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  #1  
Old 07-25-2005, 12:24 AM
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Drive belt tensioner

Hi Guys,

My Mercedes repair CDs appear to be experiencing problems booting up, so I am wondering if anyone has torque specs. for parts removed during drive belt tesioner replacement - specifically:

1. The drive belt tensioner mount. I have read this described as a "chicken foot" or "Y" bracket. I have the spec. for the big 19mm bolt, but not for the other three (13mm) bolts. one near PS pump; one, with nut on back below ps pump; and final one behind fan (the "wet" one that needs RTV to install);

2. The four fan pulley bolts;

3. The three water pump pulley bolts; and

4. The three ps pump pulley bolts.

Any assistance would be greatly appreciated.

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  #2  
Old 07-25-2005, 12:25 AM
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oops

Sorry, forgot - Car for which specs. desired is a '90 300e.
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  #3  
Old 07-25-2005, 12:35 AM
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Try This

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  #4  
Old 07-25-2005, 04:04 PM
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Quote:
Originally Posted by KermitF
Hi Guys,

My Mercedes repair CDs appear to be experiencing problems booting up, so I am wondering if anyone has torque specs. for parts removed during drive belt tesioner replacement - specifically:

1. The drive belt tensioner mount. I have read this described as a "chicken foot" or "Y" bracket. I have the spec. for the big 19mm bolt, but not for the other three (13mm) bolts. one near PS pump; one, with nut on back below ps pump; and final one behind fan (the "wet" one that needs RTV to install);

2. The four fan pulley bolts;

3. The three water pump pulley bolts; and

4. The three ps pump pulley bolts.

Any assistance would be greatly appreciated.
I just changed a belt tensioner this weekend on a friend's 300E. It was a "training job" for when the time comes that I have to do mine, and I sure learned a few lessons.

I suggest you use anti-seize compound on bolt threads that engage aluminum. This would include the (19 mm hex) tensioner lock bolt and at least one of the Y-bracket bolts. Two of these bolts engage in threaded castings and one has a nut on the backside, which is tough to reach, but you should be able to get a 13 mm combination wrench on it to hold while you unthread the bolt. Once the bolt was free I used my free hand to stick a magnetic finger down there and withdrew the wrench, nut, and magnetic finger together with everything stuck together.

The service procedure (13-345 in the M103 engine manual) torque spec for the mounting bolts on all three pulleys is 10 Nm - 7.2 lb-ft. This seems about right for the fan pulley bolts which are 6 mm pitch diameter (10 mm hex), but seems low for the water pump and PS pulley bolts, which have larger pitch diameter - I think 8 mm. In any event, that's the "book spec".

I believe the 13 mm hex bolts that secure the Y-bracket are also 8 mm pitch diameter. I have a standard torque table for SAE bolts based on grade, pitch diameter, and course or fine thread. Unfortunately I don't have one for metric, but those bolts are similar to 5/16" SAE course, and 15 lb-ft would be about right, especially it they thread into aluminum. Disassembly inspection showed no sealer on the inboard location as called for in the procedure (specifically on both sides of the bracket surrounding the bolt hole), and as far as I can tell, that thread is blind. Nevertheless, I used sealer as it can do no harm.

Here are a few other tips:

1. Prior to placing the rod and lever on the tensioner housing, thread the adjuster nut all the way to the bottom of the rod thread. (Note that the rod and adjuster nut are left hand thread.) At this point the top of the rod thread should be protruding from the top of the nut about a quarter inch.

2. Assemble the components onto the lock bolt in this order - bracket, pointer, tensioner, rod and lever. MAKE SURE the rod is forward relative to the lever. If you install it "backwards" (rod to the rear of the lever) you can damage the tensioner rod and nut when tightening the adjuster nut. DO NOT install the shock to the tensioner at this time. Position the assembly such that the adjuster nut protrudes through the bracket hole and engage the lock bolt and thread to finger tight. Now install the three (13mm hex) Y-bracket hex bolts finger tight. The nut on the one bolt is a little tricky to get started, so have that magnetic finger handy, again, in case you drop it.

