A/T - Harsh 3-2 Downshifts & Other Hard Shift Conditions: Overview
Bulletin No.: T-93-1File in Group: 7
Number: 245
Corp.Ref.No.: 277162R
Date: Oct. 1992
SUBJECT:
SERVICE INFORMATION FOR HARSH 3-2 DOWNSHIFTS AND OTHER HARSH SHIFT CONDITIONS
MODELS AFFECTED:
1985-1990 DEVILLES AND FLEETWOODS
1986-1990 ELDORADOS AND SEVILLES WITH HYDRA-MATIC 4T60 (THM 440-T4)
This bulletin cancels and replaces Service bulletin T-89-93 which should be discarded.
Some 1985-1990 DeVilles and Fleetwoods and 1986-1990 Eldorados and Sevilles may experience one or more of the following conditions:
I. Very Harsh Part Throttle 3-2 Shift At 20-40 MPH: 1985-1990 DeVilles and Fleetwoods and 1986-1990 Eldorados and Sevilles.
II. Harsh 3-2 and/or 4-2 Shifts: 1985-1990 DeVilles and Fleetwoods and 1986-1990 Eldorados and Sevilles.
III. Intermittent Harsh Part Throttle 3-2 Shift: 1985-1986 DeVilles and Fleetwoods and 1986 Eldorados and Sevilles.
IV. Harsh 3-2 Shift At High Altitude Only: 1985-1990 DeVilles and Fleetwoods and 1986-1990 Eldorados and Sevilles.
V. Harsh 3-2 Shift At Low Altitude Only: 1985-1990 DeVilles and Fleetwoods and 1986-1990 Eldorados and Sevilles.
VI. Harsh 3-2 Shift Maneuver At 17-20 MPH: 1985-1986 DeVilles and Fleetwoods and 1986 Eldorados and Sevilles.
VII. Harsh 3-2 Coast Down Bump At 15-22 MPH: 1989 DeVilles, Fleetwoods, Eldorados, and Sevilles.
VIII. Harsh shifts At High Altitude Only: 1985-1986 DeVilles and Fleetwoods and 1986 Eldorados and Sevilles.
IMPORTANT:
Before performing any service, verify the customer's concern is a 3-2 downshift and not a 2-1 downshift concern. ECM status light displays for fourth and third gears (where applicable) in diagnostics can be used as an aid to make this determination. Also, it would be an aid for diagnosis to put the transaxle in manual third or second before and after the concern to determine actual gears of the shift complaint.
This bulletin covers the listed harsh shift conditions and specific service information which apply to the correction of these conditions. Before resorting to the Service Packages available verify that the harsh shift condition is not caused by the following:
^ Incorrect Vacuum Response to the Modulator. Refer to Service bulletin T-87-33A. Remember, the available vacuum will be less at higher altitudes. Also, a restricted exhaust (plugged catalytic converter) can cause a low vacuum. Measure exhaust system pressure as outlined in Section 6F of the Service Information Manual.
^ Faulty Vacuum Modulator (19). Check for proper modulator function as outlined in Section 7 (Transaxle Unit Repair) of the appropriate Service Information Manual. Also verify the modulator valve (17) moves freely in its bore and is free of nicks or burrs.
^ Sticking 1-2 Servo Control Valve (373) in Control Valve Assembly (35).
^ Sticking 3-2 Control Valve (340) in Control Valve Assembly (35).
^ Missing or Mislocated Checkballs (2, 4, 12). (Refer to Figure 2).
^ Sticking 3-2 Coast Down Valve (218) in Oil Pump Assembly (44).
^ Improper TV Cable Adjustment. (Refer to Section 7A of the appropriate Service Information Manual for the proper TV cable adjustment procedure.)
IMPORTANT:
DO NOT USE General Motors limited slip differential lubricant additive or equivalent to eliminate undesirable "shift-feel" to reduce harsh shifting or to minimize torque converter apply/release "shudder". This will prematurely distress the clutches and bands in the transaxle/transmission by elongating shift engagement times and increasing slippage between surfaces, even though it might provide better shift-feel for the customer at first. The customer will be pleased at first but will be back to the dealership with a distressed transaxle/transmission.