Usage:
784B (5RK) Tractors;
785 (8GB),
785B (6HK),
789 (9ZC),
789B (7EK),
793 (3SJ),
793B (1HL) Off-Highway Trucks
New clutch piston and clutch balance piston assembly can be ordered through the normal distribution system or the existing clutch piston and clutch balance piston assembly can be modified.
To modify the existing clutch piston and clutch balance piston assembly, two transmission clutch piston return springs must be added to the No. 1 transmission clutch and add eight transmission clutch piston return springs to the No. 2 transmission clutch in the 793 and 793B Off-Highway Trucks. In the 784B Tractor; 785, 785B, 789, and 789B Off-Highway Trucks six piston return springs must be added to the No. 2 transmission clutch.
The clutch piston return springs have been added to provide additional piston retraction force to prevent possible clutch dragging when the balance piston cavities are empty.
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Clutch dragging would most likely occur when the machine is first started. However, the most critical time period is during the refill of the transmission oil system after an oil change. The engine should be cranked over with the starter or started for a very brief period of time until the transmission oil level drops to the bottom of the sight gage. The oil level should not be allowed to drop below the lower sight glass. The tank should be refilled and the procedure continued until the oil level has stabilized at the desired level.
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The existing clutch piston and clutch balance piston assembly may be modified to the improved design by machining additional piston spring pockets in the clutch pistons as shown in the illustrations. 4J-1386 Springs must be added to the piston spring pockets. A spring bore diameter of either 21.0 mm (.83 in) or 21.5 mm (.85 in) may be used depending on available tooling.
Procedure For Modification Of Existing Clutch Piston And Clutch Balance Piston Assembly
Illustration 1. Modification of 6Y-3481 No. 1 Clutch Piston (793 and 793B Off-Highway Trucks).
Drill two holes in 6Y-3481 No. 1 Clutch Piston at Callout E as shown in Illustration 1. For dimensions refer to Chart 2.
Illustration 2. Modification of 6Y-2979 No. 1 Clutch Balance Piston (793 and 793B Off-Highway Trucks).
Drill two holes in 6Y-2979 No. 1 Clutch Balance Piston at Callout H as shown in Illustration 2. Refer to Chart 3 for dimensions.
Illustration 3. Modification of 6Y-3482 No. 2 Clutch Piston (793 and 793B Off-Highway Trucks).
Drill eight holes in 6Y-3482 No. 2 Clutch Piston at Callout M as shown in Illustration 3. For dimensions refer to Chart 4.
Illustration 4. Modification of 6Y-2981 No. 2 Clutch Balance Piston Assembly (793 and 793B Off-Highway Trucks).
Drill eight holes in 6Y-2981 No. 2 Clutch Balance Piston at Callout T as shown in Illustration 4. Refer to Chart 5 for dimensions.
Illustration 5. Modification of 9W-3897 No. 2 Clutch Balance Piston Assembly (784B Tractors; 785, 785B, 789 and 789B Off-Highway Trucks).
Drill six holes in 9W-3897 No. 2 Clutch Piston at Callout BB as shown in Illustration 5. Refer to Chart 6 for dimensions.
Illustration 6. Modification of 3T-0053 No. 2 Clutch Piston (784B Tractors; 785, 785B, 789 and 789B Off-Highway Trucks).
Drill six holes in 3T-0053 No. 2 Clutch Piston at Callout HH as shown in Illustration 6. Refer to Chart 7 for dimensions.