3. Once the tensioner is installed back the lock bolt off just enough from finger tight so the pulley can be moved back and forth. Make sure the pulley moves smoothly through its arc of travel from the inboard (left as you look at it) to outboard limits and note the approximate midpoint of the arc. As you move the pulley back and forth the tensioner nut should slide smoothly through the bracket hole. Now hold the pulley to the inboard limit with your left hand and use the fingers on your right hand to turn the tensioner nut clockwise. Once the base of the nut contacts the bracket, it should continue to turn easily and the pulley should move outboard. Keep turning the nut until the pulley moves to about the midway point of travel that you noted earlier. If the action is not smooth and low effort, the lever might be on backwards. You can use a flashlight to verify that the adjuster rod is positioned between the tensioner housing and lever, not behind the lever. Once this test is passed the assembly sequence should be correct and you can torque the three (13 mm hex) Y-bracket bolts.

4. Run the tensioner nut (CCW) back to the bottom of the rod thread or at least until the top of the rod is flush with the top of the nut. Compress the shock all the way (takes considerable force) and attach to the tensioner housing with the (11 mm hex) bolt finger tight. Move the tensioner pulley to about halfway between its limits of travel, align the shock with the upper mount and tighten the lower mount bolt. (You can't reach this bolt once the fan pulley is on.)

5. Install the pulleys. They can be torqued to 10 Nm by just holding the pulley with your hand, and make sure the fan pulley is in line with the others. It's easy to install backwards, which will offset it from the other pulleys. After making sure all the pulleys are in line, move the tensioner pulley to the inboard limit of travel (left as you look into the engine compartment and you may have to wiggle unattached top of the shock to keep it from interfering with anything solid so you can move the tensioner pulley all the way to the inboard limit) and install the belt.

6. With the belt installed, turn the tensioner nut CW until the base of the nut contacts the bracket, which takes all the slack is out of the tensioner rod, then set the pointer at the bottom of the ramp. (It "rachets" in small increments.) Now continue to turn the tensioner nut CW (I used a 13 mm socket on a speed handle) and notice that the pointer begins moving up the ramp. If the pointer does not move up the ramp in equal incrments with each consistent increment (say a half turn) of CW adjuster nut rotation, the rubber sleeve bushing (between the inner and outer tensioner segments) holding torque is below minimum and the tensioner must be replaced. Same if the pointer makes it only part way up the ramp and then stops moving as the adjuster nut is turned CW. If this is the case you can bend the adjuster rod if you keep turning the nut because the rod will eventually interfere with the adjuster housing. Assuming you have a good tensioner continue to turn the tensioner nut CW until the pointer reaches the top of the ramp or just a little beyond and then torque the lock bolt to 56 lb-ft.

7. At this point I suggest you recheck torque on all the pulley bolts, then expand the shock until you can install the upper mount bolt and nut. (The shock requires considerable force to compress, but very little to expand.)

8. At this point you're done except to reinstall the fan and shroud.

I'm still working on the old tensioner post mortum, but one thing I noticed is that I could use the tensioner swivel and rod to lever against the housing to turn the inner an outer tensioner segments relative to each other. This was not possible on the new tensioner with comparable torque, but there was nothing visually to indicate that the old tensioner bushing had failed. It's the friction torque of the rubber bushing that holds the belt tension, and if this friction torque degrades the pulley will move back inboard along its arc of travel, however, the "shock" (It's actually a damped gas spring.) should keep the belt from going completely slack, so the "shock" is basically a fail-safe device.

The way you can TELL if the tensioner is failing is to observe the pointer.

If the tensioner bushing holding torque degrades, the pointer will move BEYOND the top of the ramp. If the pointer moves to or beyond the solid line beyond the top of the ramp, the tensioner holding torque is below spec, so the pointer should be observed when you check oil or otherwise open the hood. Of course, this assumes that the pointer was properly set prior to tensioning and that tensioning was done correctly to place the pointer at the top of the ramp.

