HARSH 1-2 AND 2-3 UP SHIFTS THM 400 (VR-VV)

VEHICLES AFFECTED: 1985-86 THM 400 Transmissions

A 1985 THM 400 'VR', 'VS', 'VV' or 1986 THM 400 'LRA', 'LSA' model transmission may exhibit a harsh 1-2 and 2-3 upshift condition. This condition may be caused by improper operation of one or more of the following items:

- Engine tune - Engine vacuum and/or vacuum supply to the modulator (Minimum 18 in. Hg. vacuum at hot engine idle speed) - Transmission pump operation and line pressures

A preliminary check of the items listed above should be made using diagnosis procedures contained in the appropriate Service Manual.

If the cause for the firm shift condition cannot be found, replacement of the control valve and/or modulator assemblies may be necessary. At the start of production for the 1986 model year, all 'LRA' and 'LSA' models were built with the new control valve assembly part no. 8655160 listed in the chart below. The vacuum modulator assembly (part no. 3030643) started in production on November 19, 1985 or Julian Date 323, (refer to Figure 1) for all 1986 'LRA' and 'LSA' models. Refer to the chart below for the new assemblies to be ordered from GM Parts.

Model Part year Model Part Name number Required --- ----- --------- ------ -------- 1985 VR, VS, VV Control Valve Assembly 8655160 Yes Vacuum Modulator Asm. 3030843 Yes

1986 LRA & LSA Vacuum Modulator Asm. 3030843 Yes Before Julian Date 323

Refer to the appropriate Service Manual for removal and installation procedures.

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.