it_is_nerdy

Does anyone have tips on diagnosing steering pump vs. proportioning valve with DAS?
Thanks for reading my post! I am trying to diagnose if it is the power steering pump or the proportioning valve causing power steering assist to reduce to almost no assist at low RPMs after driving for more than 5 minutes or so. In the beginning of the day, the pump is quiet and steering feels like normal. After driving for a while the pump starts to whine and the feel is very heavy at lower RPMs.(I would say 1500rpm and below.)Here is what I have checked so far:
No apparent foam / air in the fluid after driving a while. Completely flushed and put in new fluid from the dealer. Yes, I know I could have saved $7-10, but - I was a block from the dealer when I decided to try that first.
I have and older STAR diagnostic computer in case there is a way to check that way.

It's an '04 W211 E500 4matic with 108,000 miles roughly.BTW, yesterday the voltage regulator completely failed and I barely got home! Stuck for a couple of days until I get the new one from Autohausaz, thankfully I have my S210 still

Thanks for any suggestions!

it_is_nerdy

In this cars case, it turned out to be the pump; the independent that did the work tried changing the reservoir first, in case the built-in screen was blocking the flow of the fluid but that did not do the trick so he checked pressure coming from the pump and it was not high enough.
The independent I used is Mercedes Repair by Maurice in Costa Mesa, CA if anybody needs a good one in Southern California:
http://www.mercedesrepairbymaurice.com