Power steering fluid is a type of hydraulic fluid designed specifically for use in power steering systems on most modern automobiles, aircraft, and other vehicles. When serving its intended purpose, power steering fluid is the medium through which power is transferred from the steering wheel to the vehicle's steering mechanisms (in cars, the front wheels).
Power steering fluid, like all hydraulic fluids, must have low compressibility, meaning that the volume of the fluid will not be significantly reduced under pressure. To reduce the risk of combustion, power steering fluid typically has low volatility and high fire resistance. Quality power steering fluids are also highly viscous and flow well in a wide range of temperatures, which helps lubricate and protect moving parts within the hydraulic system.
In a typical power steering system, the power steering fluid is contained in a network of sealed pipes and reinforced hoses that are connected to a series of pistons. When one piston presses down into the hydraulic fluid, it forces the fluid level to rise in another part of the system due to the fluid's low compressibility.
Some components of a power steering system, such as rubber seals and metal components that may corrode over time, will eventually release contaminants and small particles into the power steering fluid reservoirs. Those contaminants can cause accelerated wear on the system and may eventually cause it to break down. To safeguard against that, power steering fluid is often formulated to protect the system by acting as a good lubricant, serving as a detergent and guarding against corrosion.
Power steering fluid is only used in vehicles and aircraft designed with power steering systems. The first commercial application was in the power steering system of the 1951 Chrysler Imperial, and other automobile and aircraft manufacturers gradually made power steering systems standard features. Today, nearly all new street-legal vehicles have power steering and require this fluid.
Since power steering fluid inevitably picks up contaminants and loose particles over time, it should be drained, flushed and replaced as part of routine maintenance. Some automobile and aircraft manufacturers may specify how often the fluid should be changed in a specific system, but the most reliable way to tell when it needs replacing is to look at the color of the fluid. On most vehicles, this is easy to check by removing the power steering fluid cap under the hood. If the fluid is brown, black or not at all transparent, it's due for a change.
