Main.TrailIndexPage | Steering System | Power Steering Box

Power Steering Box

This component is part of Steering. German: Servolenkung


Describe, in general terms the function of this component. Meaning what is it there for and what role it plays. Describe how it works, the inside mechanism. Use diagrams to explain.


Describe common maintenance procedures, and common faults that may occur. Describe how these may be diagnosed and resolved. Again, include diagrams, photographs and explanations. Where possible, include measures, tolerances, weights etc.

Link to related components where appropriate.

Power Steering Box & Coupling

Reassembly of power steering box, torque settings

There are a number of torque settings that need to be observed when re-assembling the power steering box.

  1. To set the torque of the front bearing, the large hex nut that secures the bearing insert (round part with pin wrench holes on steering worm) should be loosened to allow bearing insert to be screwed in against the bearing face and Teflon seal. The insert should be screwed in until the steering worm shaft turning toque reaches correct torque—see below. Once this turning toque is reached, the bearing insert should be locked in position by tightening the large hex nut while holding the bearing insert in position with a pin wrench.
  2. There is a large round “nut’ with pin wrench holes that sits atop the pressure spring assembly. The pressure spring screw—this is the adjusting screw- runs through this and is secured with a regular hex nut and a domed nut. To properly adjust this, put the steering shaft arm in the straight ahead position. This can be done by removing the small bolt on the top of the box and observing the small detent that will be visible at the bottom of the screw hole. You can lock the box in this position by grinding down to a fine taper a suitable 8mm bolt. Be careful you select an 8mm bolt with fine threads that match the small bolt you removed. Once the box is locked in this position, the round nut with the two pin wrench holes should be screwed in an out several times and then tightened finally against the bottom stop with .1MKP. The adjusting screw that runs through this round nut should then be backed off ¼ turn and then tightened to a torque of .6 mkp. A dial test guage is then placed on the top of the adjusting screw and set to zero. The adjusting screw then gets turned in approx 1/8 turn and the hex nut on it is then torqued to 2.5-3MKP while holding the large nut with the pin wrench holes in position. The gauge ((Dial test Gauge) is supposed to go back to zero.
  3. Use a torque wrench to check the torque of the pitman arm. It should peak between 220-330cmkp

Torque settings

  • Turning torque of steering shaft before tightening the bearing insert: 0.8 -1.0cmkp (.69-.86 in-lbs)
  • Increase in turning torque after the bearing insert is tightened: 1.6 – 1.8 cmkp (1.38-1.56in lbs)
  • Friction torque of pitman arm after assembly: 220-300cmkp (15.9-21.7 Ft-lbs)

Helpful diagram by Dave H

Old Yahoo content

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Most recent setback. I took my ‘68 SL out for a quick spin the other day and noticed an intermittant feel in the power steering. Stopped and popped the hood to find all sorts of red fluid spattered about the engine compartment. As it's pretty darn cold now and my garage is too small to allow any work to be done inside, I haven't had the opportunity to check for the cause of the problem. My hope is it's just a hose. Anyone out there have any tips on the steering system? If I'm going to replace hoses, maybe I should do them all.

Frank says: the low pressure hose isn't very expensive, but a new high pressure hose will cost over $100. Suggest you first refill & bleed the system, then see if/where it is leaking.

Will: I assume you know where the PS reservoir is. There are 2 hoses attached to it. Both low pressure. The easiest hose to see goes from the reservoir back to the power steering gear box. The less easy hose goes forward from the reservoir to the PS pump. This short hose makes a 90 degree turn. Either hose could be leaking. Both hoses are clamped in place so check for a loose hose clamp. The high pressure hose goes from the PS pump to the PS gear box. It could have a leak. The low pressure hose is bulk hose, cut to length. The high pressure hose is a special made hose with aeroquip style fittings on the ends. You use wrenches to take this hose off. Remove the cover of the reservoir and fill with fluid. You can use ATF, but Mercedes does supply official power steering oil and it is not red. Leave the cover off and start the car. Be ready for a face full of fluid if the leak sprays up or if the fan catches it and blows it back. The leak should show up quickly. If not and you find the reservoir low on fluid or sucking air, pour in more fluid until you reach the line embossed in the side of the reservoir. Fluid in the reservoir is not under pressure and will not splash out. Escaping air bubbles will make the fluid foam until all the air is purged. Prior to or after the fix you may need to run a small plastic hose (1/4" clear vinyl like you find at the hardware store) from the bleed screw on top of the PS gear box to the open reservoir. Open the bleed screw and bleed air/oil into the reservoir. Turn the steering wheel lock to lock to bleed air. I suggest you use Mercedes hose designed for PS. Anything else may melt internally and gum up the very expensive and hard to rebuild PS gear box.

