What is power steering fluid, as well as its types and differences

Hydraulic power steering (GUR) is a system that is part of the steering of a car and is designed to reduce the driver's efforts when turning the driving wheels. It is a closed circuit, inside which there is a power steering fluid. In the article, we will consider the types of power steering fluids, their characteristics and differences.

What is power steering

First, we will briefly consider the power steering device. As already mentioned, the system is closed, which means it is under pressure. The power steering includes a pump, a steering rack with a hydraulic cylinder, a reservoir with a fluid supply, a pressure regulator (bypass valve), a control spool, as well as pressure and return pipelines.

When the steering wheel is turned, the control valve rotates to change the hydraulic flow. The hydraulic cylinder is integrated with the steering rack and works in both directions. The pump is belt driven by the motor and creates operating pressure in the system. The bypass valve regulates the pressure, draining excess fluid as needed. A special oil is used as a liquid in the system.

Hydraulic booster fluid

Power steering fluid transfers the pressure generated by the pump to the piston of the hydraulic cylinder. This is its main function, but there are others:

• lubrication and cooling of power steering system units;
• corrosion protection.

On average, about one liter of fluid will intervene in the power steering system. It is poured through a tank, which usually has level indicators, sometimes recommendations for the type of fluid.

There is a large selection of liquids on the market that differ in chemical composition (synthetic or mineral) and color (green, red, yellow). Also, the driver needs to navigate the abbreviations and names of fluids for power steering. Modern systems use:

• PSF (Power Steering Fluid) - power steering fluids.
• ATF (Automatic Transmission Fluid) - automatic transmission fluids.
• Dexron II, III and Multi HF are trademarks.

Types of fluids for power steering

Power steering fluids must have different properties, which are provided by additives and chemical composition. Among them:

• required viscosity index;
• resistance to temperatures;
• mechanical and hydraulic properties;
• corrosion protection;
• anti-foam properties;
• lubricating properties.

All these characteristics, to one degree or another, are possessed by all power steering fluids on the market.

In turn, the chemical composition is distinguished:

• synthetic;
• semi-synthetic;
• mineral oils.

Let's look at their differences and scope.

Synthetic

Synthetics are based on hydrocarbons (alkylbenzenes, polyalphaolefins) and various ethers. All of these compounds are obtained as a result of directed chemical synthesis from oil. This is the base to which various additives are added. Synthetic oils have the following benefits:

• high viscosity index;
• thermo-oxidative stability;
• long service life;
• low volatility;
• resistance to low and high temperatures;
• excellent anti-corrosion, anti-foam and lubricating properties.

But even with these characteristics, fully synthetic oils are rarely used in power steering systems due to the multitude of rubber seals that synthetics can attack aggressively. Synthetics are used only if approved by the manufacturer. Another disadvantage of synthetics is the high price.

Semi-synthetic

To neutralize the aggressive effect on rubber parts, manufacturers add a variety of silicone additives.

Mineral

Mineral oils are based on various petroleum fractions such as naphthenes and paraffins. 97% is a mineral base, the other 3% are additives. Such oils are more applicable for power steering, as they are neutral to rubber elements. Working temperature in the range from -40 ° С to 90 ° С. Synthetics work up to 130 ° C-150 ° C, the lower limit is similar. Mineral oils are affordable, but in other respects they are inferior to synthetic oils. This applies to service life, foaming and lubricating properties.

What kind of oil to pour into the power steering - synthetic or mineral? First of all, the one that is recommended by the manufacturer.

Differences in color

As already mentioned, oils also differ in color - red, yellow, green. They are both mineral and synthetic and semi-synthetic.

Red

They belong to the ATF class, that is, transmission. Most often used for automatic transmissions, but sometimes also applicable for power steering. Red marks Dexron II and Dexron III are the development of the carmaker General Motors. There are other red brands, but they are manufactured under license from General Motors.

Yellow

The development of the Daimler AG concern, respectively, is most often used in the brands Mercedes-Benz, Maybach, AMG, Smart and others. They belong to the class of universal ones for hydraulic boosters and hydraulic suspensions. Mineral yellow oils are used for power steering. Popular yellow brands are Mobil and Total.

Green

The development of the VAG concern, respectively, is used in the brands Volkswagen, Porsche, Audi, Lamborghini, Bentley, Seat, Scania, MAN and others. They belong to the PSF class, that is, they are used only in power steering.

Daimler also manufactures its green PSF counterparts under the popular Pentosin brand.

Can I mix different colors

It should be said right away that it is generally better not to allow mixing of different oils, even if this is allowed. Synthetic and mineral oils should never be mixed due to their differences in chemical composition.

You can mix yellow and red in color, since their chemical composition is in many ways similar. The additives will not react with other substances. But it is better to change this mixture to a homogeneous one.

Green oils cannot be mixed with others, as they have a universal chemical structure, that is, synthetic and mineral constituents.

The oils have to be mixed during refilling, when the level of the liquid in the reservoir drops. This indicates a leak that needs to be identified and repaired.

Leakage signs

Signs that may indicate a power steering fluid leak or talk about the need to replace it:

• falling level in the tank;
• leaks appeared on the seals or oil seals of the system;
• a knock is heard in the steering rack when driving;
• the steering wheel turns tightly, with effort;
• the power steering pump emits extraneous noises, hum.

To fill the power steering fluid, you must first of all use the manufacturer's recommendations. Try to use one brand without mixing. If you have to mix different oils, remember that mineral and synthetic oils are incompatible, even if they are the same color. It is also necessary to regularly monitor the oil level and its condition.