A/T - Harsh 1-2 and 2-3 Shifts
Number: 86-37Section: 7C
Date: FEB., 1986
Subject: 1985 AND 1986 THM 400 TRANSMISSIONS
HARSH 1-2 AND 2-3 SHIFTS
Model and Year: 1985-86 C AND G MODEL TRUCKS WITH
4.3L V6 ENGINE (VIN CODE N)
TO: ALL CHEVROLET DEALERS
A 1985 THM 400 "VR", "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 #8655160 listed in the chart below. The vacuum modular assembly (part #3030843) 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 for new assemblies to be ordered from WDDGM.
Model Year Model Part Name Part 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
TRANSMISSION NAMEPLATE INFORMATION:
Refer to the appropriate Service Manual for removal and installation procedures.