A/T - Harsh Shift Conditions: Overview
90buick06Bulletin: 90-7440-7A
Date: Apr. '90
Ref. No.: 90-070A-7440
Corp. Ref. No.: 977162
SUBJECT: HARSH SHIFT CONDITIONS AND ON CAR SERVICE INFORMATION FOR HARSH 3-2 DOWNSHIFTS
MODELS AFFECTED: 1985-1990 THM4T60 (HYDRA-MATIC 440T4) TRANSAXLES
THIS BULLETIN IS BEING REVISED TO CORRECT THE PART NUMBER LISTINGS. SOME PARTS ARE LISTED IN WRONG AREAS. PLEASE DISCARD BULLETIN 90-7440-7.
IMPORTANT: Before performing any service, make sure to diagnose the specific shift conditions and altitude that affects vehicle performance. DO NOT assume that every condition listed applies to the vehicle.
NOTICE: Do not use General Motors' Limited Slip Differential Lubricant Additive or equivalent to eliminate undesirable "shift-feel", reduce harsh shifting, or to minimize torque converter apply/release "shudder". This will prematurely distress the clutches and bands in the transaxles/ transmissions by elongating shift-engagement times and increasing slippage between surfaces, even though it might provide better shift-feel for the customer at first. Also, the static capacity (A measure of load-holding capacity) will be reduced by at least 50%, i.e. The transaxle/transmission will only be able to handle 1/2 the combined gross vehicle weight as it did before without slipping.
BULLETIN COVERS
Harsh shift conditions and the specific service information that applies to each of these conditions. However, some other items that affect transaxle operation which may also contribute to these harsh shift conditions are:
- Vacuum leaks (low vacuum supply to the vacuum modulator). 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 Manual.
- Paint in the modulator tube and/or carbon at the throttle body fitting (vacuum signal should respond quickly to throttle movement) restricting vacuum to the modulator, causing a delayed response to throttle movement.
- Sticking modulator valve
- Sticking 1-2 servo valve
- Missing checkballs (# 2, # 4 or # 12)
- Improper TV cable set (3-2 downshifts only)
Before performing any service on the transaxle, verify that the customer concern is a 3-2 downshift or 4-2 and not a 2-1 down-shift or harsh TCC/VCC apply/release. A TECH 1 or other scan tools can be used as an aid to make this determination. The TECH 1 or other scan tools will not service all model years; instead use GM Dealer Equipment TCC tester model GM-1 or GM Dealer Equipment Torque Convertor Clutch Tester part number 091-00001 to determine if the shift concern is TCC/VCC related. Also, it would be an aid for diagnosis to put the transaxle in Manual 4th, 3rd or 2nd before and after concern to determine actual gears of the shift complaint. This will help ensure that the customer concern will be addressed.