asz fan belt tensioner issues

[mercedes190d]

i bought fan belt tensioner and fan belt double sided ribbed.
when i took off my old one (tensioner) it has grooves for the ribbed belt to sit in
on the new genuine part its just smooth nylon not grooved took it back to vw and told me that its the right and only part,
will this cause the belt to come off?
alternator pulley is close so assume it will be fine
as anyone changed one from grooved to smooth.


2003 tdi 1.9 asz engine

thanks for any info

 

[mercedes190d]

tensioner changed a week ago 1000 miles done and all is good so if you buy one and its different from the one on there it will be fine