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Finding GMLAN wiring problems

Some owners of 2007-2010 Cadillac Escalades, Chevy Avalanche, Silverado, Suburban and Tahoe, and GMC Sierra, Yukon, Yukon XL, Yukon Denali and Yukon Denali XL vehicles may complain about issues ranging from: IPC indicators on, gauges inoperative, no crank, harsh transmission shifting, no shifting, door locks cycling, etc.

When checking for DTCs there may be a loss of communication codes for the High Speed GMLAN modules (examples include U0100, U0101, U0102, U0106, U 0109, U0117, U0121, U0122, U0132, U0137, U0140 and/or U0073.

These issues may be intermittent, making it difficult to diagnose. Following is a list of common areas where the High Speed GMLAN wiring may have been shorted, chaffed, pinched, terminals backed out, or poor pin fit. These areas have been identified in the field.

* Adjustable pedal motor: As the brake pedal is applied, the DLC wiring harness may rub on the adjustable pedal motor and short out. This concern will normally occur when the brake pedal is depressed.

* Rear of transfer case: Wiring harness chaffing on the transfer case.

* Terminating Resistor: Review PIT3993C for more information.

* Pinched harness between Pickup Box and Frame.

* Inline connector X115 or C115: Inspect for backed out, bent or poor terminal fit using the correct test probe.

* EBCM connector: Inspect for corrosion/water in the connector/harness, backed out, or poor terminal fit, using the correct test probe.

* PTO connector: Inspect for corrosion/water in the connector, connector not fully seated (even though the lever on the connector is locked down), and poor terminal fit.

* FPCM (fuel pump control module): Inspect for corrosion/water in the connector or module, connector not fully seated (even though the connector lever is locked down) and poor terminal fit.

* VCIM (On Star module): Inspect for aftermarket equipment that may be mounted to the lower center part of the dash. The screws used to mount the equipment have been know to pierce the VCIM harness or module. Inspect the VCIM connector for poor terminal fit, using the correct test probe. Inspect the two VCIMs listed in the DTC module list with the Tech 2 under Vehicle Control Systems / Vehicle DTC Information / DTC Display, as this may indicate a faulty VCIM. If the VCIM is suspect, remove the “Info” fuse, which is the power feed to the VCIM and see if the High Speed GMLAN concern is gone.

* Inline connector X109: Inspect for backed out, bent or poor terminal fit using the correct test probe. Also check for the connector not being fully seated together, even though the connector lever is locked down.

* 6L80/6190 Auto Trans 16-way Round Pass Through Connector: Review GM bulletin 08-07-30-021B for more information.

* Chaffed harness near parking brake pedal/sill plate area: Refer to GM bulletin 07-08-49-018.

* Chaffed harness at the Left IP Junction Block Mounting Bracket: Remove the junction block and inspect for any shorted/chaffed wiring.

* Pickup chassis harness chaffing at the body mounts along the left frame rail.

* Trucks with diesel engines: Inspect the engine wiring harness for chaffing on the connector/harness mounting bracket at the X108 connector break out.

The DLC wiring harness may rub on the adjustable pedal motor. Inspect for rubbing/shorts.
<p>The DLC wiring harness may rub on the adjustable pedal motor. Inspect for rubbing/shorts.</p>

Even though the connector lever may be locked down, inspect the inline connector X109 for proper seating.
<p>Even though the connector lever may be locked down, inspect the inline connector X109 for proper seating.</p>

Inspect the wiring harness along the left frame rail.
<p>Inspect the wiring harness along the left frame rail.</p>

On Diesel engine equipped vehicles, inspect the engine wiring harness for chaffing at the X108 connector mounting bracket.
<p>On Diesel engine equipped vehicles, inspect the engine wiring harness for chaffing at the X108 connector mounting bracket.</p>

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