Scan Tool Does Not Communicate with High 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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High Speed GMLAN Serial Data (+)
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1
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U0100-U02FF*
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1
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-
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High Speed GMLAN Serial Data (-)
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1
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U0100-U02FF*
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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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* No communications with one or more high speed GMLAN modules. An
open in only one high speed GMLAN serial data circuit may allow
degraded communication between the modules. An open between the
data link connector (DLC) and the first splice will only effect the
communication with the scan tool. The vehicle modules will still
communicate.
1. No communication with any high speed GMLAN module
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|
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Circuit/System Description
The serial data is transmitted on two twisted wires that allow
speeds up to 500 Kb/s. The twisted pair is terminated with two 120
ohms resistors, one is internal to the engine control module (ECM)
and the other can be a separate resistor in a connector assembly or
in another control module. The resistors are used as the load for
the High Speed GMLAN buss during normal vehicle operation. The high
speed GMLAN is a differential bus. The high speed GMLAN serial data
bus (+) and high speed GMLAN serial data (-) are driven to opposite
extremes from a rest or idle level of approximately 2.5 V. Driving
the lines to their extremes, adds one volt to the high speed GMLAN
serial data bus (+) circuit and subtracts one volt from the high
speed GMLAN serial data bus (-) circuit. If serial data is lost,
control modules will set a no communication code against the
non-communicating control module. Note that a loss of serial data
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: |
|
- |
A control module
on the data communication circuit was disconnected while the
communication circuit is awake. |
|
- |
Power to one or
more 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. |
• |
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 body control module (BCM) and the scan tool with the
high 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 DLC ground
circuit terminal 5 will allow the scan tool to operate but not
communicate with the vehicle. |
• |
The engine will not start when
there is a total malfunction of the high speed GMLAN serial data
bus. |
• |
Technicians may find various
Local Area Network (LAN) communication Diagnostic Trouble Codes
(DTC) and no low speed GMLAN communications with the scan
tool. |
• |
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: |
|
- |
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
Connector End View Reference
Component Connector End Views
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
Note: Each control module
may need to be disconnected to isolate a circuit fault. Use the
schematic to identify the following:
• |
Control modules
the vehicle is equipped with |
• |
High speed GMLAN
serial data circuit terminating resistors |
• |
Control module
locations on the high speed GMLAN serial data circuits |
• |
Each control
module's high speed GMLAN serial data circuit terminals
|
1. |
Attempt to communicate with
all control modules on the high speed GMLAN serial data circuit
Refer to
Data Link References . Communications should not be available
with two or more control modules on the high speed GMLAN serial
data circuit. |
⇒ |
If only one control module is not communicating,
diagnose that control module only. Refer to
DTC U0100-U02FF . |
2. |
Disconnect the scan tool from
the X84 data link connector (DLC). The following tests will be done
at the DLC connector. |
3. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. 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 circuits listed below and
ground: |
⇒ |
If greater than the specified range, test the
serial data circuit for a short to voltage. Refer to Testing the
Serial Data Circuits for a Short to Voltage. |
5. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for greater than 100 Ω between
the serial data circuits listed below 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. |
Test for 50-70 Ω between
the serial data circuits terminal 6 and terminal 14. |
⇒ |
If less than 35 Ω, test for a short between
the serial data circuits. Refer to Testing the Serial Data Circuits
for a Short between the Circuits. |
⇒ |
If 35-50 Ω there may be a third terminating
resistor between the serial data circuits. This can happen if the
incorrect control module is installed. Some control modules are
available with and without the terminating resistors installed to
reduce the need of terminating resistors in the wiring
harness. |
⇒ |
If greater than 70 Ω but less than infinite,
test the serial data circuit for an open/high resistance. Refer to
Testing the Serial Data Circuits for an Open/High Resistance. |
⇒ |
If infinite, test the serial data circuits between
the DLC and the first connection to the serial data circuit for an
open/high resistance. |
Circuit/System Testing
Note: Some control modules
with an internal terminating resistor have a loop in the harness
that connects the internal terminating resistor to the serial data
circuit. When wired this way, test these loop circuits for the
appropriate failure mode short to voltage, short to ground, or
open/high resistance prior to replacing the control module for each
of the following tests.
