DRIVE LINE; IDLE AND/OR PARK VIBRATION (DIAGNOSIS/REPAIR)

Subject: DRIVE LINE IDLE AND/OR PARK VIBRATION

Model and Year: 1988-91 CORSICA OR BERETTA WITH 2.8L OR 3.L1 ENGINE AND AUTOMATIC TRANSMISSION

Model and Year: 1988-91 "U" cars with 2.8L (VIN W, RPO LB6), or 3.1 L (VIN T, RPO LHO), 125C or 3T40 (RPO MD) Automatic Transaxle and FE1 Front Suspension.

Some customers of the above specified Corsica/Beretta may experience a vibration felt from the seat, floor, and/or steering wheel with the engine at idle and the vehicle in gear. The vibration may also be described as a noise or boom and could be more noticeable with the vehicle on an incline, with the air conditioning operating, and/or with the vehicle in park.

It has been found that this condition could be a third order engine vibration (firing frequency) being transmitted through the drive axles into the passenger compartment from the steering linkage and suspension.

When attempting to diagnose this condition, the following preliminary items should be checked first:

1. Inspect the engine and transaxle mounts for grounding or binding. Loosen the mounts, start engine and run at idle. Shift the engine through its gear ranges, then tighten the engine mounts to 67 N.m (50 lbs. ft.).

2. Inspect the exhaust system for grounding and align as required.

If either of the two preliminary checks listed above do not repair the vibration condition, replace BOTH cross-groove type drive axles with the newer designed tri-pot ("free-motion") type drive axles. Due to their design, the cross-grove type drive axle could act as a direct transmitter of the engine vibration. The tri-pot ("free-motion") drive axle has the ability to translate axial (in and out) motion in its housing and not transmit much of the vibration.

Refer to Service Parts information in this bulletin for the proper drive axle replacement part numbers. For the proper removal and installation procedure, refer to Drive Axle (Section 4D) of the Corsica/Beretta Service Manual. Warranty information is listed at the end of this bulletin.

If a "launch shudder" or wobble on wide open throttle acceleration is felt after installing the tri pot (free-motion) axles, the front end trim height should be measured and adjusted. Lowering (reducing) the vehicle trim height may reduce the shudder condition to an acceptable level. The front end trim heights may be adjusted by replacing the front coil springs with the next lower load springs. Each spring rate, listed in the Spring Chart in the

Parts Catalog, lowers the vehicle approximately 13.Omm (0.5in.). After BOTH new springs (LH and RH) are installed, the vehicles trim height must be remeasured to assure that the trim heights have been decreased.

For the proper front coil spring part numbers, refer to the Spring Charts in the Parts Catalog. To check vehicle trim height, refer to Steering, Suspension, Tires and Wheels Diagnosis (Section 3) of the Corsica/Beretta Service Manual. For the coil spring removal and installation procedure, refer to Front Suspension (Section 3C) of the Corsica/Beretta Service Manual.

Service Parts Information

Part Number Quantity Description ---------- -------- ------------------------ 26015000 1 RH Front Drive Shaft Kit 26015026 1 LH Front Drive Shaft Kit

Parts are currently available from GMSPO.

For the recommended proper front coil spring part numbers, refer to the Spring Charts in the Parts Catalog.

Warranty Information:

Labor Operation Number: T1737 Labor Time: Trouble Code Engine Mounts and 92 Exhaust System inspection 0.6 Hours

add time A: To align Exhaust 0.3 Hours

Engine Mounts and Exhaust T1737 92 System Inspection and replace both Drive Axles 1.5 Hours

add time A: to align exhaust 0.3 Hours

add time B: with locking wheel covers/lug nuts (one or all) 0.1 Hours

Engine Mounts and Exhaust T1737 92 System Inspection and replace BOTH drive axles and springs 3.5 Hours

add time A: to align exhaust 0.3 Hours

add time B: with locking wheel covers Aug nuts (one or all) 0.1 Hours

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.