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1970+ Corvette: Service Bulletin: Positraction Chatter

Subject: Positraction Chatter
Model and Year: NA
Source: Chevrolet Dealer Service Technical Bulletin
Bulletin Number: 78-T-5, Section IV
Date: Jan., 1978

TO:  ALL CHEVROLET DEALERS

In some cases after extended expressway driving in warm ambients, a slip stick condition (chatter) occurs in the clutch plates of positraction axles during tight turns.  Rubbing or moaning noises may accompany the chatter noise.  Chattering conditions may be corrected by flushing the rear axle per the following procedure.  If chatter recurs at a later mileage, add additive only.

  1. Drain all original lubricant (must be hot) from differential housing, clean off magnet.
  2. Refill with rear axle lube (GMPD P/N 1052271, 24-ounce container or P/N 1052272, 15-gallons).
  3. Raise both rear wheels off floor.
  4. With car properly supported, operate vehicle in drive range (or high gear) for three to four minutes.  Do not exceed 30 MPH on speedometer or accelerate or decelerate rapidly.
  5. Drain all lubricant from axle.
  6. Refill with lubricant per Step #2 or equivalent, plus 4-ounces of rear axle lubricant additive (GMPD P/N 1050428).
  7. Drive car in a figure-eight manner, making at least ten (10) complete loops, to thoroughly work new lubricant into the clutch pack.

NOTE:  If it is not possible to perform Step #7, owner should be advised that it will require a minimum of 25 miles of normal driving to thoroughly work new lubricant into the clutch pack.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.

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Comments (1)
Comment by Gary Ramadei on Sat, Aug 2nd, 2014 at 8:37 AM
Following this procedure could lead to 1/2 shaft binding. With the rear wheel off the ground the suspension will hang, operating the car in this manner may cause the u-joints to bind in operation. Posi clutch hammer is cause by overtight side gear shimmg many times, also the stock clutch configuration from 1971-1979 were weak and many broke in use. If flushing and using the additive does not solve the problem then the posi should be removed and properly setup to eliminate hammering.
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