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What is the difference between automatic and manual transmission fluid?

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While automatic and manual transmission fluids are both lubricants used in automotive vehicles, they differ in the complexity of the roles they perform. Understandably, automatic transmissions are far more complicated items of equipment than manual systems. As a result, the fluids they require to function correctly are equally complex. In this blog, we’ll look at the functions that automatic and manual transmission fluids are tasked with to highlight their differences.

Understanding the functions of automatic transmission fluids (ATFs)

ATF must perform multiple functions, including acting as a hydraulic fluid, providing proper frictional requirements, reducing heat, and protecting gear against wear.

Serving as a hydraulic fluid

An automatic transmission uses pressurised fluid to accomplish gear changes. ATFs act as a hydraulic fluid. When a vehicle’s onboard computer requires a gear shift, it transmits an electrical signal to the correct transmission solenoid. This solenoid then directs fluid via a complex network of passages within the valve body and engages the correct gear. The ATF hydraulicly squeezes several plates together within the clutch pack and connects the engine to the car’s transmission output shaft to route power to its wheels.

When a transmission is functioning correctly, this process occurs instantly and largely unobserved. However, if the transmission fluid’s viscosity is too high, it will not flow swiftly to create the confident crisp shifts desired. As a result, ATF has a lower viscosity index and is much thinner than manual transmission fluid.

ATF fluid which accumulates foam can also negate its usefulness as a hydraulic fluid. As foam bubbles break down under pressure, elongated and inconsistent shifts occur along with wear to gears. To combat this, ATFs also include additives called foam inhibitors.

Providing the right frictional requirements.

As mentioned, pressurised ATF squeezes clutch packs together to engage the right gears. Clutch packs are composed of bare metal plates and other plates which are coated with friction material. Disengagement and engagement must happen seamlessly to optimise driving experience for the vehicle user.

The ATFs frictional properties decide whether this complex choreography of fluid and moving metal produces precise shifts or has drivers changing transmission fluid. As a result, ATF is formulated to deliver specific frictional properties which manual fluids don’t require.

Battling Heat

The nemesis of an ATF is heat, which breaks down the fluid via oxidation. Degraded ATFs lead to varnish and sludge deposits which can jam narrow oil passages and contribute to hard clutch shifts, jerks and hesitations. Running hotter than manuals, automatic transmissions require fluids with enhanced heat protection.

Protecting gears against wear

Finally, automatic transmissions feature an array of components like sun, ring and planet gears that need lubrication to defend against wear. The ATF must form a robust fluid film on all metal surfaces to stop metal-to-metal contact and inhibit wear.

Understanding manual transmission fluid

While less complex, manual transmission fluids must still empower smooth shifts, facilitate shift feel, fight wear, and protect components.

Enabling smooth gear shifts

A manual gearbox that shifts smoothly connects drivers and their vehicles closer. As a result, many auto enthusiasts avoid transmission fluids that sever this link. Manual and automatic fluids both deliver smooth shifts, but the process involves different component architecture.

Manual transmissions are mostly fitted with synchronizers. The synchro is engineered to equalize its speed with the gear being engaged, enabling a smooth gear shift. Without this, the spinning gears rotating at different speeds would never interconnect as required.

The synchronizer unit has two key components: the blocker (sometimes called the synchroniser ring) and the sleeve. When a driver selects the first gear for instance, the sleeve moves to first gear and then locks onto the gear’s engagement teeth. When the clutch pedal is depressed, and second gear is selected the sleeve moves the other way and selects second gear. However, before the sleeve can lock on, the spinning speed of the components must be synchronised to avoid any clashing.

If the viscosity of a fluid is too high, it can prevent shifting until the vehicle’s transmission warms up or abnormally high temperatures can occur during operation. However, if viscosity is too low, it can cause the synchroniser and teeth to engage too swiftly, causing hard shifts, grinding and extreme transmission wear.

Combatting wear

As with an ATF, manual transmission fluid must also protect parts from wear. Manual transmission fluids tend to have a higher viscosity index, which helps the fluid to offer a thick and durable film in order to deliver high protection levels.

Protecting brass synchronisers

Manual transmission synchronisers are commonly composed of brass, a softer material than most metals. Some lubricant additives are not suitable for brass and can cause damage to the synchronisers. As a result, high-quality manual transmission fluids are formulated to be compatible with brass synchronisers to ensure they remain protected.

For optimum driving experiences and the highest levels of performance and protection, always use the correct type of fluid for your vehicle’s transmission.

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