A/T - Harsh 1-2 and 2-3 Upshifts
NUMBER: 86-B-37GROUP: 7A-Automatic Transmission
DATE: February, 1986
SUBJECT: HARSH 1-2 AND 2-3 SHIFTS
MODELS: 1985-86 C, G WITH 4.3L ENGINE AND THM 400 AUTOMATIC TRANSMISSION
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.
Figure No. 1 - THM 400 Transmission Nameplate Information:
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 P/N 8655160 listed in the chart below. The vacuum modulator assembly (P/N 3030843) started in production on November 19, 1985 or Julian Date 323, (refer to Figure No.. 1) for all 1986 "LRA" and "LSA" models. Refer to the chart below for the new assemblies to be ordered from WDDGM:
Model Year Model Part Name Part Number Required
1985 VR, VS, VV Control Valve Assembly 8655160 Yes
Vaccum 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.