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Discussion Starter #1
my belt blew apart this weekend and clearing the mess, the idle pulley was pretty badly chewed up. Here is the question, the dealer insists that the pulley is smooth and not grooved and yet the inside of the belt routes across the surface. Does that make sense that the notched side of the belt runs across the smooth idle pulley?
 

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its an 08 ml320 cdi. the idle pulley in question is the pulley between the tensioner and the water pump located pretty much in top center of the engine. the diagram shows that routes across the top of the pulley which puts the inside of belt against the smooth surface of the pulley (6422000570). I thought about using the 2722021019 grooved pulley but it is a larger diameter, not sure of that is an option.
 

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This might be just a design thing. I found a diagram, that shows the same part (part 8) used for both an inside and outside idle pulley
. the old belt had around 100k on it, so it was due for replacement. my concern is that the top pulley was a real mess with grooves warn half way through it.
 

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This might be just a design thing. I found a diagram, that shows the same part (part 8) used for both an inside and outside idle pulley
. the old belt had around 100k on it, so it was due for replacement. my concern is that the top pulley was a real mess with grooves warn half way through it.
Your number 5 is actually the pulley on the tensioner assembly. It's not separately available and is definitely different from the idler pulley.


The EPC shows two part numbers for the 164.122 and they are based on engine number.

P/N 6422000570 through engine no. 40777759

P/N 6422001070 from engine no. 40777760

Both of them are smooth pulleys.

I'd order the pulley based on the EPC and your engine number. There should be a routing diagram in your owner's manual.

Good luck.
 

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Discussion Starter #7
the diagram i posted is misleading in that it had the red 5 marked for the tensioner for another purpose. The pully that i am questioning is the number 8 at the top of the engine. Notice how it is on the inside of the serpentine and yet is a smooth pulley. It seems strange to me that a smooth pulley would be used on the inside of a grooved belt, but that must be the design. I have a theory as to why my pulley was so messed up. When i pulled the old belt parts out, the water pump wasn't spinning freely. i assumed that was just a result of some of the crud from the belt and pulley, but i am thinking that it was the cause of the problem. if it wasn't spinning freely, the belt would have been sliding hot across that pulley and melting the idler. everything freed up now, i installed the smooth pulley as per the EPC and so far it all looks good, i just thought it rather strange to use a smooth pulley on the inside of a serpentine belt. thanks for your help.
 

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thats very interesting! there is no part number that i can find that matches an 08 CDI grooved pulley for part number 8 at the top the diagram. both Part number 8's are smooth (see check codes post), which leads me to think that the belt wears the grooves into the pulley and over time, that pulley will almost certainly wear through as it did in my case. it just doesn't make sense that a smooth roller would be spec'd on the inside of a serpentine belt! thanks for checking.
 

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Are you in the US? I am wondering if there is a different part for non-US 642 engines. Mine is an 08 CDI, grooved pulley and according to parts diagram A6422000570 is in fact a grooved pulley on mine
 

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thanks for checking on your end, this is getting even stranger. I am in Canada, A6422000570 is a smooth pulley here and used twice on the vehicle (part 8 in the diagram). Are you sure your A6422000570 was a grooved pulley when it was new or is just one now, as mine will soon be in a few more miles. Where are you seeing the diagram that shows the part as grooved? when I google the part number and select images, i see a lot of smooth rollers.
i suspect that the pulley is installed smooth and grooves are being worn in, which does not sound like a good idea for either the belt or pulley?
 

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I took a second look and I was wrong in my post.

The pulley, indeed, is smooth as I was looking at the wrong one. Took the engine cover off this time and here's the top #8 pulley.

ImageUploadedByAutoGuide1423011517.422550.jpg

Kind of a funky spot, but the surface is slightly grooved.

Here's the pulley I originally looked at which is left of the first photo. Clearly designed with grooves.

ImageUploadedByAutoGuide1423011767.212456.jpg

Skippy
 

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that is some investigating! After looking around I see there are a few others who do the grooved pulley over the smooth plastic roller as well. I would suggest when you are doing any service work around belt related components, to have a look at that top pulley. With the belt heating, cooling and wearing the pulley down, eventually it develops pretty sharp edges that in my case, i believe tore the belt to shreds. thanks for all your assistance.
 

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A couple of months back I replaced the idler pulley and the belt, the pulley is smooth, with the grooved part on the smooth pulley (original pulley) I saw markings which misleads you to believe it is a grooved pulley
 

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that is some investigating! After looking around I see there are a few others who do the grooved pulley over the smooth plastic roller as well. I would suggest when you are doing any service work around belt related components, to have a look at that top pulley. With the belt heating, cooling and wearing the pulley down, eventually it develops pretty sharp edges that in my case, i believe tore the belt to shreds. thanks for all your assistance.
how did you get pulley off as my belt just snapped and pulley badley scored
 

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I just found this tread because I'm having a hard time with this also. I just snapped a belt and when inspecting my pullies, I found the surface of the #8 pulley looking completely mutilated. The pully appears to be grooved, but when I searched for a replacement, as has been noted, the only options are smooth pullies. From this tread, I now see that the belt created this pseudo grooved pully. This makes no sense and I think there must have been a mistake somewhere along the line. Having a smooth pulley putting pressure on the outside of the ribs can't be good for the belt. It's basically flattening the ribs. If its wearing the plastic pulley so severely, think about what its doing to the softer belt. I've attached photos of the upper (#8) and lower idler pullies, including them stacked to show the difference in wear.

The pulley has ~150k miles, while the belt snapped at ~70,000 miles, which was 10k miles before my scheduled replacement.
 

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Nilknarf,

I suspect both of the pulleys you have are smooth. The one with the buildup is really just belt material.
Do you have any oil leaks going onto the belt or the pulleys? Might be a reason for the material transfer.

Skippy


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