In light of recent thread activity on the forum, here is a rather lengthy write-up of a recent experience with my OE idler pulley. Also included is a write-up of the installation for a new idler pulley.
For those who don’t want to read the whole thing, scroll to the conclusions section.

Background:
Case: I have a 2002 C32 AMG with 76K miles. No over- / under-driven pulleys. The poly V belt and all pulleys are in good shape esthetically, and there is minimal rotational wobble in any of the pulleys at idle. I have never removed any of the pulleys and the vehicle’s records (VMI) are also clear of any pulley work.
Introduction: I noticed a ‘louder’ sound coming from the engine bay at start-up one recent morning. Like many of my dealership experiences, I could never reproduce it to grab some video! Aside from the 5 minutes of weird whine, I never heard it again. I initially dismissed the sound as ‘normal’ due to the passage of cooler and better air through the intakes and supercharger. The evidence that partially led to its identification was the fact that it got louder with increasing engine revs.

Hypothesis:
My POS idler pulley is transmuting into a potentially catastrophic projectile.

Materials and Methods:
Stethoscope / tubing, mechanic’s gloves, Torx sockets sized E10 and E12, 15mm socket and wrench, torque wrench (15 ft.lb / 180 in.lb), credit card, and ½ beer (job didn’t take long enough for a full one!).

First, let’s alleviate any confusion by first identifying the pulley in question:
It is pulley #4 in the diagram below. This pulley is also known to MB aficionados/techs as a ‘Tightening’ pulley. It’s the same thing.



Checking for abnormal pulley sounds: Obviously I visually checked and put my ear up against all the pulleys; however, I wasn’t able to hear anything of note. That’s where the stethoscope / piece of rubber tubing is of use. With the engine running, place the piece of tubing very close to the pulley and you will be able to hear any abnormal sounds (please be careful doing this!). For my idler pulley, there was a nasty raspy sound coming from the pulley – most likely an early bearing problem.

Removing the idler pulley: This takes less than 5 minutes (seriously). For those of you not familiar with the procedure, check out the lame video I made of the various steps. Note that you don’t have to remove the upper SC guide pulley, but I wanted to remove it so I could check its integrity and it helps to have it out of the way.

1. Make sure the engine is OFF and preferably not hot! Some people may find it prudent to disconnect the ground on their battery and / or unplug your radiator fan.
2. Remove engine cover.
3. Get the air lines/tubes out of your way by pulley away from the engine (firm but gentle) and remove the torx-head fasteners (2 x E10 torx). Push the air lines away to the left of the engine. If you think the procedure will take some time, you may want to cover the open ends of the tubes and inlets.
4. Slip the belt off the idler pulley by turning the belt tensioner counter-clockwise (E10 torx). After slipping the belt off, slowly allow the tensioner to spring fully back clockwise.
5. If you are checking the SC guide pulley, remove the black bracket that fastens the pulley and holds the belt in place. Superior (top) fastener is an E10 torx and anterior (facing the engine) is E12 torx. The SC guide pulley comes off with the assembly.
6. Remove the idler pulley using a 15mm socket wrench.
7. Reinstall in the reverse order starting with the idler pulley. Use a new bolt and tighten this bolt to a maximum of 15 ft.lb (180 in.lb). Use Locktite if you feel it needs a little more holding power!
8. Obviously an extremely important step when reinstalling the belt is to make sure it is in the correct orientation and that it is aligned exactly with the grooves on the pulleys. You will experience a big truck-load of FAIL if you do not do this.
Here’s a low-quality video showing the various steps.



Results:
Importantly, there was no tell-tale screeching of the pulley. It was just a slightly higher pitch whine/drone than the motor sounds and it lasted for 5 minutes. I eventually identified the source using my stethoscope. There was a nice normal whine coming from all pulleys except one. A nasty raspy whine coming from the idler pulley (#4) – again, through the stethoscope (it wasn’t otherwise audible).
On my car, the OE part number is 112 202 00 19. This part has been replaced by part number 112 202 01 19. The pulley was $72 and included a new bolt and cap.
My old idler pulley (112 202 00 19) has a bearing from NTN. The bearing type is 6303 LHA (and the color is yellow). The bearing in the new idler pulley is 6303 RDD (C3 5C OR5) (color is green).
Here’s a composite of the new and old idler pulleys:



Visual test: no abnormalities could be seen superficially. The bearing seal was intact and I couldn’t see any bulging / deformities or grease.
Spin test: deplorable! As you can see from the video, this pulley is not a happy camper and is not spinning freely. Clearly the bearings are failing (for comparison, compare the video of the new idler pulley rotating). That finding was definitely sufficient for me to get a new pulley.
Here is a video showing the old and new idler pulleys spinning:



Torque value for tightening the idler pulley bolt: 14-15 ft.lb. I got this value straight from the techs at my MBZ dealership.
This torque value is extremely important because stress is placed on the bearing if you over torque this bolt. Increased stress ultimately means decreased life of the bearing. Since this torque value is relatively low, I used Locktite on the bolts for extra peace of mind.

Conclusions:
Your STOCK idler pulley bearings may be failing – even if you don’t hear or see any problems right now! Note that you don’t need any power-adding modifications for your stock idler pulley to fail!

For the sake of 15 minutes, if you have a higher mileage M112K, I think it is worth it to check your idler pulley (and others) to make sure they are structurally and functionally sound. To do this, you need to have a stethoscope (or length of hosing) to amplify the sound coming from the pulley.

While you’re at it (especially you guys who have changed your pulleys in order to accommodate a larger crank pulley), check for play in the belt. If the belt is off, lateral forces can definitely contribute to the failure of the bearing. It’s simple physics.