The complexity of this design defies what I consider to be good engineering, especially when compared to the belt tensioners on late model Ford and GM products.

I know I can change the belt on a Corvette or Mustang in about a minute, and I've never seen one of those simple mechanical spring tensioners fail. They don't have any rubber parts that can degrade and cause loss of belt tension.

I'd like to get my hands on the Kraut who designed this g...d d...d Mercedes drive belt tensioner and wring his freaking neck!!!

The design of these cars in many areas shows excellent consideration for maintenance such as the brakes (pad changes) and cooling system changes (The nipple on the end of the block and radiator drains makes attaching hoses easy for spill free coolant changes.), but this belt tensioner appears to be an exercise in making something that should be simple as complex and expensive as possible. An elegant design is one that does the job with the least cost and complexity. That is the essense of good design engineering!

Duke

Last edited by Duke2.6; 07-26-2005 at 12:38 PM.
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  #5  
Old 07-26-2005, 12:04 PM
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Quote:
I'd like to get my hands on the Kraut who designed this g...d d...d Mercedes drive belt tensioner and wring his freaking neck!!!
Second the motion.
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  #6  
Old 07-26-2005, 05:02 PM
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ah huh

Quote:
Originally Posted by haasman
Try This

Haasman

thanks for the link. I desperately needed that.
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  #7  
Old 07-27-2005, 09:29 PM
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drive belt tensioner

Haasman,

Thanks very much for the link. Good resouirce for other diy tasks as well.



Duke,

Thanks very much for the excellent write up - including the bit on testing before re-assembly - this is a great write up; I cannot believe that Haasman has not requested this be posted in the helpful tips section. Your write up, along with that of anthonyB back on 10-08-2001 got me through this time-consuming, but satisfying to complete task.

BTW, a couple of things that worked well, in addition to you good advice were:

- used a pretty good size "magnetic finger" to hold the tensioner's lower bolt while starting the threading process - could not really get 13 mm wrench to give enough grip - magnetic finger held nut fast until threading began, then switched to wrench.

- used a little anti-seize for the tensioner cap (under the bracket to ensure smooth operation later) (read this on an earlier post while I was researching topic).

Larry Bible,

Thanks for assist in diagnosing. Overheating, periodic squealing now but a memory - which is especially great given the temp here in Atlanta now - almost up there with Texas temps.


Really appreciate everyone's help.

KermitF
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  #8  
Old 07-28-2005, 11:00 PM
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As a footnote to all this, I had to change the belt tensioner on the 93 190e 2.6 recently. The center rubber looked good and what confused me was that the inner portion rotated inside the housing.

Weird. It took several minutes to finally figure out what was wrong. It wasn't until I noticed it was not in the same orientation as the replacement and then turninging the bad one with a long wrench did the failure appear.

I would always recommend checking for this.

Haasman
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  #9  
Old 07-29-2005, 01:40 AM
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You can't see the internal rubber bushing (the "tensioning element") from the outside. The "rubber rings" you see on the front and rear faces of the tensioner are the ball bearing seals, not the internal rubber tensioning element.

The key is to watch if the pointer backs down the ramp as you relieve tension. On an old tensioner, it probably will not. In this case remove the tensioner and observe the orientation of the flats relative to the pulley axis. If not oriented as stated and confirmed in this thread: Drive belt tensioner failure analysis the tensioner is not serviceable and must be replaced.

If you attempt to reuse it, you may end up bending the tensioner rod, and even if it does appear to tension the belt, after a few miles of driving you'll hear the belt screech due to lack of tension.

You'll be doing the job again!

Duke

Last edited by Duke2.6; 07-29-2005 at 12:10 PM.
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  #10  
Old 07-29-2005, 08:50 AM
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Duke - You da man!

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