REVERSE SHUDDER OR 3-2 TIP-IN FLARE AT 21-24 MPH

Model and Year: 1990 W CAR WITH 3.1L, 4T60 AUTOMATIC TRANSMISSION

THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 90--40-7A, DATED AUGUST 1989. THE CAR MODEL DESIGNATION HAS BEEN CORRECTED. ALL COPIES OF 90-40-7A SHOULD BE DISCARDED.

CONDITION:

---------- Some early 1990 Chevrolet Lumina vehicles built before breakpoints 9148046, 1135062, equipped with 3.1 L engine and 4 speed automatic transmission may exhibit one or both of the following reverse shudder conditions:

A. "Apply" Shudder - When the driver accelerates from a standstill (with transaxle in reverse) the vehicle shudders.

B."Lift-Foot" Shudder - When the driver accelerates at light to medium throttle (with transaxle in reverse) then quickly lifts their foot off the throttle the vehicle shudders.

Some of these vehicles may also exhibit a 3-2 tip-in flare condition at 21-24 mph.

SERVICE PROCEDURE: (Figures 1 and 2) ------------------ To repair either of the reverse shudder conditions install a new vacuum modulator pipe assembly which incorporates an in-line .031" brass orifice.

In the unlikely event that the new modulator pipe doesn't eliminate the reverse shudder, an updated calibration package may be installed on any OCJH, OCHH, OYLH, or OYKH model transaxle built before (Julian date 086) March 27, 1989. All units built after Julian date 086 were built with the updated calibration. The updated calibration package cannot be used on any other models.

If the new modulator pipe and updated calibration package doesn't eliminate the shudder then reference the 440-T4 Transaxle Diagnosis section in the W-car Service Manual for other potential causes.

To repair a 3-2 tip-in flare condition, on a vehicle equipped with a OCJH, OYLH, OCHH, or OYKH model transaxle built before (Julian date 086) March 27, 1989, an updated calibration package may be installed.

SERVICE PARTS:

Part Number Description Model Usage ----------- ----------- ------------ 10150082 Vacuum Modulator Pipe W/.031 orifice

8662993 Calibration Pkg.- Reverse OCJH, OYLH Shudder/3-2 Flare - Valve, Line Boost - Bushing, Line Boost Valve - Spring, Pressure Regulator Modulator Boost - Valve, Reverse Boost - Bushing, Reverse Boost - Spring, Reverse Boost - Plate, Valve Body Spacer - Gasket, Spacer Plate/Channel Plate - Gasket, Spacer PlateNalve Body - Instruction Sheet

8662992 Calibration Pkg.- Reverse OCHH, OYKH Shudder/3-2 Flare - Same Description as 8662993

Parts are currently available from GMSPO.

Use applicable labor operations and times.

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.