Even though I heard an abnormal pulley bearing sound for about 5 minutes, I’m sure the bearings on the pulley would start to make much more noise when it is getting closer to complete failure. We may be playing with fire if we want to wait for audible confirmation that our idler bearings have failed.

If I have omitted any important details, let me know so that noobs aren’t led astray.

 

Bravo excellent write up

 

Wow! Good information and write up!

 

Hmmm... I am in an almost identical situation with mileage. My car is making some odd sounds also. This thread is great. I will be following this advise right away.

 

Mercedes has recently issued a major recall on the Idler/guide pulleys on various different models, PLEASE CHECK with your dealership, this SHOULD be a free replacement via recall replacement. Double check with your dealership.

 

First class wite up my friend.

How much does ANYONE want to bet that NO one torque that bolt to only 15 ft/lbs?

Thanks for the info

See yeah

 

Very good DIY write up Patrick!!! The Pictures are very always helpful , One thing that is hardy ever written into text is the use & application of adheasives , Sealants and the used of liquid thread locking stuff. I always use a small drop of RED loctite on all screw threads that go into an Alum Housings. Thanks again , We hope for many more write ups. Well maybe not? Hopefully not on your car!!! Cheers ___PTEngineering

 

C32 Thread of the Year!

Thank you boohoo.

 

superbly done- thank you. mine been wobbling a bit since i put on my evosport ODPS. Dale fabbed a spacer but made it worse. i've been watching it and there has been no change since original install but will not have it checked out.

i guess the pullied cars use a different idler?

i have close to 88k and about to drive 400 miles round trip on a biz trip this pm.

 

Awesome Thx!

 

First class thread I read since I came to MBworld!

This is the kind of stuff everyone wants/needs to know!

 

Most excellent.
15 Ft Lbs is light; locktight is a good idea and should be required.

 

+1 , awesome write-up and explanation. Do you suppose us guys with a different wrap would have the same problem?? Brandon, chime in here?? This ought to be a C32 sticky for sure!!!


Glenn

 

Does anyone know if this particular pulley # 4 was one of those recalled??? I have a different wrap than this one, BUT would assume the idler is the same??

 

I'm at 55K. Should I check?

 

Absolutely, everyone should check. Greatest thread in the past few months!!! Great job Ramblers!!!

 

Pulleys #6 and #9 use the same bearing you might want to change these as well.

 

Thanks for the kind words guys!

The recall does NOT apply to the 2002 C32 AMG. I talked to several people at the dealership and confirmed it with MBUSA just this morning. However, you may want to investigate further if you have a later model.

Good point! - it's definitely a 'weak link' on these pulleys - if you torque the bolt excessively, it's definitely going to shorten it's lifespan.

That's correct! If you have no immediate plans to remove the idler pulley, I would use the Red Loctite. Especially given that 15 ft.lb torque setting. Also, at 15 ft.lb, it shouldn't be too much of a bear to get off with Red Loctite. I currently have Blue Loctite on mine because I am working on an idler pulley 'emergency'repair' project that I will write up soon.

I would imagine the motors with overdrive pulley systems have their own proprietary idler pulley systems designed by that company. That said, some have reverted back to the stock idler pulley due to various issues and used long belts to compensate. It just depends on the pulley system.

Obviously Brandon is the one to ask this; however, it would probably depend on the degree of difference in wrap. i.e. if the wrap / contact area is not significantly altered, it probably shouldn't matter that much. That said, all pulleys should be operating within their rpm grease limits. Another factor to consider is the direction of the force acting on the pulley by the belt.

As alluded to above - it may be a different case for '03 and '04 models, but the '02 C32 is NOT on the recall list.

You should want to check!
A) It only takes a few minutes.
B) It's fun. I get an unhealthy dose of enjoyment from doing anything with my car!

I'm pretty sure #9 (tensioner) uses the same pulley - not sure about #6 (SC guide pulley). When I checked it, I couldn't get an ID on the bearing (see photo below).
Either way, they pass the stethoscope and spin tests with flying colors! Good point though - it's probably a good idea to keep an eye on those also.

 

Subscribed. Thanks!

 

i have 76K miles on my 2002 C32, but just had all of my pullies replaced (under warranty) about a month or two ago. am i safe?

 

How difficult is it to remove and replace S/C clutch bearing. I'm not talking about C3 pulley bearing. I noticed this is where an abnormal whining metallic sound at idle is coming from.

 

I should hope the idler pulley and the others are fine if they were replaced at the dealership a few months ago!


Well, having checked my own SC pulley and clutch, I think it is reasonable to assume that the sound of rotation coming from the entire SC assembly is different than the rest of the pulleys. i.e. expect it to sound somewhat abnormal. That said, it shouldn't be making a terrible din! Try the procedure in this little video to check that assembly:
[Please use appropriate gloves and use extreme caution with moving parts!!]

 

Thanks for the post

 

Here's a replacement alternative to your stock size idler pulley (smooth one below the SC).

Click the link below and scroll down to P/N's MTB460089 & MTB460089B toward the bottom of the page. These are DOUBLE bearing pulleys and aren't pricey at all. Our cars use a 90mm idler stock and will fit w/out modification (other than a longer bolt). There is also a 100mm version of this. If you're running the Gatorback belt with the Code3, your belt is too long to give sufficient tension; the 100mm should remedy this. Also, I'm not sure if this will fit on the belt tensioner, but that pulley is the same as the stock idler, so it might be a good idea to replace that at the same time. This will not replace the idler pulley that came with your larger crank pulley since this is too big.

http://www.rpmoutlet.com/03cobraasp.htm#2

There are numerous other 90mm pulleys available from other vendors as well, since this is a very popular and necessary upgrade for the Cobra guys. They are available in many different colors but these are the only double bearing versions I've seen.

 

Any more information on this recall? I just got a check engine light and I'm wondering if it is related.