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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2005 Pathfinder LE
2001 E55
1998 M3
1982 320i