In Mercedes 126 Repair: Forgotten Fluids, Part I we looked at the rear differential. Now let's turn our attention to another fluid that most owners never bother to change – power steering fluid.
POWER STEERING FLUID CHANGE
Most Mercedes 126 owners will never take apart their recirculating-ball steering gear box. But those that do gain a new-found appreciation for the importance of clean fluid in the power-steering system. In addition to the main worm gear assembly and the interface between the ball nut and the sector gear that actually turns the Pitman arm, there are several sets of needle bearings. All these moving parts are lubricated by steady flows of power steering fluid. The more heavily contaminated that fluid is, the more quickly these parts will wear. And a worn steering box translates into sloppy steering, potential safety hazards, and an expensive repair or replacement. (The power steering pump, by contrast, is much, much easier to replace, and good used units can be sourced for as little as $ 50.)
There has been some debate about the propriety of using automatic transmission fluid (ATF) in this system. While ATF was specified in the owners manual, ATF is not what it used to be and has become somewhat hostile to the many seals within the steering system. Ideally, ATF should be eschewed in favor of plain old power steering fluid. While synthetic fluids are available, conventional fluids are perfectly fine.
To change the fluid, support the front of the car on jack stands so that both front wheels are off the ground. Undo the hose clamp securing the return hose to the return "snorkel" screwed into the pump, and direct the hose into a suitable container for catching the old fluid. Either plug the snorkel or loosen it with a 19mm wrench and turn it so that it points upwards, allowing you to fill the reservoir without losing fluid from the return fitting. The standard procedure here is to start the engine and add fluid while the pump quickly forces the old fluid out. But this procedure has some major disadvantages. It is most emphatically not a one-man operation. The flow rate is so fast that there is a great risk of the pump running dry and sucking air without a steady supply of new fluid is poured in. If things get out of hand, you need a helper to shut the engine off right away. If you do not have a helper and do not want to risk damaging the pump or pulling air into the system, you need a better way.
Fortunately, it is perfectly possible to pump out the old fluid in a highly controlled fashion simply by turning the steering wheel from stop to stop. Keep the fluid level in the reservoir above the filter to prevent the ingress of air, and continue until you are satisfied that the fluid leaving the system through the return hose is clean. You need 2-3 quarts to fully replenish the fluid, though if you're trying to remove all traces of (red) ATF you may find it takes a little more to end up with a completely clean reservoir. When you're happy, reconnect the return hose and thoroughly bleed the system by turning the steering wheel back and forth with the engine running.
This is, of course, the perfect opportunity to change the filter in the pump and inspect the rubber return hoses, replacing if necessary. The fact that Mercedes cave us a filter in the power steering pump tells us something about the importance of clean fluid. How many other car manufacturers do this?