1998 ML 320, 99K miles, developed a constant power steering whine, although the PS fluid level was correct. The whine was clearly audible within the passenger compartment, not temperature dependent, increased in intensity as the engine speed increased and changed little when the wheels were turned left or right. Outside of the noise, the PS system operated flawlessly. Inspection of the hoses and rack revealed no evidence of fluid leaks or kinked/collapsed hoses.

The PS oil was renewed with no effect. The pump was then renewed but the noise is still present, although the intensity is lower. Fluid drained from the system at the time of the fluid change and subsequent pump change was clean and free of particulates. The fluid reservoir was also inspected and found to be free of debris. Hoses are firm and not spongy. The noise is clearly pump related... it disappears when the engine is run with the poly-V-belt disconnected. Again, other than the constant whine, the PS system appears to be operating properly.

Is the new pump defective... or is something else amiss? The dealer advises they sell a fair number of these pumps but doesn't recall seeing a bad one out of the box.

An obstruction within the system... perhaps the rack? If so, how does one locate and confirm an internal obstruction when there are no outward clues as to its location? Replacing hoses piecemeal is one thing but rack renewal is too expensive to perform based on just a quess.

You sure it's not just the belt?

My car developed a whine as well, but I have yet to figure out where it is coming from. From my experiance these are a bit tough to track down. Mine appeared after a timing chain, sprocket and guide change. It also DOES not dissapear when the belt is removed, originally I did not know you can test it that way, but before this, I thought it was the PS pump as well, then the idler pulley which was changed with not results.

I just finished replacing the guide rails one more time to see if it will go away and still nothing.

Since the engine is all metal, the sounds travel and resonate in different places, In this case, I think it would be best to make sure the whine is first tracked down to it's point of origin, then tested to see if it goes away, then make any replacements, but who on earth would go through a lengthy procedure like that?

xp

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When you had the pump replaced did the reservoir get replaced as well??
IF not then that is probably the problem! The reservoir has a FILTER made into it & they do plug-up & cause noises.

MB part number 000-460-01-83, $27. list

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Replaced the fluid reservoir but the noise is still present. There was some debris present on the screen of the old reservoir but it was not close to being clogged.

The noise is definitely coming from the left side of the engine and is engine-speed dependent... the higher the rpm, the louder the noise. At idle there is little change in the noise as the steering wheel is rotated either left or right.

The noise disappears when the poly-V-belt is removed... and the only belt-driven components on the left side of the engine are the A/C compressor and the PS pump. The compressor pulley sounds a bit dry when rotated by hand but with the belt installed the noise seems to come from the pump, not the compressor pulley.

External inspection of the PS system reveals no gross faults - none of the hoses are crimped, the oil cooler and rack are free of damage and there are no leaks.

The power steering system works flawlessly, other than the noise.

Any suggestions? Install yet another pump? And if its not the pump, what should be examined next?

the description of your problem is exactly what I am experiencing on my 99 ML 320. I changed the pump, no change. I am still searching for an answer. Let me know if you find out anything. I am going to check the filter in the resivior as stated in the thread.
JP

I get a few of these every week. Not just ML's either. 203's, 211's, 220's, etc. It seems that MB has figured out a way to reproduce a "P/S pump type" noise from the A/C compressor . The way we diagnose it is simple. You'll need MB poly-belt #011-997-96-92. Just install this belt in place of the original, leaving the A/C compressor out of the loop. For diagnostic purposes, the engine can be run in this condition with no problem. If the noise goes away, the compressor is to blame. This procedure works on M112 and M113 engines.

As far as ML's go, I replaced a noisy P/S pump once, and the noise was still there. I looped the A/C compressor out, but the noise didn't go away. I drove it around for awhile to make sure all the air was bled out, but no luck. I was certain the noise was coming from the new pump, so I ordered another one. The next day, when I started it up to drive it into the shop, the noise was gone. Nothing. Not even a hum. I guess some air was trapped and it just rose to the top while the vehicle sat overnight.

Anyhow, the trick with the belt works great. MB dealers all over have been doing it for years. Good luck

I just replaced my A/C clutch (kit comes with pulley, hub, and coil) because the bearing in the pulley was shot. Made a noise on a cold engine for about 3 seconds at startup then silence. Got it apart and can't figure how it lasted as long as it did...looked like the snapring was the only thing holding the balls in their races.

You may want to replace it to save yourself some trouble later on. Might even fix your noise.

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