I got a chance to look into my previously reported power steering problem. I thought it might be a hose, but I found differently. When I drove the car (only about 20 feet forward and 20 feet back) the power steering made an odd "moaning" sound. When I shut the enging off, I heard a spitting sound from under the hood. I thought, good, I can see where the leak is. Not so good. There was fluid spurting from under the lid to the power steering resevoir and around the wing nut that holds the lid down. I could not see any other leaks from hoses, pump or steering box. Any ideas what might cause such a thing? I haven't had much of a chance to debug as I have to work in the driveway and it's still pretty cold in Philly. I'd appreciate any advice on procedure to locate the bad component. Could something be clogged?

Frank: You need to bleed it thoroughly from the bleed fitting on top of the box. First, though, change the filter in the reservoir if it has't been done recently.

Will adds: like Frank said, you need to bleed the system. There is a bleed screw on top of the steering gear box (this is what the steering column attaches to). Put a clear vinyl hose on it and run the other end to the open reservoir. Open the bleed screw and start the motor. You should see bubbles of air in the clear hose. Turn the wheels back and forth from lock to lock until there are no more bubbles. Only use Mercedes power steering fluid. You should be able to remove the lid from the power steering reservoir while the engine is running. No fluid should come out.

Walter Klatt adds: I had the same problem with my 1968 280 SEL. It was working fine for a long time and then out of the blue it started making funny gurgling and moaning noises, I thought it was the gas in the tank doing that. When I looked under the car I could see a bright red puddle slowly getting bigger, and opened the hood to see the steering fluid bubbling and foaming out the top of the reservoir. I tried the bleed method when putting new fluid in, but it still did the same thing. Then when I replaced the pump with a new unit the problem went away. My guess is if you had no problem before and then it shows up-the pump is bad like mine was.

Then Will again: if your pump is doing what Walter's did, perhaps the front seal is leaking. I am just guessing, but it could be the front seal is allowing air to be drawn into the system and this is getting pressurized. If you have to replace or repair the pump you have to get the pulley off before you can remove the pump. The pulley is on a taper with a key. The special tool to remove the pulley works best and prevents damage to the pulley by trying to pry it off.

I have a pump that is working well, but leaking. It will need replaced or resealed at some point. What "special tool" do you refer too? Is there a part number? Is it expensive?

Spent some time with my Haynes and MB chasis manual and got the picture of what was suggested by this forum. I had ordered a new PS filter from my friendly neighborhood MB dealer ($19.95). Had to suck out the old fluid with a turkey baster to figure out how to get the filter cover off. This was a good plan as the PS reservoir was full of gunk. Attached a length of clear hose from PS box bleed nipple and set the other end in the resevoir. Filled with new fluid and started the car. Opened the bleed nipple and watched the fluid circulate. Once there seemed to be no more bubbles, I did what was suggested and turned the wheel. That's where the adventure began. I read in Haynes to make sure one held the hose onto the nipple as pressure will increase when the wheel is turned and the hose could shoot off. Well, with arm deep into the engine compartment and eyes watching for telltale squirts, my wife slowly turn the wheel. All was fine until the other end of the hose liberated itself from the reservoir and shot PS fluid all over the place. Classic Three Stooges. Once the car and me were cleaned up, a quick road test changed my mood. Steering was perfect.

Will: good work on the PS system. My best story about the power steering is one day a man drives to my place and wants to show me his 230 SL he has owned for about a year. I show him the girlfriend's 230 SL and we compare notes. He finally asks if the steering on her 230 SL is as hard as his. I say the steering can be turned with a couple of fingers. He says he has to really work to steer the car and that is why the wife can't drive it. I get in, start the engine and turn the steering wheel. If you have ever driven a car where the power steering did not work, you know how hard it is to move the steering wheel. This was his car. I check the reservoir and it is bone dry and has been dry for sometime. I fill it with fluid, turn the wheel, bleed the box (takes just a few minutes) and wonder of wonders, his PS works. He is so excited he calls the wife on the cell phone. What amazed me is where did the fluid go? Why did the PS work after I filled it? I figured the pump would be ruined. And I never heard anything else on the PS, although he has called and visited since. To this day it is working fine and holding fluid.

