I recently r/r'd the water pump on my 1993 300se and after reassembly I have been unable to adjust the tension on my serpentine belt. I can adjust the rod all the way in or out and I get no movement on my tensioner pulley. I noticed the adjusting rod can be put back on the tensioner more than one way. I do not know if I reassembled this incorrectly or not. I have removed and installed it in as many ways that will allow and still no pulley adjustment. The pulley rests hard up against the water pump pulley. Could the shock absorber that is attatched to it be a problem as it appears to be an opposing spring force? Or has the tensioner just gone bad for no apparent reason? I find it hard to believe that the tensioner quit as the local Mercedes dealer says he rarely ever sells one.

If you've got the tensioner and adjusting rod and collar mounted properly, there's almost no way it can not move the adjuster pulley towards tensioning the belt. The tensioner mechanism itself consists of an inner and outer housing with a rubber tensioning medium in between.

The rubber can take a set after years of use because of heat and constant tension to the point that it will not rebound much, if at all, when you loosen the adjusting rod. If that's the case, the only fix is a new belt tensioner. With the tensioner center bolt loosened 1/4 - 1/2 turn, see if you can rotate the tensioner around the center bolt by loosening the adj. rod nut, grabbing the tensioner puller and pushing or pulling it in either direction. You should feel resistance from the shock, but you should also see the adjusting rod move up and down. If not, remove the tensioner and adjusting rod assembly and make sure they're both in good shape.

The adjusting rod and collar can go on in one of two directions. The proper way is with the adjusting rod/collar pivot oriented to your left (9 o'clock) when facing the engine from in front of the car.

Also, make absolutely sure that you're installing the belt in the right configuration (see pic below). If you're using a new belt, make certain that it's the same length as the one you took off. The belt length may be marked on the outside of the belt in millimeters. Depending on tensioner type, tensioner pulley size and accessories, belts can vary in length, and it's possible you may have been sold the wrong one.

Hope this helps

Gary

Attached Thumbnails

 

I did have the center through bolt loose and have been able to move the pulley with a screw driver. Its just that I can run the adjusting rod all the way down (tight) or all the way loosened and that pulley never moves a millimeter. The belt is the same one I removed for the water pump change. It is not a replacement. I'm leaning towards a bad belt tensioner. The tensioner can only go in one way so I don't see how that can be an issue. Trying the adjuster bolt collar swapped 180 deg out didn't make a difference. I have a drawing of the serpentine belt install (thank you anyway) prior to install and I know that is correct. The only thing I can think of is if there is another manner by which to install the adjuster bolt collar. Otherwise I eat another $200.00+ for the tensioner.

Try this; remove the belt, position the tensioner roller where you can easily get your hands on it to move it both directions and then tighten the center bolt. Now try moving the tensioner roller/outer housing in either direction. If it moves and doesn't return to it's original place, then the rubber is sheared or otherwise destroyed. Replace the tensioner. If you can move it just a little bit, and it returns to the original starting point, then it may be ok and you need to look further into it. Check to see that when you turn the adj. rod nut, (with the center bolt slackened and the belt still off) that it actually rotates the inner part of the tensioner. Normally, the outer part should move with it.

If you need to replace the tensioner, they can be found for under $90. I just replaced mine about 2 weeks ago, and the quality appears to be the same as the original. PM me if you'd like the link for the supplier.

Gary

With the tensioner out and in a vice I put a pair of channel lock pliers on the collar end and turned. The collar end recoiled back to its original spot. It seemed as though it had some pretty good resistance. With the tensioner oriented in its correct position in the car the flat sides of the collar end are virtually perpendicular to the ground. Is this in the correct position or could have there been any internal slippage? Now with absolutely no tension on the adjusting rod the tensioner pulley is resting up against the water pump pulley. I then crank the nut nearly all the way down, the collar is turning, end of rod is @ the 9'o' clock position, but the pulley never moves. It appears to be getting tighter against the water pump pulley instead of pushing away. That is why I questioned the position of the rod end and collar. No matter how I positioned it it seems to be pushing the pulley in the wrong direction. I feel like I'm losing my marbles.

It sounds like the tensioner is working, since it springs back with good tension. Here's a couple of things I can think of: make sure that the pivot point where the adj. rod connects to the collar is at the 9:00 position (or to the left) when viewed from the front of the engine. (see the photo in the following post) When mounted in this position the adj. rod nut, when turned CW as viewed from the top (i.e. the "normal" tightening direction) will push the adj. rod downward turning the tensioner outer housing, and consequently the tensioner pulley, CCW, away from the water pump pulley. If the opposite is happening, check to make sure that the collar on the adj. nut is under the adj. bracket, not on top of it.

