A/T - Piston Seals Leaking
88chevy12Number: 88-94-7A
Section: 7A
Date: Dec., 1987
Subject: PISTON SEALS CUT/LEAKING - SERVICE AND PARTS INFORMATION FOR NEW ALUMINUM DIE CAST PISTONS
Model and Year: 1978-88 MODELS WITH THM 400/475 TRANSMISSIONS
TO: ALL CHEVROLET DEALERS
This bulletin covers service and parts information for new aluminum die cast pistons used in the THM 400/475 forward, direct, and intermediate clutches. The new pistons reduce the occurrence of a cut and/or leaking seal.
FIGURE 1 - THM 400 TRANSMISSION IDENTIFICATION INFORMATION:
DATE OF CHANGE: (Figure 1)
July 21, 1987, (Julian date 202) for all 1987 and 1988 THM 400/475 transmission models.
FIGURE 2:
SERVICE ACTION:
When servicing a 1978 through 1987 THM 400/475 transmission, and replacement of a piston is necessary, use the stamped piston and apply ring assembly until current service stock is depleted. Once service stock has been depleted, the stamped piston assemblies will be superseded with the new die cast pistons.
For all 1988 THM 400/475 transmissions, the new die cast pistons have replaced the stamped piston and apply ring assemblies. These units should be serviced with the appropriate die cast piston listed under "Service Parts Information". Important: All other clutch assembly components, including the piston seals, remain the same (Figure 2).
ALUMINUM DIE CAST PISTON - 1988 FORWARD CLUTCH APPLICATION CHART:
ALUMINUM DIE CAST PISTON - 1988 DIRECT CLUTCH APPLICATION CHART:
ALUMINUM DIE CAST PISTON - 1988 INTERMEDIATE CLUTCH APPLICATION CHART:
SERVTCE PARTS INFORMATION
FIGURE 3:
SERVICE NOTE: (Figure 3)
With the introduction of belleville plates, (used with the Neutral/Drive garage shift package, referenced in dealer service bulletin 88-19 (Section 7A), the THM 400/475 forward clutch is manufactured with two distinct clutch assemblies depending on the cushion plate used. When the piston is followed by a dished (belleville) plate (612), the dished plate is followed by a steel plate and then alternate with composition plates to complete the assembly. When the piston is followed by a waved plate (611), the waved plate is followed by a composition plate and then alternate with steel plates to complete the assembly.