Transfer Case - High Shift Effort: Overview
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GMC NUMBER: 91-7D-142
GROUP: 7D Transfer Case
DATE: September, 1991
CORPORATE
NUMBER: 167401
SUBJECT:
HIGH TRANSFER CASE SHIFT EFFORT (INSTALLATION OF EASY SHIFT PACKAGE)
MODELS:
1988-91 T1 TRUCKS AND UTILITY VEHICLES
Some 1988-91 T-Trucks may experience high transfer case shift effort or total block-outs when "shifting on the fly" from 2H to 4H under cold weather conditions.
This condition may be corrected by the installation of the synthetic 75W90 gear lube described in Corporate Bulletin Number 967402R (GMC 90-7D-112). This fluid maintains a more constant viscosity under cold weather conditions, allowing quicker front axle synchronization and easier transfer case engagement.
If the installation of the synthetic fluid does not result in a satisfactory shift effort, an "Easy Shift" package may be installed in the transfer case. This package features a roller pin (53) rather than a solid pin on the mode shift fork (54) and revised ramp angles on the sector (55) to reduce engagement forces (refer to figures 2 and 3). This package was installed in the 1991 model year and will provide reduced transfer case shift effort under cold weather conditions.
VEHICLES INVOLVED:
Group A:
Some 1988-91 T-Trucks equipped with transfer cases identified by the following part numbers:
15568078 15627441
15568079 15627442
15568597 15653450
This part number can be found on the identification tag (80) located on the rear case half (figure 1).
Group B:
Some 1991 T-Trucks equipped with transfer cases identified by the following part numbers and built before 10/22/90:
15653414 15659232
This part number and build number can be found on the identification tag (80) located on the rear case half (figure 1).
SERVICE PROCEDURE:
Group A:
These transfer cases require the replacement of the parts indicated in the Group A column of the PARTS INFORMATION section.
Group B:
These transfer cases require the replacement of the parts indicated in the Group B column of the PARTS INFORMATION section.