Here's a picture showing the collared part of the adj. nut (coated with silver anti-seize) under the bracket:

Attached Thumbnails

 

Here's a picture of my old tensioner. It was working, but had taken a "set" from age and heat. I've taken the picture as it would be mounted on the car, but from the rear (as viewed from the driver's seat). Note that when the flats are perpendicular to the ground, the pulley is at about 4:30 - 5:00. Viewed from the front, this would be around 7:00 - 7:30. A new one may be closer to 6:30 - 7:00. When the adj. nut is turned CW, it will move the pulley toward the 6:00 position. If you have this type of tensioner and it deviates significantly from the picture, replace the tensioner. If it's the original it's already been subjected to tension and heat for 10 years. If there's a significant number of miles on it in addition, you might want to replace it just as preventive maintenance.

Hope this helps

Gary

Attached Thumbnails

 

Those are excellent pictures. Thank you. I will try this tommorrow. I do believe I had it assembled that way but it may have been with the nut on top of the bracket. I don't recall. So many assemblies, disassemblies, combinations, I've lost track. I tried just about anything. I will reassemble tommorrow and try as per your photos. While adjusting the tension, is the adjusting rod nut supposed to draw through the larger cutout on the bracket or does it remain slid back in the narrower cutout? Thanks again.

The nut goes through the larger hole, and will slide into the smaller hole as you put tension on the bolt. It must be inserted from underneath the bracket with the nut screwed down on the bolt.

You will also have to pull the tensioner over to get the belt on (this held me up for quite a while when I did the TE a couple weeks ago). The shock pushes it all the way tight.

I bought a new tensioner, no way one with 200,000 miles is gonna be good anymore -- rubber was cracked pretty bad.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

One important note to make here:
On that last image Gary posted, you will notice the parallel flat spots on the inner metal piece of the tensioner. Now if you look lower and to the right, you will see the threaded hole that the idler pulley bolts to. If you can imagine a centerline through the central bolt hole that also runs parallel to the flat spots, this imaginary line should "point" at the threaded hole for the idler pulley (+/- 10 degrees or so). Maybe Gary can "doctor" his image to illustrate this. For refgerence, as Gary stated, his old tensioner was about shot, not pointing at the center bolt hole for the idler anymore. I believe the actual reason for this is degradation of the rubber and the inner and outer parts slip. I don't think it's accurate to describe this damage as "taking a set". But in either case, this method of inspection should always be done to a tensioner when removed from the engine.

Gilly

Gilly,

Actually, the threaded hole is where the shock mounts. The tensioner pulley is visible as the dark half-round protuberance and the bottom of the tensioner. You can see it a bit better in this front view:

Attached Thumbnails

 

Here's my "doctored" image. per Gilly's request. The green line indicates approximately where (+/- 10 deg.) the center of the pulley wheel should be on a good unit. The red line indicates where it ends up after 17 years and 130,000 mi. The end result in not enough tension on the belt to keep it from slipping. Actually, this one was right at the end of it's useful life. It would provide just enough tension, but was at the limit of the adjuster.

Gary

Attached Thumbnails

 

Gary
Thanks, that's an excellent image, I'll have to remember that image exists here as i have tried to explain that exact image to other people.
I had the correct hole in mind when i described it, guess it was a lousy description, I forgot the shock mounting hole was more obvious, but I knew which one was correct.

Gilly

Great pictures. I assembled the tensioner onto the car, all looked good. Tensioner pulley was not resting on water pump pulley. As soon as I put the belt on it pulled the tensioner pulley up against the water pump pulley again and there it rests, AGAIN. I have tried to adjust it. As soon as I start to run the nut down on the adjusting rod (ccw), it pulls through the bracket. Once its through the bracket it just runs down the rod. No adjustment. Same for the opposite direction. It appears as if the bracket would serve no purpose, as if I was just running a nut up and down the rod. I don't get it. What am I not getting? I've been fixing cars and airplanes for 20+ years and for some reason I can't get this back together. Is there such a thing as "mechanics block" like writers block? Help, I am out of patience. One more day and I send it to the local dealer. I have spent way too many hours on this.

By your written description, I think I know your problem. The wide/chamfered part of that nut goes UNDER the sheetmetal bracket, not on top of the bracket. Only the cylindrical and hex-shaped part of the nut should be on top, this part "pokes through" that kind of key-hole shaped hole in the bracket. Look at that image that was first posted.
You need to thread the nut onto the bolt, put the collar onto the tensioner (rod goes towards the engine side), and put this whole mess up through the bracket. One tip is to thread the nut down so the tip of the rod is about even with the top of the nut 9up even with the top of the hex part).
Hope this helps

Gilly

ps Look at the bottom part of the nut, it's UNDER the bracket:
http://www.peachparts.com/shopforum/attachment.php?postid=482113