A/T - Park or Neutral Buzz or Vibration Noise

91chevy20

GMC NUMBER: 91-7A-116

GROUP: 7A Automatic Transmission


DATE: July, 1991

CORPORATE
NUMBER: 177110R


SUBJECT: BUZZ OR VIBRATION NOISE IN PARK OR NEUTRAL

MODELS: 1991 C/K, R/V, AND G TRUCKS WITH 4L80E/4L80EHD TRANSMISSION (NEW SERVICE PACKAGE)


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.

Figure 1:

DATE OF PRODUCTION CHANGE:
(Figure 1) On 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

Parts are currently available through GMSPO.

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

Figure 2:

Figure 3:

Figure 4:

ON-VEHICLE SERVICE PROCEDURE:
(See Figures 2, 3, 4)

1. Remove seventeen 10mm 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 10 mm Screws (27), torque 8-14 N-m (6-10 lb.ft.).

Labor Operation: T7281

Labor Time: .7 Hours

SERVICE MANUAL REFERENCE:

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