T-150 Transfer Case Phantom Shift - keywords 4HI 4WD HI4 light NP1 position shift switch unwanted

This PI was superseded to update Recommendation/Instructions and administrative details. Please discard PIP3558G.

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Unrequested transfer case mode shift. Condition is likely intermittent.

One known concern is water intrusion under the carpet in the area of the Transfer Case Control Module. The carpet in these vehicles has a water resistant backing and there may be water trapped under it that would not be evident by feeling the carpet on top. Remove the door sill trim plate and feel under the carpet for any evidence of water. If water intrusion is found, check for the source. Two known concerns have been with the shift cable grommet just below the brake pedal and also the grommet for the hood release cable. These grommets may be the source of water intrusion if no others are found. Seal the grommets to prevent additional water intrusion.

After inspecting the electrical connectors make sure that they are fully seated. Listen for an audible 'click' as the connectors are reinserted.

Inspect the TCCM part number, if the numbers are Production 24239016 or Service 98052378 and the concern can be readily duplicated please contact the technical assistance center at the number you have been provided. Please provide the conditions and any details under which the concern can be duplicated.

Note: If the vehicle has either of the part number TCCM's already installed and the concern can NOT be duplicated please do not attempt any further repairs at this time as engineering is still investigating this concern.

If the part number on TCCM is neither of the numbers listed above replace the TCCM, use part number P/N 98052378. This module incorporates a 'range hold' feature that may help prevent the occurrence of an unwanted or phantom shift. This part is currently available through GMSPO.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.