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Transmission Fluids
Background & Properties
July 2017
Types of Transmissions and Diagnosing Problems
A premium-performing hydrostatic transmission/hydraulic fluid must:
• Protect against metal-to-metal contact;
• Prevent wear in vane and piston pumps, transmission spur gears, final drive spur, and spiral bevel gears and axle
and transmission bearings;
• Provide proper frictional characteristics for wet brakes and clutches;
• Have exceptional water tolerance to maintain function and prevent corrosion in the presence of 1-2% water,
which may be found in these machines;
• Offer excellent extreme pressure protection for gears;
• Provide antiwear protection for hydraulic pumps; and
• Provide adequate lubrication over a wide range of temperatures.
Diagnosing Hydrostatic Transmission Problems
Improper fluid usage in a hydrostatic transmission typically causes brake squawk, harsh shifts, poor low temperature
performance and inadequate water tolerance. An ATF used in some transmissions will result in brake squawk,
inadequate water tolerance and insufficient film strength for gear protection.
Manual Transmissions
Unlike automatic, power shift and hydrostatic transmissions, manual transmissions do not use fluid to transfer power.
Power from the engine is distributed through the transmission by engaging and disengaging gears through the clutch
and collars. Gears will move freely unless engaged by a collar.
Premium manual transmission fluids are designed to:
• Prevent metal-to-metal contact and wear;
• Provide cold flow characteristics and maintain proper viscosity over a wide
temperature range;
• Be compatible with seals;
• Maintain thermal stability throughout drain interval; and
• Control oxidation.
Diagnosing Manual Transmission Problems
The use of an inadequate fluid in most manual transmissions will produce burnt and/or slipping clutches, insufficient
wear protection for gears, harsh shifts, premature thermal degradation, poor low temperature performance, mediocre
transmission response, sludge and deposit formation and copper and bronze corrosion.
Friction Characteristics:
Similar to a gear oil, manual transmission fluids need to protect gears from metal to metal contact, disperse heat, and
seal the component. Friction modifiers are utilized in manual transmissions and are needed for proper synchronizer
friction. These friction modifiers utilize a low static coefficient of friction to allow smooth shifting between collar and
gears. Typically, manual transmission fluids will require a 50 weight or a 75W-85 transmission fluid. Newer ones are
moving toward lower viscosity.
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