NOISE IN PARK OR NEUTRAL NEW SERVICE PACKAGE

Subject: BUZZ OR VIBRATION NOISE IN PARK OR NEUTRAL (NEW SERVICE PACKAGE)

Model and Year: 1991 C, K, R, V, G TRUCKS WITH 4L8OE/4L80EHD AUTOMATIC TRANSMISSION

SERVICE INFORMATION:

Some HYDRA-MATIC 4L80-E/4L80-EHD Transmissions may experience a buzz or vibration noise while in park or neutral. This is caused by line pressure instability in the pressure regulator valve line-up. Changes made to the Pump Assembly will eliminate this condition. A service package is released for an on-vehicle correction to the Pressure Regulator Valve line- up in the Pump Assembly.

DATE OF PRODUCTION CHANGE: (Figure 1) --------------------------------------- Starting March 15, 1991 (Julian Date 074) all HYDRA-MATIC 4L80-E/4L80-EHD models were built with the new Pressure Regulator and Reverse Boost Valve assembly.

SERVICE PARTS INFORMATION:

Part Number Part Description ----------- ----------------- 8682998 Package, Pressure Regulator and Reverse Boost Valve

Pump cover and Body Assemblies will have new numbers with updated parts at a future date.

ON-VEHICLE SERVICE PROCEDURE: (Figure 2,3,4) --------------------------------------------- 1. Remove seventeen 10 mm Pan Bolts (27), Seal (29), and Pan (28).

NOTE: Seal (29) is reusable.

2. Remove Filter Assembly (31).

3. Remove Retainer Ring (226), Reverse Boost Bushing (227), Reverse Boost Valve (228), Pressure Regulator Spring Retainer (229), Pressure Regulator Spring(230), and Pressure Regulator Valve (231)

4. Install NEW Pressure Regulator Valve (231). Retain in position with TRANSJEL tm J-36850.

5. Pre-assemble Reverse Boost Valve Bushing (227), New Reverse Boost Valve (228), New Regulator Spring Retainer (229), added Isolator Pressure Regulator Spring (238), and Pressure Regulator Spring (230).

6. Install Pre-assembled parts into pump bore.

7. Install Retainer Ring (226) while holding Reverse Boost Valve in place. Make sure Retainer Ring is in the groove.

8. Install filter (31).

9. Seal (29) and Pan (28) with seventeen 10mm Screws (27), torque 8-14 Nm (6-10 lb.ft.).

SERVICE MANUAL REFERENCE:

Refer to the Unit Repair Service Manual Section of the appropriate service manual for replacement procedures.

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.