A/T - Boom-Beat Vibration After TCC Lock-Up
90chevy21Number: 90-134-4B
Section: 4B
Date: January 1990
Corp. Bulletin No.: 964102
Subject: BOOM - BEAT VIBRATION AT 38-40 MPH WITH TCC LOCK-UP
Model and Year: 1990 M VAN WITH 4.3L (VIN CODE Z) ENGINE AND 4L60 AUTOMATIC TRANSMISSION
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 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 38-40 mph.
A service prom is available which delays converter lock-up until 45 mph. There are resonant frequencies present at 38-40 mph which are not present at higher speeds, so the new prom is effective preventing the subject boom-beat.
Service Prom Numbers:
45 mph TCC Lock-up Prom Part # 16148810 Federal
16148808 California
Parts are currently available from GMSPO.
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 VIN:
LB132066
The prom modification does not apply to 1990 M vans built after October 30, 1989, with a serial number greater than that above.
For vehicles repaired under warranty, use:
Labor Operation Number: T0500
Labor Time: 0.6 Hrs.