will set a loss
of communication DTC for each module effected, and the other
modules will still communicate. |
• |
An open in the data link
connector (DLC) ground circuit terminal 5 will allow the scan tool
to operate but not communicate with the vehicle. |
• |
The engine may not start when
there is a total malfunction of the low speed GMLAN serial data
circuit. |
• |
Technicians may find various
Local Area Network (LAN) communication Diagnostic Trouble Codes
(DTC) and no low speed LAN communications with the scan
tool. |
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These conditions may be caused by the installation of an
aftermarket navigation radio module (see bulletins). Some customers
may comment of one or more of the following concerns: |
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Vehicle cranks
but will not start |
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Vehicle stability
enhancement system warning lights and messages |
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PRNDL gear
indicator position errors |
Reference Information
Schematic Reference
Data Communication Schematics
Connector End View Reference
Component Connector End Views
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Circuit/System Testing
Note: Use the schematic to
identify the following:
• |
Control modules
the vehicle is equipped with |
• |
Control module and
splice pack locations on the low speed GMLAN serial data
circuit |
• |
The low speed
GMLAN serial data circuit terminals for each control module,
component, or splice pack |
1. |
Attempt to communicate with
all control modules on the low speed GMLAN serial data circuit.
Refer to
Data Link References . Communications should not be available
with two or more control modules on the low speed GMLAN serial data
circuit. |
⇒ |
If only one control module is not communicating,
diagnose that control module only. Refer to
DTC U0100-U02FF . |
⇒ |
If one or more control modules are communicating
but not all, refer to Testing the Serial Data Circuit for an
Open/High Resistance. |
2. |
Disconnect the scan tool from
the DLC. The following tests will be done at the DLC
connector. |
3. |
All access doors closed,
ignition OFF for 60 seconds, test for less than 10 Ω between
the ground circuit terminal 5 and ground. |
⇒ |
If greater than the specified range, test the
ground circuit for an open/high resistance. |
4. |
Ignition ON, test for less
than 4.5 V between the serial data circuit terminal 1 and
ground. |
⇒ |
If greater than the specified range, test the
serial data circuit for a short to voltage. Refer to Testing the
Serial Data Circuit for a Short to Voltage. |
5. |
Ignition OFF for 60 seconds,
test for greater than 100 Ω between the serial data circuit
terminal 1 and ground. |
⇒ |
If less than the specified range, test the serial
data circuit for a short to ground. Refer to Testing the Serial
Data Circuits for a Short to ground. |
6. |
Disconnect the harness
connector at the first splice pack closest in the circuit to the
DLC. |
7. |
Test for less than 2 Ω
between the DLC serial data circuit terminal 1 and the splice pack
connector serial data input terminal. |
⇒ |
If greater than the specified range, test the
serial data circuit for an open/high resistance. If the circuit
tests normal, replace the splice pack. |
Testing the Serial Data Circuits for a Short to Voltage
1. |
Ignition OFF, disconnect the
appropriate harness connectors at the following components:
|
|
• |
All low speed serial data
splice packs |
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• |
Any module or component
between that may be between the splice packs |
2. |
Ignition ON, test for less
than 4.5 V between the DLC serial data circuit terminal 1 and
ground. |
⇒ |
If greater than the specified range, repair the
short to voltage. |
3. |
Test for less than 4.5 V
between each serial data circuit at each component connector and
ground. |
⇒ |
If greater than the specified range, perform the
appropriate test listed below: |
|
• |
Serial data circuits with one
control module, test the serial data circuit for a short to
voltage. If the circuit tests normal, replace the control
module. |
|
• |
Serial data circuits with 2 or
more control modules, test each section of the serial data circuit
for a short to voltage. If the circuits test normal, replace the
control module that causes the short to voltage when
connected. |
Testing the Serial Data Circuits for a Short to Ground
1. |
Ignition OFF, disconnect the
appropriate harness connectors at the following components:
|
|
• |
All low speed serial data
splice packs |
|
• |
Any module or component
between that may be between the splice packs |
2. |
Test for infinite resistance
between the DLC serial data circuit terminal 1 and ground.
|
⇒ |
If not the specified value, repair the short to
ground. |
3. |
Ignition OFF for 60 seconds,
test for greater than 100 Ω between each serial data circuit
at the splice pack and ground. |
⇒ |
If less than the specified range, perform the
appropriate test listed below: |
|
• |
Serial data circuits with one
control module, test the serial data circuit for a short to ground.
If the circuit tests normal, replace the control module.
|
|
• |
Serial data circuits with 2 or
more control modules, test each section of the serial data circuit
for a short to ground. If the circuits test normal, replace the
control module that causes the short to ground when
connected. |
Testing the Serial Data Circuit for an Open/High
Resistance
1. |
If equipped with a second
splice pack, ignition ON, verify the scan tool communicates with
one or more control modules connected to the second splice
pack. |
⇒ |
If all control modules on the second splice pack
do not communicate, test the serial data circuit between the first
and second splice packs for an open/high resistance. If the circuit
tests normal, test the second splice pack for an open/high
resistance. If the splice pack tests normal, replace any module or
component that may be between those two splice packs. |
2. |
If equipped with a third
splice pack, ignition ON, verify the scan tool communicates with
one or more control modules connected to the third splice
pack. |
⇒ |
If all control modules on the third splice pack do
not communicate, test the serial data circuit between the second
and third splice packs for an open/high resistance. If the circuit
tests normal, test the third splice pack for an open/high
resistance. If the splice pack tests normal, replace any module or
component that may be between the those two splice packs. |
3. |
Ignition OFF, disconnect the
splice pack containing the serial data circuits to the modules that
are not communicating. |
4. |
Install a 3 A fused jumper
wire between the splice pack connector serial data input terminal
and a low speed GMLAN serial data circuit that is not
communicating: |
5. |
Ignition ON, verify the scan
tool communicates with the control modules connected to the low
speed GMLAN serial data circuit. |
⇒ |
If any control module does not communicate, test
each section of the serial data circuit for an open/high
resistance. If the circuits test normal, replace the control module
that does not communicate when connected. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the diagnostic
procedure.
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