BOOM-BEAT VIBRATION @ 38-40 MPH WITH TCC LOCK-UP (PROM)

VEHICLES AFFECTED: 1990 M VAN WITH 4.3 ENGINE

With the introduction of all new rear axles in the 1990 M van, the pinion angle was lowered. This angle change resulted in a boom-beat vibration in some 1990 M vans at 60-65 km/h (38-40 mph), following torque converter lock-up. The boom-beat is annoying but does not affect component life. Drivers can drive through the boom-beat by accelerating and avoiding TCC lock-up at 60-65 km/h (38-40 mph).

A service prom is available which delays converter lock-up until 70 km/h (45 mph). There are resonant frequencies present at 60-65 km/h (38-40 mph) which are not present at higher speeds, so the new prom is effective preventing the subject boom-beat.

Service Prom-Numbers:

70 km/h (45 mph) TCC Lock-up Prom Part # 16148810 Federal

The potential for the boom-beat vibration was corrected in production on October 30, 1989 when Baltimore Assembly began building vehicles with the rear pinion angle raised. This correction was made effective with the following VINS:

GMC LB509442 Chevrolet LB132066

The prom modification does not apply to 1990 M vans built after October 30, 1989, with serial numbers greater than those above.

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.