HARSH SHIFT/3-4 SHIFT CYCLING DUE TO LOW VACUUM. GMC Truck GMC MD P Bus Chassis 1990

SUBJECT: HARSH SHIFT AND/OR 3-4 SHIFT CYCLING

MODELS: ALL 1980-1990 MEDIUM TRUCKS EQUIPPED WITH ALLISON AT-500 OR MT-600 SERIES TRANSMISSIONS AND CARBURETED 350,366 AND 427 GASOLINE EN- GINES (RPO'S LF5, L86 AND L43) WITH VACUUM MODULATOR (VIN CODES A, B AND E)

THIS BULLETIN CANCELS AND SUPERSEDES BULLETIN 90-7A-23 (CORPORATE NUMBER 967102) TO CORRECTTHE PART NUMBER FOR THE VACUUM MODULATOR. ALL COPIES OF 90-7A-23 SHOULD BE DESTROYED.

Some of the subject vehicles may experience a harsh shift and/or 3-4 shift cycling due to low vacuum.

To correct these conditions, a new "Service Only" vacuum modulator kit has been released.

PARTS INFORMATION:

Part Number Description ----------- ---------------------- 29501199 Vacuum Modulator Kit

Kit Includes:

29501199 Vacuum Modulator 8658110 O-Ring

Use the modulator kit only after appropriate trouble shooting has been completed. Verify the carburetor is perforining properly and the vacuum lines are in good condition (no leaks or splits) prior to installing the new kit.

The new vacuum modulator kit is purple in color and can be purchased through Allison Transmission Division Parts Distribution System.

For vehicles repaired under warranty use: Labor Operation: K6180 Labor Time: 0.2 Hrs.

Parts are currently available from your Allison Transmission Distributors and Dealers.

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.