Seems like my bleeding of the PS system didn't solve the problem. I think it's sucking air in somewhere. When I run the engine and turn the steering wheel, I get some leakage of PS fluid from the reservoir. When I open the lid with the engine running, the PS fluid in the reservoir is "frothing". I was hoping it wasn't a pump seal. Looking at where the pump is, looks like a lot of work to replace.

Does anyone know whether the steering pump of my 1970 280 SL, when connected to the fluid reservoir and filled with ATF, should shoot the fluid out at the output port after I have disconnected that fitting directly at the pump (and started the engine, of course). I just did that and even before running engine, the fluid started flowing out of the high pressure hose that comes from the steering box, but it never seemed to get pumped out of the high pressure outlet at the pump. As the fluid was flowing out of the high pressure hose, the level in the reservoir quickly dropped, even though the hose from the reservoir to the pump inlet was connected and tight. Here is a little more background information about my steering system. The system worked fine until I decided to install all new seals in the power steering pump and steering box and put everything back together correctly (certainly on the pump as these are fairly simple). Now everything's installed again on a fully restored car and the system is not working (there are no leaks anywhere). When I filled in the ATF and started bleeding the system according to MB procedure, it seemed like air bubbles were properly escaping out the bleed hose which I had inserted into the fluid in the reservoir at first, maybe not quite as much as I have usually seen when bleeding the steering. After several weeks, even months of fiddling with it, hoping that some stray air pocket may find its way out and everything would be fine, I am concluding that I must have made some mistake. As you know it is very difficult to work on the pump and steering box once everything is installed. However, at the present I happen to have the radiator out to fix another leak, so I have access to the pump at least.

Today I disconnected the high pressure hose directly at the power steering pump which I had resealed a while ago. The engine was running when I did this and I expected that the ATF would come gushing out fairly noticably even at idle. Actually it merely trickled out, in my opinion probably much the way it would just from the static pressure of the higher mounted fluid reservoir as it was emptying out via the pump. Even when I revved the engine higher the flow of ATF increased only marginally. Can I assume that I should expect a much stronger flow than I have on a working pump? Or is the flow of these vane pumps very small and the pressure high? I hope somebody out there can help me from their experience before I go ahead and pull the pump out and try a replacement.

I got the hoses all repaired on my 230SL and took it out for a run over the weekend. Things felt great, it was running well, despite a year in the garage. Yes, things went well until...... The power steering pump went out. It looks like there is a relief valve that used to be on the high pressure side of the pump. There is a very round hole, approximately 1/8 inch in diameter there. All the fluid was lost out of that hole. So, what do I do now? Do I have to repair the pump or can I simply replace whatever it was that went in the hole? Pulling the pump out is a real bear.

You can get a pressure valve kit, a seal kit for the rest of the pump, or get a new pump.

Well, my week is not going too well.... after a ride the other day I noticed the power steering had a minor leak. I replaced the missing fluid and took another ride. When I got back the leak was worse, in fact severe. The leak seems to coming from the bottom of the pump itself. I wonder if it is practical to rebuild the pump?

Joe Alexander: Here is my experience with these Vickers power steering pumps; They are very expensive new, however they are usually bomb proof. Complete failure needing replacement is rare. Seal kits for the pump are cheap. Major rebuild kits are available but the seal kit will usually work. Check to make sure that the bolts holding the pump together are not coming loose and causing leaks. Few people realize that their is a replaceable filter for the fluid in the reservoir. It is in the bottom of the resevour underneath the sheetmetal bottom. Used pumps are readily available since the same pump was used on most Mercedes cars of the same era. There were two basic types early and late. The early ones had a straight shaft with key that the pulley mounted on and the later have a tappered shaft with key that the pulley mounted on. Mercedes changed sometime in the mid 60's to the tappered shaft version. They are interchangeable as long as you keep the correct pulley with the correct pump. The steering box has a bleeder to bleed out air, however the system will purge all the air out itself if you give it a chance. Removal of the pump can be a little tricky since the pulley must be removed before the pump body can be detached from the mounting bracket. Be careful not to loose the spacer on ther rear mounting bolt between the pump body and the bracket during pump removal.

