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1991 350SD
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Discussion Starter #1
Well, as with all things in life, don"t get too comfortable or smug. After spending some time changing out rotors and calipers all the way around, I was getting ready to take the old girl out for a spin but I noticed a bit of the edge of the V belt laying in the engine compartment. Sure enough, it's coming apart and needs replacing. Why not get a new tensioner and pully while I'm at it, right? With all of the repair documentation I can find, there is no mention of having to remove the radiator to do this job (except on the 603.96 Turbo engine where radiator removal is indicated. I have the 603.970 engine). Looking at the work space between the radiator and the fan, it seems pretty difficult to do this without removing the radiator. Does anyone have any experience with this? I've looked all over the W126 forum and can't find this particular job referenced in any detail. Is removing the radiator the best bet to change out the belt, tensioner and tensioner pulley? Thanks in advance for your input.
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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What year and model? I was able to change the tensioner and fan clutch without removing the radiator on mine. I did remove the plastic shroud, though.
 

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1991 350SD
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Discussion Starter #3
She's a 1991 350SD. I lifted the shroud up and back but there isn't much room. I can see on the cooling fins that someone in a prior life must have changed it with the radiator in place as the fins are buggered up a bit right where the fan bolt is.
 

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If you are going to change the coolant anyway it is fairly easy to get the radiator out, just a few clips, trans lines and hoses and it will come up between the condenser and shroud, then you can pull the fan and anything else you want to get while you are in there. A good time for a metal impeller water pump if not equipped, I'm not sure if the 603s had metal or plastic though. There is also a fully aluminum aftermarket replacement radiator for these. I just pulled the tensioner assembly on a similar I-6 setup in my w126 without having to remove the fan or radiator, it may be easier than you think. Once you get the spring and shock off you can move the tensioner pulley enough to clear the removal of the bolt and pulley. Slip a piece of cardboard or plywood between fan shroud and radiator for protection if needed. If you have to pull the fan I'd remove the radiator, on mine it is pretty tight between fan and fins.
 

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1991 350SD
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Discussion Starter #5
She's a 1991 350SD. I lifted the shroud up and back but there isn't much room. I can see on the cooling fins that someone in a prior life must have changed it with the radiator in place as the fins are buggered up a bit right where the fan bolt is.
I'm only about 7K into my last coolant change so I may try to preserve that if possible. I had the water pump changed out by a mechanic in Dallas about two years ago as a preventive step before my move to New Mexico. Not a lot of MB support in west Texas and eastern NM and I didn't know the history of the water pump. Is there any way to tell externally if it's all metal? Also, before I get too far along, am I right in assuming that once everything is reassembled that the tensioner gets the tension right without any further adjustment? Nothing I read indicates needing to actually adjust the belt tension. With that said, have you ever replaced a tensioner and if so is it fairly straightforward? Any traps along the way?
 

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The tension is set by the spring. Get a new one if you want to be certain.
 

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Discussion Starter #8
The tension is set by the spring. Get a new one if you want to be certain.
John,
If you are so inclined, would you be able to explain the relationship between the belt tensioner lever, the tensioner shock and the tension spring? What initiates what action?
 

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The spring pulls the idler pulley into the belt and determines the tension. The "shock" is there to dampen vibrations caused by the power pulses from the engine. Not sure what you mean by the lever. Could you mean a tool to relieve the tension?
 

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1991 350SD
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Discussion Starter #10
The spring pulls the idler pulley into the belt and determines the tension. The "shock" is there to dampen vibrations caused by the power pulses from the engine. Not sure what you mean by the lever. Could you mean a tool to relieve the tension?
I've probably got the terms incorrect. What I see that's involved in the V belt tension system is 1. The Idler Pulley, 2. The Tensioner (I called it the Tensioner Lever prior), 3. The Tension Spring and 4. The Damper.The Tensioner is mounted on the Bearing Pin. The Tensioner has the Spring attached on the Right side and the Damper on the left. The Tensioner seems to be some sort of fulcrum. The Idler Pulley is attached to this Tensioner.
 

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V-belt?

Anyhow, the spring pulls on the lever arm forcing the pulley into the belt to take up the slack. The damper is there to dampen any vibrations induced between the power pulses, the belt and the spring.
 

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Discussion Starter #12
V-belt?

Anyhow, the spring pulls on the lever arm forcing the pulley into the belt to take up the slack. The damper is there to dampen any vibrations induced between the power pulses, the belt and the spring.
John, I use the "Service Manual for Diesel Engines 602 and 603" when trying to figure out how to get something fixed on my 350SD. MB Work Order #13-342 is titled "Removal and installation of Poly V Belt".
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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Apparently, the terms: 'Poly-V belt' & 'Serpentine' mean the very same.. Like the brand: Kleenex & facial tissues, etc.

The manual says to remove the radiator for the R&R tensioner job on a W126.

M


Screen Shot 2020-01-29 at 6.04.06 PM.png
 

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AlI I can say is I had the tensioner, fan and fan clutch all out without removing the radiator on my '90 with 603.970. Of course, I did it without reading the manual... ;)

I would react differently if I saw Poly-V belt than I would if I saw Poly V-belt. Either way, Mercedes has muddied the waters. If you google "V-belt" or "V belt" you will get something very specific and very different from what is used on these cars.
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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I would react differently if I saw Poly-V belt than I would if I saw Poly V-belt. Either way, Mercedes has muddied the waters. If you google "V-belt" or "V belt" you will get something very specific and very different from what is used on these cars.
I hear ya, John & I couldn't agree more.
Even my 1930 Ford has a V-belt to drive the engine accessories.

M
 

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1991 350SD
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Discussion Starter #17
I hear ya, John & I couldn't agree more.
Even my 1930 Ford has a V-belt to drive the engine accessories.

M
V belts have been around for 100 years, having generally replaced the old style flat belts that drove machinery around the turn of the 19th century. It's called a V belt simply because of its general cross-section trapezoidal shape. A poly V belt incorporates many v's in one belt (hence the addition of "poly" as an identifier). which allows the belt to be thinner and more flexible. Almost all poly V belts are also identified as "serpentine" as they snake around many pulleys and travel in different directions. The accessory belt on my 350SD is a poly V serpentine belt.
 

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I guess I've been living under a rock. Until now I had never heard "V" in the context of a serpentine belt. Regardless, if you drop the "poly" part, then the meaning is unambiguously not a serpentine belt.

Now, back to our regularly scheduled program. Everything else I said about the tensioner and damper applies.
 

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Discussion Starter #19
I guess I've been living under a rock. Until now I had never heard "V" in the context of a serpentine belt. Regardless, if you drop the "poly" part, then the meaning is unambiguously not a serpentine belt.

Now, back to our regularly scheduled program. Everything else I said about the tensioner and damper applies.
John, You're absolutely correct sir (as far as I know). Fan belts, as they used to be called, were V belts based on their cross sectional shape. Pulleys were also re-engineered to work more effectively in conjunction with a V belt as now there were three sides of the belt transferring energy (and contact) with the pulley. That's why there are sides to pulleys, in addition to making the belt more secure on the pulley. In the old days, pulleys and belts were flat. The original V fan belts were quite rigid and this rigidity didn't lend itself to the new(er) serpentine belts. As mechanical engineering evolved, some bright guys figured out a way to make the v belts pliable so a single belt could snake around all of the accessory pulleys and viola, the birth of the serpentine belt. My cars from the 60's and 70's had 4 or 5 belts, each turning an accessory.
 
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