Testing the Serial Data Circuits for a Short to Voltage
1. |
Ignition OFF, disconnect the
harness connectors with the high speed GMLAN serial data circuits
at an easily accessible control module. |
2. |
Ignition ON, test for greater
than 4.5 V between each serial data circuit at the control module
connector that was just disconnected and ground. Verify that one or
more serial data circuits are greater than 4.5 V. |
⇒ |
If all serial data circuits are less than the
specified range, replace the control module that was
disconnected. |
3. |
Ignition OFF, disconnect the
harness connectors with the high speed GMLAN serial data circuits
at another control module, in the direction of the circuit shorted
to voltage. |
4. |
Ignition ON, test for greater
than 4.5 V between the serial data circuits at the control module
connector that was just disconnected and ground. Verify that one or
more serial data circuits are greater than 4.5 V. |
⇒ |
If all serial data circuits are less than the
specified range, replace the control module that was just
disconnected. |
5. |
Repeat step 3 until one of the
follow conditions are isolated: |
|
• |
A short to voltage on the
serial data circuit between 2 control modules or splice packs, if
equipped. |
|
• |
A short to voltage on the
serial data circuit between a control module and a terminating
resistor. |
Testing the Serial Data Circuits for a Short to Ground
1. |
Ignition OFF, disconnect the
harness connectors with the high speed GMLAN serial data circuits
at an easily accessible control module. |
2. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 1k Ω between each
serial data circuit at the control module connector that was just
disconnected and ground. Verify that one or more serial data
circuits are less than 1k Ω. |
⇒ |
If all serial data circuits are greater than the
specified range, replace the control module that was
disconnected. |
3. |
Disconnect the harness
connectors with the high speed GMLAN serial data circuits at
another control module, in the direction of the circuit shorted to
ground. |
4. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 1k Ω between the
serial data circuits at the control module connector that was just
disconnected and ground. Verify that one or more serial data
circuits are less than 1k Ω. |
⇒ |
If greater than the specified range for all serial
data circuits, replace the control module that was just
disconnected. |
5. |
Repeat step 3 until one of the
follow conditions are isolated: |
|
• |
A short to ground on the
serial data circuit between 2 control modules or splice packs, if
equipped. |
|
• |
A short to ground on the
serial data circuit between a control module and a terminating
resistor. |
|
• |
A short to ground on the
serial data circuit between the DLC and the first control module or
splice pack. |
Testing the Serial Data Circuits for a Short between the
Circuits
1. |
Ignition OFF, disconnect the
harness connectors with the high speed GMLAN serial data circuits
at an easily accessible control module that is not
communicating. |
2. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 110 Ω between each
pair of serial data circuits at the control module connector that
was just disconnected. Verify that one pair of serial data circuits
are less than 110 Ω. |
⇒ |
If each pair of serial data circuits is greater
than the specified range, replace the control module that was
disconnected. |
3. |
Connect the harness connectors
at the control module that was disconnected. |
4. |
Disconnect the harness
connectors with the high speed GMLAN serial data circuits at
another control module, in the direction of the circuits shorted
together. |
5. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 110 Ω between each
pair of serial data circuits at the control module connector that
was just disconnected. Verify that one pair of serial data circuits
are less than 110 Ω. |
⇒ |
If each pair of serial data circuits is greater
than the specified range, replace the control module that was just
disconnected. |
6. |
Repeat step 3 until one of the
following conditions are isolated: |
|
• |
Serial data circuits shorted
together between 2 control modules or splice packs, if
equipped. |
|
• |
Serial data circuits shorted
together between a control module and a terminating
resistor. |
|
• |
Serial data circuits shorted
together between the DLC and the first control module or splice
pack. |
|
• |
A shorted terminating
resistor. |
Testing the Serial Data Circuits for an Open/High
Resistance
1. |
Ignition OFF, disconnect the
harness connectors with the high speed GMLAN serial data circuits
at an easily accessible control module that is not
communicating. |
2. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for greater than 130 Ω between
each pair of serial data circuits at the control module connector
that was just disconnected. Verify that one pair of serial data
circuits are greater than 130 Ω. |
⇒ |
If each pair of serial data circuits is less than
the specified range, replace the control module that was
disconnected. |
3. |
Connect the harness connectors
at the control module that was disconnected. |
4. |
Disconnect the harness
connectors with the high speed GMLAN serial data circuits at
another control module, in the direction of the open
circuit. |
5. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for greater than 130 Ω between
each pair of serial data circuits at the control module connector
that was just disconnected. Verify that one pair of serial data
circuits are greater than 130 Ω. |
⇒ |
If each pair of serial data circuits is less than
the specified range, replace the control module that was just
disconnected. |
6. |
Repeat step 3 until one of the
following conditions are isolated: |
|
• |
An open/high resistance on the
serial data circuit between 2 control modules or splice packs, if
equipped. |
|
• |
An open/high resistance on the
serial data circuit between a control module and a terminating
resistor. |
|
• |
An open/high resistance
terminating resistor. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the repair.
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