Ok, So heres the deal my Smog pump goes out and I go to replaece it, and all is still wonderful and beautiful in the world. I forget to mention that this a 1992 300E. I got the fan and belt out of the way in record time and got the new pump in. I go to retension the belt and I got nothing and it feels like the tension adjustment nut is stripped. So I enlist the help of a friend to tighten the secureing bolt ( the 19mm one) while I with a long 2X2 in hand apply pressure to the tensioner pulley (the ungroved one) in the attempt to restore at least a little bit of tension into the system. At first this revised tensioning method show some promise bescause my pushing gets quite a bit of tension back into the belt. Then my world crumbles!! My friend tightens the bolt sufficiently and I ease off my 2X2 in hand., and the belt slacks up. Can anyone help? What's going on internally in that tensioning device that makes it slack up even when that so called securing bolt is tightened? Come to my rescue.

Thanks, a violinmaker that loves to work on his car,
Tucker

I don't think you're doing it right. Loosen the 19mm bolt; some say half a turn, some say until it's finger tight. Then tighten the tensioner adjustment nut -- it's the brass colored nut on a vertical axis behind the power steering reservoir until the indicator on the tensioner body (above and behind the 19mm bolt) is at the max point on the scale.

I don't know what you're going by applying force to the pulley, but I'm pretty sure it's not the right way to set belt tension.

Sixto
91 300SE
81 300SD

The belt tighteners on these engines aren't very durable. There is probably internal slippage in the tightener assembly which is preventing the idler from being moved far enough to tighten the belt. Once you loosen the 19mm nut the tension is lost, and that's it for the tensioner. Now that it slipped, the tensioner nut can be turned until the tensioner rod contacts either an object, such as a bolt, or the rod will start contacting the side of the tensioner itself, usually breaking the tensioner cam, but occasionally the rod strips out, as yours may have. You need a new tensioner is the short of it, and now you probably need the tensioner tightening cam and rod, and I usually recommend the tensioner shock absorber as well. I usually make a cursory inspection of the belt tightener before removing the serp belt in this manner: If the threaded rod is very close to the bottom of the tightener nut, I make sure the service advisor knows that the tightener is suspect of being worn out; the farther down in the nut the rod is, the more the previous mechanic had to turn it to get the belt to proper tension.
Gilly

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

I mentioned that I think that the 13mm adjuster nut is stripped, not allowing me to adjust it? My question was, is this theaded arm and the hollow 13mm adjustment nut that is attached the tensioner the thing that keeps the tension in place because it doesn't seem as if the 19mm bolt is keeping any tension in place, because even with that bolt tight I can yank on the tensioner pulley and move back and forth (slack and tight) without much effort. The other question, is the thread on the 13mm a reverse thread, when I turn left the nut goes away from me and when it's a good distance down the tension seem looser than when the nut is up close to me (getting close to the end of the threads). It's at this point when more turning in a right clockwise direction just results in stripped thread sensation. Do you think that I just need to replace that flanged hollow 13 mm adjustment ( the threads on the bolt look fine) and cal it a day.

Tucker

No, I still think the tensioner is shot. if you tighten the 19mm bolt and you can grab on to the smooth pulley and move it around without much resistance, the tensioner is shot. The nut is probably stripped because the cam on the back of the tensioner, which the threaded rod attaches to, hit a bolt head behind the tensioner and then wouldn't move any more. You probably kept turning anyways (which I did also a few times when I was new at this) and stripped the nut. Usually the adjusting cam will just break, but not always.
Gilly

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

If you can't tighten it with the tension nut or manually with a 2x2 as you mentioned, then the tensioner is shot. The tensioner has two main parts, the middle thru which the bolt goes and the outer housing. These two parts are joined by a rubber wall for dampening purpose. When this wall breaks there is nothing you can do, since the outer housing, which holds the pulley is free to move regardless of tightening the middle bolt. I once made a temporary fix (it takes a while to get parts out here) by drilling holes thru both parts and hammering in a few pins.

Vin

Thanks All,
The repalcement parts will be arrvieng tommorow, I hope that there aren't any more hitches. I'll let you know how it goes. Thanks again everybody. I know what's going on now.

Tucker

Tensioner was diffenently kissed took about 2 and a half hours to swap everything out. Thanks again,
Tucker

I need help in changing the V Belt Tensioner and Tightening Rod on my 1989 300e. I don't know where to start, do I need to remove the radiator, etc... I'm looking for some advice.

Thanks Mercedesphiles

First, do a search on 'M103 serpentine belt.'

Here are the steps:

- detach fan shroud from radiator and rest over fan
- take an 8mm hex to the front of the fan and a 3mm allen key or similar size bent wire to the back of the fan at the 1 o'clock position to hold the fan to remove the fan and shroud
- loosen the 19mm head bolt at the 8 o'clock position of the power steering pulley. DO NOT remove the bolt. A full turn should do it
- loosen the 13mm hollow shaft positioned vertically behind the power steering pump
- the belt should be loose enough to remove. DO NOT remove the belt until you note how it is routed and/or have a diagram from the shop or owner's manual
- installation is the reverse of removal noting that there is a tension indicator above the 19mm head bolt. Tighten the 13mm hollow shaft until the pointer is at but not beyond the full left/CCW mark on the indicator

If the belt is loose even with the indicator reading proper tension, the tensioner should be replaced.

I don't know what the shock does. My old one was as stiff as the new one so I didn't replace it.

Put a sheet of cardboard over the radiator fins so you hurt yourself of the radiator if a tool slips.

Good luck.

Sixto
91 300SE
87 300SDL
81 300SD

Thanks sixto for the advice. But does looseing the 19mm bolt remove the tensioner? Are there other bolts holding the tensioner in place, I have a new tensioner that needs to go there.

Thanks Again.

Doh! You did say tensioner. There are a few more steps but I'm not going to remember what they are exactly off the top of my head so check the archives.

I've heard that you have to detach the power steering pump to replace the tensioner but I didn't. The trick is to blindly engage the threaded rod at the end of the hollow shaft onto the cam at the back end of the tensioner. I got it in 3 tries.

The water pump, fan and power steering pulleys have to come off to gain access to the tensioner bracket bolts. You might loosen the pulley bolts a little with the belt in place or wedge a screwdriver against the other bolts while you loosen one.

There are 3 bolts on a bracket that holds the tensioner in place. 2 bolts are close to the tensioner, 1 goes into the timing chain cover and requires a dab of sealant. The little shock absorber bolts to the engine lifting eye and the back of a bracket on the tensioner.

If you remove the 19mm bolt, the bracket comes off independently of the tensioner and the tensioner comes off on it's own with the shock... I think.

My flaky memory and questionable methods might lead you to more problems than solutions so I'll stop here.

Check the fan bearing and idler bearing when the belt is off. I'd be surprised if you didn't need a new fan bearing.

Sixto
91 300SE
87 300SDL
81 300SD

Sixto-I'm about to change the serp belt and tesioner on my W201 with the 2.6 motor and was wondering what type of sealant that you used on the tesioner bracket. I'm hoping that I can use something that I can get from Kragen. Thanks.

It's been a while. I don't remember if I used Permatex gray or blue RTV. The blue stuff claims to be sensor safe, the gray stuff claims to hold torque better. I probably used the blue stuff.

Sixto
91 300SE
87 300SDL
83 300SD