belt tensioner ruined?
 

Despite having read the proper procedure a million times, and having just replaced the belt tensioner 3 weeks ago, I just screwed up. I figured out that not only did I have a radiator leak after I got everything put back together, but I also had a fan clutch pulley that sounds like it's about to go bad. When I went to loosen the belt tensioner, I forgot to loosen the 19mm bolt first. Have I definitely ruined the belt tensioner or is there hope?

Forgot to add it's an M103 engine.

19mm bolt? As I recall the only bolt anywhere on the belt tensioner is the 5mm hex bolt right in the middle. I just did this a couple weeks ago on the same engine and there was no 19mm bolt that I saw.

On my m103 the belt tensioner has a 19mm bolt around which the whole assembly pivots. This seems to be the standard from all of the other posts I've read for the m103. This is the bolt that must be removed to eventually take off the tensioner.

If this is on your '88 300 SEL, the tensioner is very similar to the 190E 2.3. Yes, it is important to loosen the large bolt in the middle first, but if you were loosening the belt it shouldn't matter...the belt would not loosen and you would realize that you forgot to loosen the large bolt.
Now if you were trying to tighten the belt, then yes, if you don't loosen the large bolt first, you could either break the Rod Assembly, or the Rod itself.

The tensioner is only damaged if the rubber bushing is torn or broken. If you by chance damaged the Rod Assembly, you can buy that separate.

eric
 

you are right about the 19mm having to be loosened to adjust the belt tension.
first a suggestion for all, weld a bolt(i used a 17mm head) to a short 19mm socket and that will give you a way to get at the 19mm with a 17mm wrench.(no damage to anything while using)
if i had your situation i would adjust the belt as tight as i could get,then pull or push on the belt to see if it repeated its position,if the rubber is broke from the center sleeve then the belt should get loose as you pull on the belt,else it stays tight and isnt slipping so tensioner should be good.
hope this helps.
larry perkins lou ky

The big danger here is that you might have stripped the threads, either on the rod or in adjusting nut. As stated earlier, if you ONLY loosened it (a bit) you might have done no damage at all.

No harm will come to the tensioner itself from this as the 19mm bolt locks it in place.

That sounds logical to me. I'll let ya'll know if it works after I get it all back together.

Well, I finished the job today. The belt tensioner worked just fine. I guess it is most important to have the 19mm bolt loose when tightening the belt tensioner, it may not matter when you are loosening your belt. Thanks all.