More grip on the pulleys surrounding the extra one? (Just a thought)...

 

Actually it seems to provide the same grip to water pump pulley only from the lower right and not the upper left!

 

I have just discovered there are two different belt routing for the serpentine belt on the Mercedes V6 engine vintage mid 2000.

The Poly-V belt routing (and number of idler pulleys) depends on the engine fitted. The WIS shows different diagrams for the following W209 petrol-engined models:

a) CLK 240. CLK320, CLK500, CLK55 with M112, M113 engine
b) CLK 200K with M271 engine
c) CLK 280, CLK 350 with M272 engine
d) CLK 500, CLK550 with M273 engine

 

The extra idler is on the M272:

http://benzbits.com/M272/BeltRouting.pdf

The M113 only has one idler:

http://benzbits.com/m112/BeltRouting.pdf

The purpose of the additional idler on the M272 is to reverse the direction of the water pump. On the M112, the water pump rotate counter to the crankshaft. On the M272, it rotates in the same direction as the crankshaft.

 

I am sure in all their wisdom MB has a rationale for that??

 

The M272/M273 engines are based on the M112/M113. The M272/M273 uses a quad-cam system instead of a dual cam. Maybe the different timing chain routing affected the design of the eater pump. Another possibility is the electronic thermostat makes a difference.

 

That may be, only the mind of their engineers know! When I build something I try to design it to use what I already have on the shelf. Of course I do not have a big budget like the MB folks have! And I do not design stuff to so that I can sell more and different parts either.