The main systems of a vehicle include the clutch system, the brake system, and the engine. These systems are composed of steel components that are set in motion when the vehicle is operating. They are, therefore, predisposed to friction and overheating. For that reason, vehicles depend on many fluids to be able to function, including clutch and transmission fluid.
What is the difference between clutch fluid and transmission fluid? Clutch fluid and transmission fluid are both fluids used in an automobile. They are similar in some respect in that they both possess lubricating properties. They also have some class of constituting components that are the same such as anti-rust and anti-oxidation compound.
Beyond this, they are quite different in a lot of ways. In this article, we shall systematically explore those dissimilarities.
Differences Between Clutch Fluid & Transmission Fluid
Regular transmission fluid is widely known for its red hue. On the other hand, clutch fluid usually appears amber in color when newly purchased. And yet both are used interchangeably.
Why? The confusion stems from the fact that there are two types of auto transmission systems. And each type has unique lubrication/hydraulic needs. Let us take a look at the significant distinctions.
Area of Operation
Clutch fluid is the same as brake fluid. Typically found in the master cylinder, which controls the slave cylinder when the pressure gets applied on it from the clutch pedal. The clutch fluid flows into the slave cylinder, and this operates the clutch fork leading to a clutch execution. When the clutch pedal is released, the clutch fluid simply flows back to the master cylinder.
Overall, the clutch fluid serves as a hydraulic to facilitate this rather weighty movement. Clutch fluid resides in the master cylinder and flows into the slave cylinder when the clutch pedal is pushed but never comes into direct contact with the gearbox cavity where the transmission fluid resides.
Transmission fluid remains in the gearbox, where it submerges the steel components, ensuring smooth mechanical motions and optimal performance. It saves the entire steel component from wear, which is a significant issue. It also prevents overheating and works to keep the transmission system from going out of order.
Transmission simply refers to the system of gears and other components which constitute the gearbox of a car. They coordinate the rotational speed and torque from the car engine. There are two types of transmission: manual transmission and automatic transmission.
Transmission fluids, unlike a clutch fluid, are basically of two types. There is the automatic transmission fluid (ATF) for the automatic transmission system, as the name implies. Like you can guess correctly, there is also the manual transmission fluid (MTF) for the manual transmission system. MTF may be ordinary motor oil, ATF, or hypoid gear oil.
The manufacturer will determine the type of MTF needed for your car. You can find this out by looking in the User's Guide. Common examples of transmission fluid include Synthetic fluid, Type-F, motor oil, High-Frequency Modified (HFM) fluid, Dexron/Mercon, hypoid gear oil, etc.
Composition and Classification
Most clutch oil in today's market is glycol-ether based. There are mineral oil, castor oil, and silicone-based fluids also. In the United States, based on the composition, they are classified by the department of transportation based on specific rating as DOT 3, DOT 4, and DOT5 and DOT 5.1. The silicone-based fluid is rated as DOT 5 and usually contains Di-2-Ethylhexyl sebacate, Dimethylpolysiloxane, or Tributyl phosphate.
The glycol-based fluid, which is rated DOT 3, 4, and 5.1, usually contain:
- Alkyl ester
- Aliphatic amine
- Diethylene glycol
- Diethylene glycol monoethyl ether
- Diethylene glycol monomethyl ether
- Dimethyl dipropylene glycol
- Polyethylene glycol monobutyl ether
- Polyethylene oxide
DOT 3 and DOT 4, unlike DOT 5, absorb moisture, a distinctive dissimilarity. Thus, you will need to prevent exposure of the fluid to air because it causes depression in the boiling point over time and makes clutch application difficult. This boiling point depression is very significant because clutch oil functions primarily as a hydraulic.
A reduction in the boiling point could cause the vaporization of the fluid. This vaporization is undesirable because gases can compress. Liquids (hydraulic fluid) have value because of they don't compress. This characteristic of hydraulics allows them to exert the needed force in a clutch or brake system.
Silicone-based based fluid, DOT 5, although nonabsorbent to moisture, should also not be left uncovered because the moisture not absorbed collects into a pocket of water and will erode the clutch system. DOT 5.1 is an upgrade from DOT 5. It contains 70 percent less silicone and is more temperature resistant. Different classes of clutch fluids should not mix because they may react badly and corrode the brake system.
This classification harmonizes with the Society of Automotive Engineers (SAE) specifications while also taking into consideration the local peculiarities. These peculiarities include the extreme climatic conditions of temperature and humidity in certain regions, for example, Russia. These conditions affect the quality of the fluid.
Some other countries have also adopted the SAE classification. The SAE standards include J1703, J1704, and 1705 and this represents the increasing performance of the clutch oil.
