I don't have an answer. I have rebuilt the mono wiper on my 300TE, and was amazed at how little grease remained inside the head, and that it was dried out and hard. I rec that you dissemble one of the wiper heads, clean and relube, and see if that helps your sluggish problem. It may be that at high setting the momentum of the wiper overcomes the resistance of the dried out lube, but at low setting, the resistance of the dried out lube bogs things down. There may also be a worn area along the track inside the head that bogs things down. The mono wiper from my TE would not retract, and inside the head, the toothed track was totally worn done, literally a pile of plastic dust!

Just an educated guess after dissembling two of these mono wiper mechanisms last weekend.

See the thread below about how th grease the 300E wiper gearbox.

Ditto the previous post - it probably needs to be cleaned and re-lubed. To do so properly, you will need to open it up and clean and then add new lubrication.

See this thread for instructions -

http://www.peachparts.com/shopforum/showthread.php?p=307591

__________________
Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon