Serpentine belt wiggle on v6

I just changed the serpentine belt, and looking at everything with a fine-tooth comb I notice (at idle speed) that the belt wiggles back-and-forth a little bit on the tensioner pulley. The amount of movement is 1/16 inch at most. The tensioner bearing felt good. No movement like this over the idler pulleys. Anybody know if this is normal? Maybe a side-effect of a brand new belt? Wish I would have looked at this before pulling the old belt off.
 
Good suggestion. In this case the only thing I removed and put back was the belt. At 36k miles, I didn't pull any of the bearings or pulleys because the all felt ok.
 

CJPJ

2008 3500 170 EXTD 3.0 V6 OM642.993 4.182
Guessing that the belt is cross rutted . One grove one pulley. slight the belt edge with a straight edge top to bottom.
More guess ,.. that it's the crank pully that's one grove off
Biggest guess , ..,that the belt is inboard towards the engine
 
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Good suggestion to check for cross-rutted. I looked carefully at all 5 grooved pulleys from under the van, and the belt is seated properly on all of them. I looked at 3 cars for belt wiggle on the tensioner. None of them have it, but I noticed than in all 3 cases there's not more than about 3 inches of belt between the tensioner pulley and a grooved pulley, on both side of the tensioner. On the OM642, there's about 10 inches of open-span belt (visual estimate) between the tensioner and the next pulley, an ungrooved idler. And about 15 inches from the tensioner to the first grooved pulley on the alternator. So I'm wondering if a small amount of wander on the idler puller might not be normal, or at least permissible? Unless I learn something new I just need to keep a close eye on it for a while. I'm just nervous how some little problem or mistake on these engines can run up a multi-thousand repair bill in the blink of an eye.
 

CJPJ

2008 3500 170 EXTD 3.0 V6 OM642.993 4.182
I routinely look at the v6 belt operation and I'll describe it as calm. The wander u describe isn't conforming to normal. Time to put the stethoscope on the idler, tensioner pulley, etc ..

Second thought; " 1/16" - is small: .. maybe clean both sides of the belt with soapy water.
The wander may be caused by a oily dirt contamination spot on the belt.
 
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Gathered a little more data today: (1) The wiggle is synchronized with the belt, i.e. one wiggle per full round trip of the belt. I think this would eliminate any of the pulleys as the cause. (2) Looking at the distance of the wiggle more closely, it's slightly less than half the width of the belt ribs. The ribs are about 1/16" wide, making the amount of wiggle a bit less than 1/32". Looking from both above and below with the engine running, the belt tracks dead straight everywhere I can see it, except on the tensioner pulley. I'm thinking I'll just drive it some and see if anything changes.
 

220629

Well-known member
Gathered a little more data today: (1) The wiggle is synchronized with the belt, i.e. one wiggle per full round trip of the belt. ...
So the belt is defective? Can you return the removed belt to service and see if the condition goes away?

If the new belt is defective I wouldn't run it. There's a chance that it can increase stress on the bearings or other parts. Why take the risk. You should be able to return it as defective. Take a short video.

:2cents: vic
 
I put the original belt back on today and it wiggles/wanders on the tensioner pulley the same as the new belt. Upon closer examination, the tensioner pulley does have a small bit of play in the bearing. The pulley can be wiggled back and forth a little bit, the very slight clank-clank when rocking it side to side. When rotating the tensioner pulley it feels great. Is a little bit of side-to-side play acceptable, or should the tensioner pulley have absolutely no play whatsoever?
 

220629

Well-known member
... Is a little bit of side-to-side play acceptable, or should the tensioner pulley have absolutely no play whatsoever?
The plastic pulley is fitted to and rides on a ball bearing. A ball bearing would have no side play. I wouldn't expect that any of the other tensioner parts that are in proper operating condition would be described as "wiggled back and forth a little bit, the very slight clank-clank when rocking it side to side".

Replace the clank-clank parts. If the condition persists, further investigation will be necessary.

:2cents: vic
 

sprint2freedom

2008 NCV3 170ext
Not an expert on this topic, but I would say that any visible play is unacceptable.

My serp belt and tensioner are original at >80k miles (I know, time to change them!) and the belt rides absolutely smooth, with no side-to-side play whatseover.
 
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I'm going to replace the tensioner, and figure I should do the idlers too. A question about the idlers: Is it possible to replace the upper idler without removing the fan belt pulley? I popped the plastic cover loose but can't pull it out because the fan pulley is in the way. The service manual says just take the old idler off and put the new one on with no mention of the fan pulley. Either I'm a klutz or the fan pulley has to come off.
 

CJPJ

2008 3500 170 EXTD 3.0 V6 OM642.993 4.182
The plastic pulley is fitted to and rides on a ball bearing. A ball bearing would have no side play. I wouldn't expect that any of the other tensioner parts that are in proper operating condition would be described as "wiggled back and forth a little bit, the very slight clank-clank when rocking it side to side".

Replace the clank-clank parts. If the condition persists, further investigation will be necessary.

:2cents: vic
: cc agree that a ball bear will have almost no side to side play within its races.
With that said: the bearing races ride on a shaft and it's normal for bearing races to have end play on the shaft. A bearing end play small doesn't effect the bear alignment with the fan belt. The small side play mentioned doesn't necessarily indicate a bad bearing. A new one may have same play.
 

220629

Well-known member
... A new one may have same play.
True, but not likely to clank-clank. With 2 different installed belts not running true, let's not discourage him from replacing parts. Eg. - Both my T1N serpentine belt systems run smooth, unless there's a problem.

:cheers: vic
 
Looking at side play vs. end play, if I try to move the tensioner pulley in and out in a straight direction, there's no movement I can detect. The only movement I pick up is rocking the pulley side-to-side.
 
Original poster back with a report. Took a few days to get new parts.

- Tensioner replaced with one from europarts-sd. Appears identical to original. Pulley on new tensioner has play that feels identical to the original tensioner.

- Idler pulleys replaced with idlers from europarts-sd. New pulleys appear identical except for having a dust shield on the rear (similar to the carrier bearing dust shield).

- Fan bearing replaced because it had a little end play. New one also has a little end play. New part from Mercedes has a larger bearing than the original, as seen from rear of the part.

- Measured radial and axial runout of harmonic balancer is between 3 and 5 thousands of an inch (setting up the dial indicator was fairly difficult). Maximum spec is 15.8 thousands.

- There is no visible axial runout on any of the 8 pulleys the belt goes over with the engine running.

Result: Belt still wanders side-to-side a small amount on the tensioner. I think the amount is a bit less than before replacing the parts. I don't have any way of getting a good measurement. I have possibly wasted a bunch of time and money replacing 4 bearings that didn't need replacing, worth it to me to know these parts aren't close to some sort of failure. I will be looking closely how the belt is tracking, especially early-on. If it goes a few hundred miles with no change I'm going to think a little wobble is normal. Maybe it's just a side-effect of the in-spec harmonic balancer runout.
 

HKs00

Well-known member
I work on a fleet of sprinters and about 20% of them have the wobble that you are describing. On average our vans see about 3.5k mi a month so we go trough a lot of maintenance repairs. Also some of our trucks run 24/7 and the only time that they come off the road is when they brake down, or 2 times a month for maintenance. When we first started noticing the wobble we literally tried everything to resolve it. One of our vans had all the pulleys tensioner, water pump, power steering pump and compressor replaced ( not all at the same time) and nothing resolved it. I believe that on some vans from factory there is a little wobble from something not being 100% aligned to the rest of the grooved pulleys and it causes the wobble. The only time that I would be concerned about the wobble is if its big enough that causes the belt to run on the edge of one of the pulleys causing it to groove and causing premature belt wear. As long as all of the pulleys feel OK and they are not making any noise and the belt tensioner is keeping the correct amount of tension on the belt I think it will be fine.
 

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