ATFs are composed of a base fluid plus a complex additive formulation intended to meet all of the physical and performance properties required of an ATF. The base fluid is usually a petroleum-based or synthetic hydrocarbon mixture with a viscosity of between 3.0 and 4.5 cSt at 100°C. Viscosity at low temperature, volatility, and oxidation stability are essential criteria in the selection of base fluid.
ATF specifications, to a considerable extent, have been determined by the manufacturing companies. There are many specifications for ATF. MERCON series for Ford company and DEXRON for the General Motors company. Below is the ATF specification for DEXRON.
General Motors ATF Specification
The first automatic transmission specification released in 1949
Type A Suffix A
Models 1957. An outdated specification, but still in use. Suffix A means improved oxidation properties.
Models 1973. The most used specification. It includes special requirements for low static coefficient of friction, high oxidation stability, and good corrosion protection in the wet chamber.
DEXRON II D
DEXRON II E
Models 1991. Includes, in addition to laboratory tests of physical and chemical properties, some special requirements for low static coefficient of friction, high oxidation stability, flash point, and fire point, antifoaming agents, good corrosion protection in wet chamber, seal compatibility requirements.
DEXRON III F
Models 1994. The specification contains upgraded characteristics to the previous, and primarily higher flash points and fire points and lover flash and fire tendency. A successor of DEXRON II D and DEXRON Il E.
DEXRON III G
This is the successor of DEXRON III (F) automatic transmission fluids. According to specification, a fluid similar to DEXRON Il E, however, with upgraded anti-oxidation and anti-wear properties. It was launched in 1997.
DEXRON III H
DEXRON III H launched in June 2003, to replace DEXRON III G fluids. They contain base oils of very high oxidation stability (groups 2 and 3). Fluids from this group have excellent friction and anti-wear properties, better flash and fire control, and longer service interval.
This specification released in 2005 to replace DEXRON III H fluids. The specification provides higher fluid slipping stability, good oxidation stability, and good antifoaming properties. Fluids that satisfy this specification can be used in an extended service interval and provide significant energy savings.
Clutch fluid absorbs moisture from the atmosphere and is said to hygroscopic. According to the society of automotive engineers (SAE), it must be changed annually or after 10,000 miles of usage. It degrades on exposure to moisture and oxygen and could damage the clutch system if not replaced.
Most automatic transmission fluid will last for a very long time if they have the gearbox hermetically sealed. Some cars have lifetime transmission fluid. This transmission fluid, the manufacturers claim, will last throughout the lifetime of the vehicle. A lifetime means 180,000 km or 112,000 miles as the lifetime of a car or transmission. However, two areas provide opportunities for entry of air: the transmission vent and the dipstick tube.
A dipstick is a rod used to inspect the level of transmission fluid in the transmission system because the gearbox is usually inaccessible. Using the traditional dipstick, however, creates a problem because it provides an entry point for air to access the transmission system leading to oxidation of the transmission fluid.
Dirt may find its way into the transmission as well. This situation worsens when the dipstick is not fully seated in the dipstick tube, or the dipstick tube plug is not fully seated.
Many modern transmission manufacturers have removed the traditional dipstick in favor of a sealed transmission. And so as expected, the transmission fluid in the sealed tube has more excellent durability than the traditional non-sealed transmission.
A transmission vent balances the fluctuating pressure changes that occur with changes in transmission fluid volumes and transmission fluid temperatures. Resultant gasket and seal leaks would arise if these pressure changes go unchecked and are allowed to build.
Traditional transmissions breather vent used Transmission Air Breathing Suppressor (TABS) valve to stop air and moisture from accessing the transmission.
Modern transmission manufacturers now use a unique breather vent, which is much smaller and can shut out moisture but allows entry of small amounts of air as needed to balance the fluctuating pressure inside the transmission.
To ensure the durability of automatic transmission fluid, use sealed containers, and buy only new fluid. A non-sealed container would expose the fluid to air and moisture and its resultant effects.
Never reuse transmission fluid. Always use clean fluid whenever you are performing a repair or refilling the transmission.
Clutch fluid is made corrosion resistant and noncorrosive to the clutch or brake system during production. Otherwise, the components of the clutch system like the master cylinder and slave cylinder can become severely damaged. Corrosion-inhibitors are usually incorporated in the mix when manufacturing clutch fluids. DOT 3 and DOT 4 are corrosive to paintwork and should not be allowed to come in contact with painted surfaces.
Rust and corrosion inhibitors constitute the makeup of all transmission fluid.