For some idiotic reason when I pulled my leaking power steering pump off yesterday, I had the bright idea of removing the three bolts that keep the metal halves of the main body together (I was just curious, and it does look pretty cool inside the pump). I had a leaking front seal (about $4.00). The two seals that join the metal halves together look like some kind of strange hybrid metal of some sort. I am assuming that if I just bolt the two halves back together its definitely going to leak along that metal edge. Can I use an RTV silicon or some other type of gasket seal to reassemble the unit. It worked perfectly fine aside from the leaky front seal.

I have a leak in the Power Steering Gearbox. Oil is coming out of the round cover on top of the gearbox, the one that is been held down by two nuts. Can I just open the lid and replace the seal / “ O “ ring inside, or is it worse ?? Also, the oil is red, and looks ATF to me, is this right ?

I am very interested to hear about how it can be repaired. I checked and a kit to replace the seals is about $20 US. I do not know how difficult it is to replace the seal but, I hope that someone can provide us some insite into how the job is done.

Bob Smith: I had to have my power steering box rebuilt because there was a constant leak of fluid. It wasn't a simple seal replacement because there was too much wander in the steering which was caused by wear of some sort in the guts of the power steering box. To reduce the steering wander even more, the steering flexible coupling was also replaced. Now the car drives straight all the time.

As far as rebuilding the power steering box, I did that as well using the MB rebuild kit consisting only of seals. I would say this is a job for an experienced shade tree mechanic, but if you want to replace exterior seals for some leaking shafts only, the task is quite simple once the box is out. After that, be very careful during complete disassembly as there are tiny recirculating balls inside the box that support the shaft. During installation, after thorough inspection and cleaning, I used a light grease to hold the balls in place for assembly. I felt a little uneasy and always had the feeling that I lost one ball in the process, but everything has worked fine for the past two years now. While I had it apart I repainted the box and cleaned up the aluminum lid etc too.

I have found that I am leaking quite a bit of power steering fluid, and the culprit seems to be the power steering pump (the steering box is fine, although I understand that this is the most common place for leaks to occur). So, one of my 'winter' (summer here) projects will be to overhaul the power steering pump. I have already aquired the repair kit from MB, but would appreciate any suggestions or experiences from the group to assist me on this one - I am at best a back yard mechanic, my only previous experiences with mechanical things was rebuilding a mini 1000 motor a few year back... or do I leave it to the experts?

Another guy wrote about this on another site. He took the pump case apart out of curiosity, and found that he shouldn't have done it. Apparently there is a special seal between the case halves that is not part of the kit (?). At any rate, I guess the advice is don't split the case if you don't need to.

I replaced my power steering pump recently. It's pretty easy if you have the right tool for removing the pully. Make sure you get one because a standard wheel pull will just bend the pully. Mine was on very tight. I found that with the right pressure (from the new pump) my worm gear began to leak. That's my next task.

Can anyone (Joe?) please tell me if I can remove the power steering pump without taking out the radiator on my 280Sl with AC? I know the pump won't come off the bracket (cradle) without removing the pulley front the front end first and it also seems like the cradle can't be unbolted from the engine block without removing the pump first. Is there an easy way to see where my PS leak is coming from? The seal isn't even very old and it's leaking pretty badly.

Joe: yes the power steering pump can be removed without tearing half the engine apart. There is a sequence to follow. First removal of the battery is not necessary but may make the process easier. You will be working from above and below. Loosen and remove the main 22mm pulley nut on the pump first. Remove the long 14mm bolt on the back side of the pump. Keep track of the spacer. The double nutted (10mm) adjuster rod must be loosened and detached. Next there are two 17mm bolts on the front of the pump, between the pulley and the pump front, there is a nut behind one. As you loosen these they will come in contact with the pulley which can be jiggled off. The pulley shaft is keyed and tapered and should allow the pulley to be removed without too much trouble. The pump can be lifted out after disconnecting the hoses. The seal replacement is a snap. Don't forget to change the power steering filter while you’re at it. Good luck!

I was just in that neighborhood last week. Because my damper pulleys are fused together(northern cars) I had to install the damper then slip in the bracket then install the pump. I think your best approach would be to remove the pulley and unbolt the pump leaving the bracket in the car. The pulley is on a really tapered shaft and I was able to break it lose with a pretty small puller. I will stick my head under my car today and see what Ideas I can come up with. It's on a lift at the moment so access is pretty good.
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