Power steering and the pumps that allow the hydraulically assisted power steering we have become accustomed to first appeared in 1938 on large trucks. Patented by Francis Davis of Pierce Arrow (1932), power steering was only used on large truck applications initially. In 1951, Chrysler introduced the first passenger car application that used a hydraulically-assisted power steering system using a hydraulic pump operated by the engine. Very quickly, the major OEMS all developed power steering systems for the heavy steel cars and trucks of the 1950s. Passenger cars that were easier to drive were now a popular purchase for the newly empowered wives, mothers, and daughters now wanting the mobility the post-war expansion was requiring.
Initially, there were four types of power steering pumps (roller, vane, slipper, and gear). The vane type is the most commonly encountered today. Regardless of the type, the function of the hydraulic pump was the same: to provide hydraulic force for driver steering assistance. The biggest difference between power steering pump systems today is how that pump is driven. With older-style conventional hydraulic power steering systems, the pump is driven by an engine belt constantly turning the pump regardless of whether the driver requires steering assist or not. With the ongoing goal of improved efficiency and the reduction of “parasitic” loss, many OEMs have gone to EPS (electric power steering). OEMs are currently using two types of EPS electrically driven power steering systems, with one being a hybrid type system using a small electric motor to drive a vane-type hydraulic pump that powers the hydraulic steering assistance. The other type of EPS system typically uses a 12-volt electric motor attached to a gearbox on the steering rack. A sensor on the shaft running from the steering wheel tells the EPS motor when and how much to “assist” the driver. This system provides power assistance only when needed, reducing the amount of power scavenged from the engine with the resulting improvement in efficiency with no steering assist that requires power steering fluid.
When power steering systems first began to appear, there was no standard fluid or fluid specification for power steering fluid. This resulted in the OEMs turning to the only automotive fluid readily available at that time that would perform as needed, GMs new Type A automatic transmission fluid. Power steering systems were a “new” technology, and the “new” automatic transmission fluid used was perfect for the new power steering systems. GM ATFs (DEXRON I, II, III) continued to be used as power steering fluids for several decades because they were readily available in automotive supply stores, and they were excellent hydraulic fluids in addition to the many other properties they possess. Using the GM-approved ATFs for power steering fluid persisted unchanged (even used by non-GM OEs) until the 1980s, when market dynamics evolved, and the focus on cost reduction and proprietary trademarked products available only from OE dealerships began to prevail. Some OEs switched to their newly licensed and copyrighted ATF for power steering fluid, while others specified a purpose-made PSF (power steering fluid) to replace the more costly ATF they had been using. Meanwhile, in Europe and Asia, the trend was to replace the GM ATF with their own proprietary ATF or higher quality power steering fluid (often made with synthetic base oils), also available only through their dealer network. Honda/Lexus became somewhat of an outlier when they chose to specify a PSF of higher viscosity than other OEMs. This meant that a dealership was the only source for these new fluids. They were generally not available in the automotive aftermarket of that time.
Power steering fluid (not unlike ATF) is composed of base oil with conventional mineral oil base stocks often used for reasons of economy and synthetic base oils being used for severe duty or extended warranty power steering fluids. PSFs also contain an additive package to reduce or eliminate foaming, prevent or reduce wear, prevent oxidation, and promote compatibility for the different types of seals, hoses, and other chemically sensitive materials used in steering systems. In most ways, power steering fluids are like hydraulic fluids used to operate other hydraulic systems. As fluid needs have changed, so have the nature and type of lubricants specified for these systems. Many OEMs decided to go to power steering fluid designed for their specific application that was often less costly than the ATF previously used and more compatible with their system materials. On top of that, they provided an additional profit center if you are the primary source for that fluid.
With the increasing use of rack and pinion systems that use the same fluid as the power steering pump system, the problem of seal, hose, and rubber component chemical deterioration also became an issue. Some ATFs can have a chemical makeup that, over time, causes certain materials to “revert” back to their original components and become soft, wear easily, and start to leak. The answer to this problem was specific power steering fluids that provide the desired seal and material compatibility with the physical properties to function well as hydraulic fluid for the pump and protection for the steering components. Another issue with newer pumps and EPS pumps has to do with efficiency and the reduction in fluid viscosity to provide adequate protection with a minimum of “parasitic” loss due to viscosity. The use of electric motors to drive the vane pump for the EPS that use PSF is often limited to using reduced viscosity PSF (usually made with synthetic base oils), especially where climate conditions are extremely cold and winters are harsh. Poor extreme cold weather fluid performance can make the power steering fluid so thick it damages the electric drive motors, disabling the power steering system. There is a lot of controversy as lubricants become thinner for improved efficiency. Engine oils and transmission fluids have already dropped in viscosity with other lubricants (power steering fluid), followed by the drive for improved efficiency (better fuel economy) and system longevity (extended warranties). Conventional wisdom used to be, “You need thicker fluids for better protection.” In many applications today, lower viscosity lubricants are performing as well or better than higher viscosity lubricants in all but the most extreme temperature conditions where a thicker lube might provide some benefit.
System design can affect fluid type but not the basic fluid requirements for system protection and material compatibility. What becomes more important after the basic requirements for wear protection and system compatibility are met is the fluid’s ability to provide sufficient uniform viscosity, stability, and anti-wear properties for the fluid’s life. Shear stability can be an issue with very inexpensive power steering fluids. Even though they may start out thick, it doesn’t take long for the cheaper base oils to shear down to a viscosity that provides very little system protection.
Scheduled preventative maintenance of power steering systems is a fairly new concept for many OEMs. For years, there were very few guide criteria other than looking at the fluid and deciding it shouldn’t be black or dark brown. With warranty costs being as high as they are, many OEMs now specify a mileage interval for preventative maintenance and provide the service at the dealership. The new fluid exchange machines that remove the fluid and replace power steering fluid from the fluid reservoir are reasonably effective at removing about 85% of the used fluid. If this fluid exchange is accompanied by the insertion of an inline filter (often magnetic and called a transmission filter) on the low-pressure return line, removal of dirt, wear metals, little pieces of hose (hoses will disintegrate in time on the inside and look good on the outside) will greatly extend the life of the system and components. If this type of service is performed every 30,000 miles, the cost is reasonable, and power steering systems and components will stay in excellent condition, far beyond the original warranty. Power steering systems and the fluids they use are the most neglected system on many vehicles and can suffer from neglect or low-quality, inexpensive power steering fluids.
The best way to prevent problems is to pay attention to what kind of fluid or fluid additives you use (power steering additives of known quality and performance can enhance and extend the life of fluid and components in most systems), change the fluid on a regular basis and use an inline filter on the low-pressure return line, and make sure the fluid you use is compatible with the viscosity and material requirements of the OEM that built the vehicle. You do not have to go to the OEM dealership for the expensive and sometimes hard-to-find power steering fluids used in vehicles today. With the aftermarket choices of quality lubricants available today, finding a fluid solution for your specific application shouldn’t be difficult and will allow you to meet or exceed the manufacturer’s requirements. On the other hand, if you decide to go to the dealer for the specific power steering fluid or service you require, you can be sure they will have a fluid solution that is specific to that application.