Service and Maintenance
The corrosion inhibitors in the clutch fluid are subject to degradation. There is corrosion in the clutch system as the fluid degrades along with the presence of excess moisture. Two years after service, the moisture content in the master cylinder reservoir may be as high as 8 percent. This phenomenon of high moisture content can lead to vapor lock and cause a complete failure of the clutch system. Therefore, routinely check your clutch fluid level.
A clutch fluid replacement usually starts with the opening of the master cylinder reservoir and monitoring the condition of the fluid. Scrutinize for any leak in the hydraulic system. And last, top off the master cylinder if there is a need. Clutch fluid replacement is a routine maintenance check and should be carried out every two years or 40,000km.
Here are some signs that could help decide if a clutch fluid replacement is necessary.
- The clutch is challenging to engage after driving aggressively.
- Grinding gears when the clutch pedal is fully pressed.
- The clutch fluid is black or dark brown.
- The clutch gives a gritty feel in between the fingers when rubbed.
Clutch fluid with different DOT ratings should not be mixed. DOT 5 should not be combined with any other the lass because the mixing of a silicone-based fluid and glycol can cause corrosion because of trapped moisture. DOT 3, DOT 4, and DOT 5.1 can be mixed since they are all based on glycol esters. DOT 2 fluid should not be mixed with any of the others. It is highly recommended to change existing fluids with new ones to obtain the ideal performance.
Many manufacturers will recommend that manual transmission fluid be changed every 30,000 to 60,000 miles. Under heavy-duty use, some manufacturers suggest changing fluid every 15,000 miles.
Fluid contamination is a much bigger problem than fluid degradation for manual transmission. Majorly because the mass of metal debris floating in the fluid increases as the components of the transmission such as gears, bearings, and synchronizers wear out. The result of this is that over time the fluid gradually loses its lubricating quality. Naturally, the solution to this is to drain out the fluid with its impurities to lengthen the lifespan of the transmission.
Checking the transmission fluid level is usually done using the dipstick. It can be a little difficult, and so, consequently, it is best to ask your mechanic to do this whenever you have a fluid replacement.
An automatic transmission does not have a clearly defined service interval, mostly because it is usually designed to last throughout the lifetime of the car without any service need. However, some authorities recommend between 30,000 miles and 100,000 miles.
More heat is generated compared to a manual transmission, and so there is a breakdown and degradation of ATF with use. There is contamination with metal debris like in manual transmission. Therefore there is a need for a drain out and replacement of ATF. Failure to do so may result in a shortening of the lifespan of the transmission and increased financial expenses. The dipstick can be used to check the ATF level for most automatic transmission cars.
Transmission oil doesn't dry out like regular engine oil. Reduction in the level of fluid always suggests a possible leak.
Viscosity is a measure of the resistance of a fluid (liquid or gas) to flow. It fluctuates in value with changes in temperature. Viscosity is measured in pascal-second (Pa-s) or dynes, which is the force needed to move a body one square centimeter in the area through a parallel body at a speed of one centimeter per second.
Viscosity is an essential factor for the road worthiness of cars because it affects the way the clutch or even the brake system functions and changes their behavior as well as their mode of operation. Clutch fluid must function optimally within a temperature range of between -40°C and +100°C.
They must operate even in extreme temperatures. Clutch fluid used for military operations, must function optimally at a more acute temperature range of -55°C and +100°C.
The need for clutch fluid to function optimally becomes even more critical in cars with stability control, anti-lock braking system, and traction control. Some manufacturers state their viscosity as a single value such as +40°C, while also stating alongside the viscosity index.
DOT 4 and DOT 5.1 clutch fluids can have low viscosity, which fulfills the maximum viscosity of 750 mm²/s at a temperature of -40°C. Not all cars installed with an anti-lock braking system or stability control approve of DOT 5.1 clutch fluid even though they come specified at a wide temperature range at a low viscosity.
Viscosity is a crucial physical property in transmission fluid design. ATF's are usually less viscous than clutch fluid. Viscosity grades in ATF's are generally not subject to approval or recommendation by the SAE or other regulatory bodies. The manufacturer determines their standard, such as General Motors' DEXRON and Ford's MERCON ATF viscosity specifications.
Viscosity Table - Measurement Data
ATF III - Mineral Automatic Transmission Fluid
Dyn. Viscosity [mPa.s]
Kin. Viscosity [mm²/s]
(Source: Anton-Parar Wiki)
In conclusion, clutch and transmission fluid are different from one another. Clutch fluid is used mainly as a hydraulic to facilitate a clutch execution while Transmission fluid keeps the gearbox lubricated. Clutch oil is composed mainly of glycol-ether or silicone, while transmission fluid is composed of a petroleum-based or synthetic base fluid. Clutch fluid is usually changed routinely, but transmission fluid can last throughout the lifetime of a vehicle.