Differential - Chatter on Acceleration, 8.5 Inch Locking
GMC NUMBER: 85-T-106SECTION: 4B - Rear Axle
DATE: December, 1986
SUBJECT: LOCKING DIFFERENTIAL CHATTER
MODELS: ALL 1981 - 1985 C, K, AND G MODELS EQUIPPED WITH AN EATON 8 1/2-INCH LOCKING DIFFERENTIAL
A condition may exist where the Eaton 8 1/2 inch locking differential may exhibit a chatter condition on acceleration from a stop. This is most likely to occur when the differential is hot, after extended highway driving. The 8 1/2-inch Eaton locking differential has been redesigned interim 1985 model year to alleviate chatter. The bell end clutch pack has been changed from seven discs to five thicker discs. This change is effective for axle date codes of January 22, 1985,
(022) and later. The date code can be found stamped on top of the right axle tube outboard of the carrier, preceded by four alpha numeric characters representing the axle and source codes. Compare the axle code stamped on the axle tube with the corporate axle codes in the GM Parts Book to determine if you have an 8 1/2-inch Eaton locking differential. Note that axle codes change for each model year.
Example: BN3 G 001 1
Axle Source Date Shift
Code Code Code Code
The new differential case assembly part number is 26005119, and the new five-disc bell end plate kit can be ordered using part number 15580825. The five and seven disc plate kits are interchangeable.
If a vehicle equipped with an Eaton 8 1/2-inch locking differential with a date code prior to 022, 1985, experiences chatter, replace the bell end clutch pack as described in the following service procedure.
SERVICE PROCEDURE
Follow the unit repair procedure for the 8 1/2 inch locking differential found on Page 4B6-4 through 4B6-7 in the 1985 Light Duty Truck Service Manual.
a) Replace the right hand (bell end) clutch pack with the new clutch pack, P/N 15580825.
Note. If the entire carrier assembly require replacement, install the new differential case assembly, P/N 26005119.
b) Refill with the recommended lubricant only, GM Part Number 1052271, or equivalent.
DO NOT ADD LIMITED SLIP ADDITIVE, GM P/N 1052358, TO THE DIFFERENTIAL FLUID. This practice delays the engagement of the locking mechanism, and may be detrimental to the life of the axle.